Biochemistry 2 PDF

Thursday, September 5, 2024 10:16 Amino Acids and the Role of pH: Amino Acids Basics Amino acids are the building blocks of proteins. Each amino acid has an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (H), and a unique side chain (R group) attached to a central...

Thursday, September 5, 2024 10:16 Amino Acids and the Role of pH: Amino Acids Basics Amino acids are the building blocks of proteins. Each amino acid has an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (H), and a unique side chain (R group) attached to a central alpha carbon. The side chain (R group) determines the unique properties of each amino acid (e.g., polar, nonpolar, acidic, or basic). Classification of Amino Acids Amino acids are categorized based on the properties of their side chains: ○ Nonpolar (hydrophobic): Tend to repel water (e.g., glycine, alanine). ○ Polar (hydrophilic): Attract water (e.g., serine, threonine). ○ Acidic: Contain additional carboxyl groups (e.g., aspartic acid, glutamic acid). ○ Basic: Contain additional amino groups (e.g., lysine, arginine). Ionization and the Role of pH Amino acids can exist in different ionization states depending on the pH of their environment. At low pH (acidic), amino acids are fully protonated (positive charge on the amino group). At high pH (basic), amino acids lose protons (negative charge on the carboxyl group). At neutral pH (around 7), amino acids exist as zwitterions (both positively and negatively charged, but overall neutral). pKa and Buffering Capacity Each amino acid has a characteristic pKa, which is the pH at which 50% of the molecule is ionized. The buffering capacity of an amino acid refers to its ability to resist changes in pH. This is important in maintaining the proper pH balance in the body. The body's pH is tightly regulated because even small changes in pH can affect protein structure and function. Isoelectric Point (pI) The isoelectric point (pI) is the pH at which the amino acid has no net charge (neutral). This is important in protein purification techniques, as proteins will behave differently depending on their charge at different pH levels. Protiens Page 1 Thursday, September 5, 2024 10:20 Protein Structure Proteins are large biomolecules made of long chains of amino acids. Their function is closely related to their structure, which is organized into four levels: Primary Structure: The linear sequence of amino acids, determined by genetic information. This sequence dictates how the protein will fold and function. Secondary Structure: Local folding patterns within a protein chain, stabilized by hydrogen bonds. The most common types are: ○ Alpha helices: Right-handed coils. ○ Beta sheets: Sheet-like structures, which can be parallel or antiparallel. Tertiary Structure: The three-dimensional conformation of the protein, driven by interactions between side chains (hydrophobic interactions, ionic bonds, disulfide bonds, and hydrogen bonds). This structure determines the protein’s biological activity. Quaternary Structure: Some proteins consist of multiple polypeptide chains (subunits) that come together to form a functional protein. Hemoglobin is an example of a protein with quaternary structure. Fibrous and Globular Proteins Proteins are generally classified into two types based on their shape and function: Fibrous proteins: These are long, insoluble proteins that play structural roles (e.g., collagen, keratin). Globular proteins: These are compact, soluble proteins with dynamic functions, such as enzymes (e.g., hemoglobin, myoglobin). Protein Folding and Stability Protein folding is a highly regulated process that ensures the correct three-dimensional structure is achieved. Chaperone proteins assist in folding by preventing misfolding or aggregation. Denaturation: Proteins can lose their structure (and thus their function) when exposed to factors like heat, pH changes, or chemicals. This process is often irreversible. The proper folding of proteins is crucial because misfolding can lead to diseases like Alzheimer’s and Parkinson’s. Post-translational Modifications After translation, proteins often undergo modifications that affect their function: Phosphorylation: Addition of a phosphate group, which can regulate enzyme activity. Glycosylation: Addition of carbohydrate groups, important for protein stability and recognition. Proteolysis: Cleavage of a protein, often to activate or deactivate it. Protein Function Proteins have a wide range of functions in the body, including: Enzymes: Catalyze biochemical reactions. Structural proteins: Provide support and shape to cells (e.g., collagen in connective tissue). Transport proteins: Move molecules across membranes or through the body (e.g., hemoglobin transporting oxygen). Hormonal proteins: Regulate physiological processes (e.g., insulin). Defense proteins: Protect the body (e.g., antibodies in the immune system). Protiens Page 2  Defense proteins: Protect the body (e.g., antibodies in the immune system). Enzymes and Catalysis Enzymes are specialized proteins that speed up chemical reactions without being consumed. They lower the activation energy required for a reaction to proceed. Active sites on enzymes bind to specific substrates, leading to the formation of enzyme-substrate complexes. Enzyme activity can be regulated by factors such as temperature, pH, and the presence of inhibitors or activators. Protiens Page 3 Thursday, September 5, 2024 10:23 Enzymes Enzymes are biological catalysts that accelerate chemical reactions by lowering the activation energy required. They do not alter the equilibrium of reactions but make it easier for reactants (substrates) to convert into products. Active Site The active site is the specific region of an enzyme where substrate molecules bind and undergo a chemical transformation. The enzyme-substrate interaction can follow two models: ○ Lock-and-key model: The substrate fits perfectly into the active site. ○ Induced-fit model: The enzyme undergoes a conformational change to accommodate the substrate upon binding. Enzyme Specificity Enzymes are highly specific, meaning they catalyze only one type of reaction or work with specific substrates. This specificity is due to the precise interaction between the enzyme’s active site and the substrate. Cofactors and Coenzymes Cofactors are non-protein molecules that assist enzymes in catalysis. They can be metal ions (e.g., Zn²⁺, Mg²⁺) or organic molecules. Coenzymes are organic cofactors, often derived from vitamins (e.g., NAD⁺, FAD). They play an essential role in transferring chemical groups between molecules. Factors Affecting Enzyme Activity Several factors influence enzyme activity, including: Substrate concentration: As substrate concentration increases, reaction velocity increases until the enzyme is saturated (Vmax). Temperature: Increasing temperature boosts reaction rates until a point where the enzyme denatures and loses activity. pH: Each enzyme has an optimal pH at which it functions most efficiently. Deviations can alter the ionization of the enzyme’s active site, reducing its activity. Enzyme Inhibition Inhibitors are molecules that decrease enzyme activity. They can be classified into: ○ Competitive inhibitors: Bind to the active site, competing with the substrate. They increase Km but do not affect Vmax. Their effects can be overcome by increasing substrate concentration. ○ Noncompetitive inhibitors: Bind to an allosteric site (not the active site) and reduce enzyme activity without affecting substrate binding. They lower Vmax but do not change Km. ○ Uncompetitive inhibitors: Bind only to the enzyme-substrate complex, lowering both Vmax and Km. Regulation of Enzyme Activity Enzyme activity is tightly regulated to ensure that metabolic processes occur efficiently. Mechanisms Protiens Page 4 Enzyme activity is tightly regulated to ensure that metabolic processes occur efficiently. Mechanisms include: Allosteric regulation: Allosteric enzymes have sites where molecules (effectors) bind and cause conformational changes, either activating or inhibiting the enzyme. These enzymes do not follow Michaelis-Menten kinetics. Covalent modification: Some enzymes are regulated by reversible covalent modifications, such as phosphorylation or dephosphorylation. Feedback inhibition: In some pathways, the end product of a reaction inhibits an enzyme earlier in the pathway, controlling the flow of metabolites. Proteolytic cleavage: Some enzymes are synthesized as inactive precursors (zymogens) and are activated by cleavage (e.g., digestive enzymes like trypsin). Clinical Relevance of Enzymes Measuring enzyme levels in the blood can be a diagnostic tool for diseases. For example, elevated levels of creatine kinase (CK) can indicate muscle damage, including heart attacks. Enzymes like lactate dehydrogenase (LDH) and alanine aminotransferase (ALT) are also used to diagnose liver and tissue damage. Protiens Page 5 Thursday, September 5, 2024 11:00 Bioenergetics Overview Bioenergetics refers to the study of energy flow in biological systems. It focuses on how cells harness energy from food (nutrients) to perform work, grow, and maintain homeostasis. Free energy (G) is the energy available to do work in a system. ○ ΔG (change in free energy) determines whether a biochemical reaction is spontaneous. ○ A reaction with negative ΔG (ΔG < 0) is exergonic and releases energy (spontaneous). ○ A reaction with positive ΔG (ΔG > 0) is endergonic and requires energy input to proceed (non-spontaneous). Enthalpy and Entropy Enthalpy (H): Reflects the total heat content of a system. ○ A reaction with negative ΔH releases heat and is exothermic. ○ A reaction with positive ΔH absorbs heat and is endothermic. Entropy (S): Represents the disorder or randomness of a system. Systems naturally tend to move towards higher entropy. ○ An increase in entropy favors the spontaneity of a reaction. Standard Free Energy Change (ΔG⁰') ΔG⁰' refers to the standard free energy change, which is measured under specific conditions (1 M concentration of reactants/products, pH 7, 25°C, and 1 atm pressure). It provides a reference point to compare the energy changes of different reactions. In cells, actual conditions may vary, so the actual free energy change (ΔG) depends on the concentration of reactants and products. ATP: The Energy Currency Adenosine triphosphate (ATP) is the primary energy carrier in the cell. It stores energy in its high- energy phosphoanhydride bonds. The hydrolysis of ATP to ADP (adenosine diphosphate) or AMP (adenosine monophosphate) releases energy, which drives many biological reactions: ATP hydrolysis has a large negative ΔG⁰' (-30.5 kJ/mol), making it a powerful source of energy for cellular processes like muscle contraction, active transport, and biosynthesis. ATP is regenerated by processes like oxidative phosphorylation and glycolysis. Coupled Reactions Endergonic reactions (ΔG > 0) can proceed by coupling them to exergonic reactions (ΔG < 0), such as the hydrolysis of ATP. For example, biosynthetic pathways that require energy are often coupled to ATP hydrolysis to drive the process forward. Types of Biochemical Reactions There are five main types of biochemical reactions in metabolism: Oxidation-Reduction (Redox) Reactions: Involve the transfer of electrons. ○ Oxidation: Loss of electrons. ○ Reduction: Gain of electrons. ○ Redox reactions are fundamental to processes like cellular respiration, where electrons are transferred from nutrients (e.g., glucose) to oxygen, producing ATP. Bioenergetics and Carbohydrate Page 6  transferred from nutrients (e.g., glucose) to oxygen, producing ATP. Ligation Reactions: Use ATP to form bonds between molecules. For example, the synthesis of oxaloacetate from pyruvate and carbon dioxide requires energy from ATP hydrolysis. Isomerization Reactions: Involve the rearrangement of atoms within a molecule. For example, in glycolysis, glucose-6-phosphate is converted to fructose-6-phosphate. Group Transfer Reactions: Transfer of a chemical group from one molecule to another. A common example is the transfer of a phosphate group from ATP to another molecule (phosphorylation). Hydrolytic Reactions: Use water to break bonds, such as the hydrolysis of peptides into amino acids or ATP into ADP and inorganic phosphate. Oxidation States of Carbon Carbon can exist in different oxidation states depending on the number of electrons associated with it. The more reduced a carbon atom is (e.g., in hydrocarbons), the more energy it contains. As nutrients like carbohydrates and fats are oxidized during metabolism, they release energy, which is harnessed by cells to regenerate ATP. High-Energy Compounds Besides ATP, other molecules like creatine phosphate, acetyl-CoA, and NADH store and transfer energy. Creatine phosphate serves as an energy reserve in muscles, while acetyl-CoA is a key intermediate in many metabolic pathways. NADH and FADH2 are electron carriers that play critical roles in oxidative phosphorylation. Bioenergetics and Carbohydrate Page 7 Thursday, September 5, 2024 11:14 Carbohydrates Overview Carbohydrates are essential biomolecules that serve as energy sources, structural components, and signaling molecules in biological systems. They are classified based on the number of sugar units they contain: ○ Monosaccharides: Single sugar molecules (e.g., glucose, fructose). ○ Disaccharides: Two sugar units (e.g., sucrose, lactose). ○ Oligosaccharides: Short chains of monosaccharides (3–10 units). ○ Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose). Monosaccharides: Structure and Stereochemistry Monosaccharides are the simplest carbohydrates, consisting of a single sugar unit. They are classified by: ○ Number of carbon atoms: ▪ Trioses (3 carbon atoms), ▪ Tetroses (4 carbon atoms), ▪ Pentoses (5 carbon atoms, e.g., ribose), ▪ Hexoses (6 carbon atoms, e.g., glucose, fructose). ○ Functional group: ▪ Aldoses (containing an aldehyde group, e.g., glucose), ▪ Ketoses (containing a ketone group, e.g., fructose). Monosaccharides exhibit chirality, meaning they exist in different stereoisomers due to the presence of asymmetric (chiral) carbons. D- and L-isomers: Monosaccharides are classified into D- or L-forms based on the orientation of the hydroxyl group (-OH) on the asymmetric carbon farthest from the carbonyl group. Most naturally occurring sugars are in the D-form. Cyclization of Monosaccharides In aqueous solutions, monosaccharides like glucose and fructose can exist in cyclic forms due to an internal reaction between the carbonyl group and a hydroxyl group. Hemiacetal or hemiketal structures are formed: ○ Aldoses (like glucose) form hemiacetals, creating a six-membered ring (pyranose form). ○ Ketoses (like fructose) form hemiketals, often creating a five-membered ring (furanose form). The formation of these rings introduces a new asymmetric carbon called the anomeric carbon. Alpha (α) and beta (β) anomers: The configuration of the hydroxyl group on the anomeric carbon can be either α (down) or β (up). Disaccharides Disaccharides consist of two monosaccharides linked by a glycosidic bond, which forms between the hydroxyl group of the anomeric carbon of one sugar and a hydroxyl group of another sugar. Important disaccharides: ○ Sucrose (glucose + fructose): Table sugar, linked by an α(1→2) glycosidic bond. ○ Lactose (galactose + glucose): Found in milk, linked by a β(1→4) glycosidic bond. ○ Maltose (glucose + glucose): A product of starch digestion, linked by an α(1→4) glycosidic bond. Bioenergetics and Carbohydrate Page 8 Polysaccharides Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. They can be classified as homopolysaccharides (one type of monosaccharide) or heteropolysaccharides (more than one type of monosaccharide). Storage polysaccharides: ○ Starch: The primary storage form of glucose in plants, consisting of amylose (unbranched, α(1→4) glycosidic bonds) and amylopectin (branched, α(1→6) glycosidic bonds). ○ Glycogen: The storage form of glucose in animals, similar to amylopectin but more highly branched, allowing rapid mobilization of glucose. Structural polysaccharides: ○ Cellulose: A major component of plant cell walls, composed of β(1→4) linked glucose units. Humans cannot digest cellulose due to the lack of enzymes that hydrolyze β-glycosidic bonds. ○ Chitin: A structural polysaccharide found in the exoskeletons of insects and crustaceans. Glycoproteins and Proteoglycans Glycoproteins: Proteins covalently attached to short carbohydrate chains (oligosaccharides). They play roles in cell recognition, signaling, and immune response. Common glycoproteins are found in the cell membrane. Proteoglycans: Consist of proteins attached to long chains of polysaccharides called glycosaminoglycans (GAGs). Proteoglycans are important for the structure of the extracellular matrix and in cartilage. Glycosylation: The process of adding carbohydrates to proteins or lipids. It is crucial for protein folding, stability, and cellular communication. Carbohydrate Function Energy source: Carbohydrates like glucose are the primary energy source for most organisms. Glucose is oxidized during glycolysis to produce ATP. Structural role: Polysaccharides like cellulose and chitin provide structural support in plants and animals. Cell recognition: Glycoproteins and glycolipids on the surface of cells play key roles in cell-cell recognition, signaling, and immune response. Bioenergetics and Carbohydrate Page 9 Thursday, September 5, 2024 12:00 The TCA Cycle The Tricarboxylic Acid (TCA) Cycle, also known as the Citric Acid Cycle or Krebs Cycle, is a central metabolic pathway in aerobic organisms. The TCA cycle occurs in the mitochondrial matrix and is essential for the complete oxidation of acetyl-CoA into carbon dioxide (CO₂), producing NADH, FADH₂, and GTP (or ATP), which are key for energy production. It serves as a hub for several metabolic pathways, including carbohydrates, fats, and proteins, feeding into the cycle through acetyl-CoA. Pyruvate Dehydrogenase Complex (PDC) Before entering the TCA cycle, pyruvate (the end product of glycolysis) is converted to acetyl-CoA by the Pyruvate Dehydrogenase Complex (PDC). This reaction is irreversible and is a key regulatory step between glycolysis and the TCA cycle. The conversion of pyruvate to acetyl-CoA involves: ○ Decarboxylation: Removal of one carbon from pyruvate, releasing CO₂. ○ Oxidation: Transfer of electrons from pyruvate to NAD⁺, forming NADH. ○ Formation of Acetyl-CoA: The remaining two-carbon unit is attached to coenzyme A (CoA) to form acetyl-CoA. PDC is a large, multienzyme complex made up of three enzymes (E1, E2, and E3) and requires five cofactors: thiamine pyrophosphate (TPP), lipoic acid, coenzyme A, FAD, and NAD⁺. Regulation of PDC: ○ Inhibition by phosphorylation: When energy levels are high (high ATP, NADH, and acetyl- CoA), PDC is phosphorylated and inhibited by PDC kinase. ○ Activation by dephosphorylation: PDC is activated by PDC phosphatase in response to high ADP and pyruvate levels (low energy). Steps of the TCA Cycle The TCA cycle is a series of eight enzyme-catalyzed reactions that oxidize acetyl-CoA to CO₂ while producing high-energy molecules (NADH, FADH₂, and GTP). The key steps are: Step 1: Citrate formation: Acetyl-CoA (2-carbon) combines with oxaloacetate (4-carbon) to form citrate (6-carbon), catalyzed by the enzyme citrate synthase. Step 2: Isomerization of citrate: Citrate is converted to isocitrate by the enzyme aconitase. Step 3: Oxidative decarboxylation of isocitrate: Isocitrate dehydrogenase catalyzes the oxidation of isocitrate to α-ketoglutarate, producing NADH and releasing CO₂ (first decarboxylation). Step 4: Oxidative decarboxylation of α-ketoglutarate: α-ketoglutarate dehydrogenase converts α-ketoglutarate to succinyl-CoA, producing NADH and releasing another molecule of CO₂ (second decarboxylation). This step is similar to the PDC reaction. Step 5: Conversion of succinyl-CoA to succinate: Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate, generating GTP (or ATP) through substrate-level phosphorylation. Step 6: Oxidation of succinate: Succinate dehydrogenase catalyzes the oxidation of succinate to fumarate, producing FADH₂. Step 7: Hydration of fumarate: Fumarase catalyzes the hydration of fumarate to form malate. Step 8: Oxidation of malate: Malate dehydrogenase catalyzes the oxidation of malate to oxaloacetate, generating NADH. This oxaloacetate can then react with another acetyl-CoA to continue the cycle. Bioenergetics and Carbohydrate Page 10 Energy Yield of the TCA Cycle For each turn of the TCA cycle, one molecule of acetyl-CoA generates: ○ 3 NADH ○ 1 FADH₂ ○ 1 GTP (which can be converted to ATP) ○ 2 CO₂ Each molecule of NADH produced during the TCA cycle yields about 2.5 ATP, and each molecule of FADH₂ yields about 1.5 ATP when oxidized in the electron transport chain. Therefore, the total energy yield from the TCA cycle per acetyl-CoA is approximately 10 ATP. Regulation of the TCA Cycle The TCA cycle is tightly regulated to ensure efficient energy production based on the cell’s needs. Key points of regulation include: Citrate synthase: Inhibited by high levels of ATP, NADH, and citrate, indicating sufficient energy. Isocitrate dehydrogenase: Activated by ADP (low energy) and inhibited by ATP and NADH. α-Ketoglutarate dehydrogenase: Inhibited by high levels of NADH and succinyl-CoA and activated by ADP and calcium ions (Ca²⁺). Anaplerotic Reactions The TCA cycle not only serves as a pathway for energy production but also provides intermediates for biosynthetic pathways (e.g., amino acids, heme, and fatty acids). Anaplerotic reactions replenish TCA cycle intermediates that are withdrawn for biosynthesis. For example: ○ The conversion of pyruvate to oxaloacetate by the enzyme pyruvate carboxylase is a key anaplerotic reaction that ensures the cycle continues even when intermediates are being used for other processes. Clinical Relevance Pyruvate Dehydrogenase Complex Deficiency: A deficiency in PDC leads to an inability to convert pyruvate to acetyl-CoA, causing a buildup of lactate and leading to conditions like lactic acidosis. This can cause neurological problems, as the brain relies heavily on glucose oxidation for energy. Thiamine Deficiency (Beriberi): Thiamine is a cofactor for PDC and α-ketoglutarate dehydrogenase. A deficiency can impair energy metabolism, affecting tissues like the nervous and cardiovascular systems. The Amphibolic Nature of the TCA Cycle The TCA cycle is described as amphibolic, meaning it plays both catabolic (breakdown of molecules for energy) and anabolic (synthesis of biomolecules) roles. Catabolic: Oxidation of acetyl-CoA to CO₂ with energy production. Anabolic: Provides precursors for the biosynthesis of amino acids, fatty acids, nucleotides, and other essential biomolecules. Bioenergetics and Carbohydrate Page 11 Thursday, September 5, 2024 12:03 Gluconeogenesis Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors, such as lactate, glycerol, and amino acids (mainly alanine). This pathway is crucial for maintaining blood glucose levels, especially during fasting, prolonged exercise, or carbohydrate starvation, as the brain, red blood cells, and other tissues rely heavily on glucose as a fuel source. It primarily occurs in the liver (90%) and kidneys (10%), with minor contributions from the intestines under some conditions. Precursors for Gluconeogenesis Several non-carbohydrate molecules can be converted into glucose: Lactate: Produced by anaerobic glycolysis in tissues like muscles and red blood cells, lactate enters the liver where it is converted to glucose in the Cori cycle. Glycerol: Released from the breakdown of triglycerides in adipose tissue, glycerol is converted to glucose in the liver. Amino acids: Alanine and other glucogenic amino acids from muscle protein breakdown provide substrates for gluconeogenesis, with alanine being a key player in the Glucose-Alanine cycle. Gluconeogenesis vs. Glycolysis Gluconeogenesis is essentially the reverse of glycolysis, but it is not a simple reversal. It bypasses the three irreversible steps of glycolysis using distinct enzymes. These key irreversible steps of glycolysis that are bypassed include: ○ The conversion of pyruvate to phosphoenolpyruvate (PEP) ○ The conversion of fructose-1,6-bisphosphate to fructose-6-phosphate ○ The conversion of glucose-6-phosphate to glucose Key Steps and Enzymes of Gluconeogenesis Step 1: Pyruvate to PEP: The conversion of pyruvate to phosphoenolpyruvate (PEP) occurs in two steps: ○ Pyruvate carboxylase: Converts pyruvate to oxaloacetate in the mitochondria. This step requires ATP and biotin as a cofactor. ○ PEP carboxykinase (PEPCK): Converts oxaloacetate to PEP in the cytoplasm, using GTP as an energy source. ○ Oxaloacetate must be transported from the mitochondria to the cytoplasm as malate or aspartate because the inner mitochondrial membrane is impermeable to oxaloacetate. Step 2: Fructose-1,6-bisphosphate to fructose-6-phosphate: The enzyme fructose-1,6- bisphosphatase catalyzes this step, bypassing the rate-limiting step of glycolysis catalyzed by phosphofructokinase-1 (PFK-1). This is a key regulatory step in gluconeogenesis. Step 3: Glucose-6-phosphate to glucose: The enzyme glucose-6-phosphatase is responsible for converting glucose-6-phosphate to glucose, a step that occurs in the liver and kidney. This step allows the free glucose to be released into the bloodstream to maintain blood glucose levels. Muscle cells lack this enzyme and cannot perform this step. Energy Requirements of Gluconeogenesis Gluconeogenesis is an energy-intensive process. To synthesize one molecule of glucose from two molecules of pyruvate, gluconeogenesis requires: Bioenergetics and Carbohydrate Page 12  molecules of pyruvate, gluconeogenesis requires: ○ 4 ATP molecules ○ 2 GTP molecules ○ 2 NADH molecules This high energy requirement makes gluconeogenesis a tightly regulated process, as it only occurs when energy and substrates are available, usually during fasting or prolonged exercise. Regulation of Gluconeogenesis Gluconeogenesis is tightly regulated to prevent simultaneous activation of glycolysis, avoiding a futile cycle. It is regulated at multiple levels, including hormonal control and the availability of substrates: Hormonal regulation: ○ Insulin: Inhibits gluconeogenesis. High insulin levels (e.g., after a carbohydrate-rich meal) suppress gluconeogenesis by promoting glycolysis and reducing the activity of gluconeogenic enzymes. ○ Glucagon: Stimulates gluconeogenesis. High glucagon levels (e.g., during fasting or carbohydrate starvation) activate gluconeogenic pathways by promoting the transcription of key gluconeogenic enzymes like PEPCK. ○ Cortisol: Also stimulates gluconeogenesis, especially during prolonged stress, by increasing the availability of gluconeogenic precursors such as amino acids through protein breakdown. Allosteric regulation: ○ Fructose-1,6-bisphosphatase is inhibited by fructose-2,6-bisphosphate and AMP, and activated by citrate, linking gluconeogenesis to the cell’s energy state. ○ Pyruvate carboxylase is activated by acetyl-CoA, ensuring gluconeogenesis is activated when there is a need for glucose production but the cell has sufficient acetyl-CoA from fat oxidation. Cori Cycle and Glucose-Alanine Cycle Cori Cycle: In this cycle, lactate produced by anaerobic glycolysis in muscle cells is transported to the liver, where it is converted back into glucose via gluconeogenesis. The newly formed glucose can then return to the muscles to be used for energy, completing the cycle. Glucose-Alanine Cycle: In this cycle, alanine generated from the breakdown of muscle proteins is transported to the liver, where it is converted into pyruvate and subsequently glucose. This cycle helps to maintain glucose levels during prolonged fasting or intense exercise. Clinical Relevance of Gluconeogenesis Hypoglycemia: Impairment of gluconeogenesis can lead to low blood glucose levels, especially during fasting or increased demand for glucose. This can result from defects in gluconeogenic enzymes (e.g., glucose-6-phosphatase deficiency, leading to von Gierke disease). Diabetes: In diabetes, gluconeogenesis can become dysregulated. For example, in type 2 diabetes, gluconeogenesis can be inappropriately active, contributing to hyperglycemia even in the fed state, as insulin fails to suppress the pathway adequately. Bioenergetics and Carbohydrate Page 13 Thursday, September 5, 2024 12:11 Glycogen Metabolism Glycogen is a polysaccharide that serves as a major storage form of glucose in the body, primarily found in the liver and muscles. Glycogen metabolism involves the synthesis (glycogenesis) and breakdown (glycogenolysis) of glycogen to regulate blood glucose levels and provide energy during periods of fasting or increased demand. Glycogenesis (Glycogen Synthesis) Glycogenesis is the process of synthesizing glycogen from glucose, occurring primarily in the liver and muscle cells. Key steps and enzymes involved in glycogenesis: ○ Glucose to Glucose-6-Phosphate: Glucose is phosphorylated to glucose-6-phosphate by hexokinase (in muscle) or glucokinase (in liver). ○ Glucose-6-Phosphate to Glucose-1-Phosphate: Glucose-6-phosphate is converted to glucose-1-phosphate by phosphoglucomutase. ○ Glucose-1-Phosphate to UDP-Glucose: Glucose-1-phosphate is converted to UDP-glucose by UDP-glucose pyrophosphorylase. ○ UDP-Glucose to Glycogen: UDP-glucose is used by glycogen synthase to add glucose units to the growing glycogen chain, forming α-1,4-glycosidic bonds. ○ Branching: The enzyme glycogen branching enzyme introduces α-1,6-glycosidic bonds to create branches in the glycogen molecule, enhancing its solubility and storage efficiency. Glycogenolysis (Glycogen Breakdown) Glycogenolysis is the process of breaking down glycogen into glucose to meet energy needs. Key steps and enzymes involved in glycogenolysis: ○ Glycogen to Glucose-1-Phosphate: Glycogen phosphorylase cleaves glucose units from the glycogen chain, producing glucose-1-phosphate. ○ Glucose-1-Phosphate to Glucose-6-Phosphate: Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase. ○ Glucose-6-Phosphate to Glucose: In the liver, glucose-6-phosphatase converts glucose-6- phosphate to free glucose, which is then released into the bloodstream. Muscle cells lack glucose-6-phosphatase and thus use glucose-6-phosphate directly in glycolysis. Regulation of Glycogen Metabolism Glycogen metabolism is regulated by hormonal and allosteric mechanisms to balance glycogen synthesis and breakdown based on the body’s energy needs: Hormonal Regulation: ○ Insulin: Promotes glycogenesis and inhibits glycogenolysis. It activates glycogen synthase and inhibits glycogen phosphorylase. ○ Glucagon: Stimulates glycogenolysis and inhibits glycogenesis in the liver. It activates glycogen phosphorylase and inhibits glycogen synthase. ○ Epinephrine (Adrenaline): Similar to glucagon, it stimulates glycogenolysis in muscle cells, providing quick energy for physical activity. Allosteric Regulation: ○ Glycogen Synthase: Activated by glucose-6-phosphate and inhibited by phosphorylation. ○ Glycogen Phosphorylase: Activated by AMP and inhibited by ATP and glucose-6-phosphate. Glycogen Storage Diseases Bioenergetics and Carbohydrate Page 14 Glycogen Storage Diseases Glycogen storage diseases are genetic disorders that result from deficiencies in enzymes involved in glycogen metabolism: Glycogen Storage Disease Type I (von Gierke Disease): Deficiency of glucose-6-phosphatase, leading to impaired gluconeogenesis and glycogenolysis, resulting in hypoglycemia and glycogen accumulation in the liver. Glycogen Storage Disease Type II (Pompe Disease): Deficiency of lysosomal α-glucosidase, causing glycogen accumulation in lysosomes, leading to muscle weakness and respiratory problems. Glycogen Storage Disease Type III (Cori Disease): Deficiency of debranching enzyme, resulting in abnormal glycogen with short branches and fasting hypoglycemia. Glycogen Storage Disease Type V (McArdle Disease): Deficiency of muscle glycogen phosphorylase, leading to exercise intolerance due to impaired glycogen breakdown in muscle. Clinical Relevance Blood Glucose Regulation: Proper glycogen metabolism is essential for maintaining blood glucose levels, especially during fasting or between meals. Dysregulation can lead to conditions like hypoglycemia or hyperglycemia. Exercise: Glycogen stores in muscles are crucial for sustained physical activity. Inadequate glycogen stores can impair exercise performance and lead to fatigue. Integration with Other Metabolic Pathways Interplay with Glycolysis and Gluconeogenesis: Glycogen metabolism is tightly integrated with glycolysis and gluconeogenesis, allowing the body to respond flexibly to changing energy demands and availability of glucose. Nutrient Sensing: The regulation of glycogen metabolism is part of a broader network of nutrient sensing and metabolic control that includes pathways like insulin signaling, AMP-activated protein kinase (AMPK) signaling, and responses to dietary intake. Bioenergetics and Carbohydrate Page 15 Thursday, September 5, 2024 12:16 Monosaccharide and Disaccharide Metabolism: Monosaccharide Metabolism: Monosaccharides are the simplest form of carbohydrates, including glucose, fructose, and galactose. They are fundamental for energy production and various biosynthetic processes. Monosaccharide metabolism involves the conversion of these sugars into forms that can enter glycolysis, the TCA cycle, or other metabolic pathways. Glucose Metabolism Glucose is a primary energy source for cells and is metabolized through glycolysis, gluconeogenesis, and other pathways. Key points in glucose metabolism: ○ Glycolysis: Converts glucose to pyruvate, generating ATP and NADH. ○ Gluconeogenesis: Converts pyruvate back to glucose when necessary, particularly in the liver. ○ Pentose Phosphate Pathway (PPP): Provides NADPH for reductive biosynthesis and ribose-5-phosphate for nucleotide synthesis. Fructose Metabolism Fructose is primarily metabolized in the liver, where it can enter glycolysis and contribute to energy production. Key steps in fructose metabolism: ○ Fructose to Fructose-1-Phosphate: Fructose is phosphorylated by fructokinase to form fructose-1-phosphate. ○ Cleavage to Glyceraldehyde and Dihydroxyacetone Phosphate (DHAP): Aldolase B cleaves fructose-1-phosphate into glyceraldehyde and DHAP. ○ Glyceraldehyde to Glyceraldehyde-3-Phosphate: Glyceraldehyde is phosphorylated to glyceraldehyde-3-phosphate, which can then enter glycolysis. Clinical Relevance: Excessive fructose intake can contribute to metabolic disorders, such as fructose intolerance, which is due to deficiencies in aldolase B, and can lead to hypoglycemia and liver damage. Galactose Metabolism Galactose is metabolized mainly in the liver, where it is converted to glucose-6-phosphate. Key steps in galactose metabolism: ○ Galactose to Galactose-1-Phosphate: Galactose is phosphorylated by galactokinase to form galactose-1-phosphate. ○ Galactose-1-Phosphate to UDP-Galactose: Galactose-1-phosphate is converted to UDP- galactose by uridine diphosphate galactose-4-epimerase. ○ UDP-Galactose to UDP-Glucose: UDP-galactose is converted to UDP-glucose, which can be further metabolized to glucose-6-phosphate. Clinical Relevance: Galactosemia is a genetic disorder characterized by deficiencies in galactose-1- phosphate uridyltransferase, leading to the accumulation of galactose-1-phosphate and galactose in the blood, resulting in liver damage, cataracts, and intellectual disability if untreated. Bioenergetics and Carbohydrate Page 16 Disaccharide Metabolism Disaccharides are composed of two monosaccharide units. The primary disaccharides are sucrose, lactose, and maltose. Key steps in disaccharide metabolism: ○ Sucrose: Composed of glucose and fructose. It is hydrolyzed by sucrase to produce glucose and fructose. ○ Lactose: Composed of glucose and galactose. It is hydrolyzed by lactase to produce glucose and galactose. ○ Maltose: Composed of two glucose units. It is hydrolyzed by maltase to produce glucose. Clinical Relevance of Disaccharide Metabolism Lactose Intolerance: Caused by a deficiency in lactase, leading to the inability to digest lactose. This results in symptoms such as bloating, diarrhea, and abdominal pain after consuming dairy products. Sucrase-Isomaltase Deficiency: A genetic disorder that affects the digestion of sucrose and maltose, leading to gastrointestinal symptoms after ingesting sucrose-containing foods. Maltase Deficiency: Leads to difficulty digesting maltose and can cause gastrointestinal symptoms. Integration with Other Metabolic Pathways Monosaccharide metabolism is integrated with other pathways such as glycolysis, gluconeogenesis, and the pentose phosphate pathway. These processes are crucial for maintaining glucose homeostasis, energy production, and providing substrates for various biosynthetic pathways. Bioenergetics and Carbohydrate Page 17 Thursday, September 5, 2024 12:20 Pentose Phosphate Pathway and Nicotinamide Adenine Dinucleotide Phosphate (NADPH) Pentose Phosphate Pathway (PPP): The Pentose Phosphate Pathway (PPP) is an alternative pathway to glycolysis that operates primarily in the cytoplasm of cells. Its primary functions are to generate NADPH (for reductive biosynthesis and detoxification) and ribose-5-phosphate (for nucleotide and nucleic acid synthesis). Phases of the Pentose Phosphate Pathway The PPP consists of two phases: Oxidative Phase: This phase generates NADPH and ribulose-5-phosphate. Non-Oxidative Phase: This phase converts ribulose-5-phosphate into ribose-5-phosphate and other sugars, which can be used for nucleotide synthesis or recycled into glycolysis. Oxidative Phase: Glucose-6-Phosphate to 6-Phosphogluconolactone: ○ Enzyme: Glucose-6-phosphate dehydrogenase (G6PD) ○ Glucose-6-phosphate is oxidized to 6-phosphogluconolactone, producing NADPH. 6-Phosphogluconolactone to 6-Phosphogluconate: ○ Enzyme: 6-Phosphogluconolactone hydrolase ○ 6-Phosphogluconolactone is hydrolyzed to 6-phosphogluconate. 6-Phosphogluconate to Ribulose-5-Phosphate: ○ Enzyme: 6-Phosphogluconate dehydrogenase ○ 6-Phosphogluconate is decarboxylated to ribulose-5-phosphate, generating a second NADPH. Non-Oxidative Phase: Ribulose-5-Phosphate to Ribose-5-Phosphate: ○ Enzyme: Ribulose-5-phosphate epimerase ○ Ribulose-5-phosphate is converted to ribose-5-phosphate. Ribose-5-Phosphate Interconversion: ○ Enzymes: Transketolase and Transaldolase ○ Ribose-5-phosphate is used to produce other sugars such as xylulose-5-phosphate, erythrose-4-phosphate, and fructose-6-phosphate, which can be integrated into glycolysis or gluconeogenesis. Functions of NADPH: NADPH is a crucial reducing agent in various biosynthetic and detoxification reactions: ○ Biosynthesis: Provides reducing power for the synthesis of fatty acids, cholesterol, and nucleic acids. ○ Detoxification: Helps in the reduction of reactive oxygen species (ROS) and detoxification of xenobiotics in the liver. ○ Phagocytosis: In immune cells, NADPH is used in the NADPH oxidase system to generate reactive oxygen species (ROS) that help kill pathogens. Bioenergetics and Carbohydrate Page 18  reactive oxygen species (ROS) that help kill pathogens. Regulation of the Pentose Phosphate Pathway: The PPP is regulated mainly by the availability of its substrates and the activity of key enzymes: ○ Glucose-6-Phosphate Dehydrogenase (G6PD): The rate-limiting enzyme of the oxidative phase. It is activated by NADP+ (substrate) and inhibited by high levels of NADPH (end product). ○ The balance between NADPH production and utilization determines the flux through the pathway. Clinical Relevance of the Pentose Phosphate Pathway: G6PD Deficiency: A genetic disorder leading to reduced activity of glucose-6-phosphate dehydrogenase, resulting in decreased NADPH production. This can cause oxidative stress and hemolytic anemia, particularly under conditions of oxidative stress (e.g., infection, certain drugs, or fava bean consumption). Cancer: Tumor cells often have increased PPP activity to support rapid cell growth by providing NADPH for reductive biosynthesis and managing oxidative stress. Diabetes: Altered PPP activity can affect cellular glucose metabolism and oxidative stress, potentially contributing to complications in diabetes. Integration with Other Metabolic Pathways: The PPP intersects with glycolysis and gluconeogenesis. The products of the non-oxidative phase can feed into glycolysis, linking the PPP with central carbohydrate metabolism. The NADPH produced in the PPP supports various cellular functions, influencing overall metabolic balance and stress responses. Bioenergetics and Carbohydrate Page 19 Thursday, September 5, 2024 12:25 Glycosaminoglycans, Proteoglycans, and Glycoproteins Glycosaminoglycans (GAGs) Glycosaminoglycans (GAGs) are long, unbranched polysaccharides composed of repeating disaccharide units. They are a key component of the extracellular matrix and connective tissues. Key GAGs and their functions: ○ Hyaluronic Acid: A non-sulfated GAG found in connective tissues, synovial fluid, and the vitreous body of the eye. It provides lubrication and acts as a shock absorber. ○ Chondroitin Sulfate: A sulfated GAG found in cartilage, tendons, and ligaments. It contributes to the tensile strength of these tissues. ○ Dermatan Sulfate: Found in the skin, blood vessels, and heart valves. It plays a role in wound healing and vascular homeostasis. ○ Heparin: A highly sulfated GAG found primarily in mast cells. It acts as an anticoagulant and regulates various cellular processes. ○ Heparan Sulfate: Found on cell surfaces and in the basement membrane. It is involved in cell signaling, adhesion, and regulation of growth factors. ○ Keratan Sulfate: Found in the cornea of the eye and in cartilage. It contributes to the structural integrity and hydration of tissues. Proteoglycans Proteoglycans are glycoproteins where GAGs are covalently attached to a core protein. They are major components of the extracellular matrix and contribute to the structural and functional properties of tissues. Structure: ○ Core Protein: The protein backbone to which GAG chains are attached. ○ GAG Chains: Long polysaccharide chains attached to the core protein. Functions: ○ Structural Support: Provide tensile strength and elasticity to connective tissues. ○ Cell Signaling: Modulate interactions between cells and the extracellular matrix, influencing cell growth and differentiation. ○ Regulation of Growth Factors: Bind and regulate the availability of growth factors and cytokines. Examples: ○ Aggrecan: A major proteoglycan in cartilage, contributing to its compressive strength and elasticity. ○ Syndecans: Membrane-bound proteoglycans involved in cell adhesion and signaling. Glycoproteins Glycoproteins are proteins with one or more oligosaccharide chains covalently attached. They are involved in a wide range of biological functions. Structure: ○ Protein Backbone: A protein to which oligosaccharides are attached. ○ Oligosaccharide Chains: Short carbohydrate chains attached to the protein. Functions: Bioenergetics and Carbohydrate Page 20  Functions: ○ Cell Recognition and Adhesion: Involved in cell-cell and cell-matrix interactions. For example, selectins and integrins are glycoproteins that mediate cell adhesion. ○ Immune Response: Glycoproteins such as antibodies have carbohydrate components that are crucial for their function and stability. ○ Hormones and Enzymes: Many hormones (e.g., glycoprotein hormones like erythropoietin) and enzymes have glycosylation modifications that are important for their activity and stability. Examples: ○ Glycoprotein Hormones: Such as human chorionic gonadotropin (hCG) and thyroid- stimulating hormone (TSH), which have carbohydrate moieties essential for their biological activity. ○ Mucins: High molecular weight glycoproteins that form protective mucus layers in various tissues, such as the respiratory and gastrointestinal tracts. Glycosylation Glycosylation is the process of adding carbohydrate groups to proteins or lipids. It occurs in the endoplasmic reticulum and Golgi apparatus and can be classified into two main types: ○ N-Linked Glycosylation: Carbohydrates are attached to the nitrogen atom of asparagine residues in proteins. ○ O-Linked Glycosylation: Carbohydrates are attached to the oxygen atom of serine or threonine residues in proteins. Clinical Relevance Glycosaminoglycan Disorders: Genetic disorders such as mucopolysaccharidoses involve defects in the enzymes responsible for GAG degradation, leading to the accumulation of GAGs in tissues and resulting in various clinical symptoms. Proteoglycan Disorders: Conditions like Osteoarthritis can be associated with changes in proteoglycan composition and function in cartilage. Glycoprotein Disorders: Congenital disorders of glycosylation (CDG) are a group of inherited metabolic disorders caused by defects in glycosylation, leading to a range of symptoms including developmental delays and organ dysfunction. Integration with Other Metabolic Pathways GAGs, proteoglycans, and glycoproteins interact with various metabolic pathways and cellular processes, including cell signaling, immune response, and tissue repair. They play roles in maintaining extracellular matrix integrity and facilitating cellular interactions. Bioenergetics and Carbohydrate Page 21 Thursday, September 5, 2024 12:32 Introduction Lipids are a diverse group of hydrophobic or amphiphilic molecules that are soluble in nonpolar solvents but generally insoluble in water. They play essential roles in biological systems, including energy storage, cell membrane structure, and signaling. Lipids can be classified into several categories: Triglycerides (Triacylglycerols): Structure: Composed of a glycerol backbone and three fatty acid chains. Function: Main form of energy storage in animals and plants. They are stored in adipose tissue and can be broken down into fatty acids and glycerol for energy. Phospholipids: Structure: Composed of a glycerol backbone, two fatty acid chains, and a phosphate group attached to a polar head group. Function: Major components of cell membranes, forming the lipid bilayer. They contribute to membrane fluidity and act as a barrier between the cell’s interior and exterior environment. Steroids Structure: Characterized by a structure of four fused carbon rings. Function: Includes cholesterol, which is a precursor for the synthesis of steroid hormones (e.g., estrogen, testosterone), bile acids, and vitamin D. Cholesterol also stabilizes cell membranes. Glycolipids: Structure: Composed of a glycerol backbone, one or more fatty acid chains, and a carbohydrate group. Function: Found in cell membranes, where they play roles in cell recognition and communication. Waxes Structure: Composed of long-chain fatty acids esterified to long-chain alcohols. Function: Provide protective barriers in plants and animals (e.g., plant cuticles and earwax in humans). Sphingolipids Structure: Based on a sphingosine backbone, with one fatty acid chain and a variety of polar head groups. Function: Involved in signaling and cell recognition, especially in nerve cells and the immune system. Functions of Lipids: Energy Storage: Triglycerides serve as a long-term energy reserve. Structural Components: Phospholipids and cholesterol are integral to cell membrane structure and function. Signaling Molecules: Steroid hormones and other lipids act as signaling molecules to regulate various physiological processes. Insulation and Protection: Lipids, particularly in the form of adipose tissue, provide thermal insulation and cushioning for organs Lipid Metabolism Page 22 Thursday, September 5, 2024 12:29 Dietary Lipids Dietary lipids include triglycerides, phospholipids, and cholesterol, which are essential for energy, cell membrane structure, and hormone synthesis. Lipid metabolism involves digestion, absorption, transport, and utilization of these lipids to meet the body’s energy needs and other physiological functions. Digestion and Absorption of Dietary Lipids Digestion: ○ In the Mouth: Lingual lipase begins the digestion of triglycerides into diglycerides and free fatty acids. ○ In the Stomach: Gastric lipase continues the digestion of triglycerides, though its role is limited compared to pancreatic lipase. ○ In the Small Intestine: Bile acids emulsify dietary fats, increasing their surface area for enzymatic action. Pancreatic lipase, in combination with bile salts, hydrolyzes triglycerides into monoglycerides and free fatty acids. Absorption: ○ Micelle Formation: Digested lipids form micelles with bile salts, allowing them to be absorbed by the intestinal mucosa. ○ Enterocyte Uptake: Monoglycerides and free fatty acids are taken up by intestinal cells (enterocytes) and reassembled into triglycerides. ○ Chylomicron Formation: Triglycerides are packaged into chylomicrons, which are lipoprotein particles that enter the lymphatic system and eventually the bloodstream. Transport and Metabolism of Lipids Chylomicrons: ○ Function: Transport dietary triglycerides and other lipids from the intestine to peripheral tissues. ○ Lipoprotein Lipase (LPL): On the surface of capillary endothelial cells, LPL hydrolyzes triglycerides in chylomicrons into free fatty acids and glycerol, which can be taken up by tissues for energy or storage. VLDL (Very Low-Density Lipoprotein): ○ Function: Transports endogenous triglycerides from the liver to peripheral tissues. ○ Lipoprotein Lipase (LPL): Similar to chylomicrons, VLDL triglycerides are hydrolyzed by LPL, releasing fatty acids for uptake by tissues. IDL (Intermediate-Density Lipoprotein): ○ Function: Formed from VLDL after triglyceride removal. It can be converted into LDL or taken up by the liver. LDL (Low-Density Lipoprotein): ○ Function: Transports cholesterol to peripheral tissues. It is often referred to as "bad cholesterol" due to its role in atherosclerosis. ○ LDL Receptors: Peripheral tissues take up LDL through receptor-mediated endocytosis. HDL (High-Density Lipoprotein): ○ Function: Collects excess cholesterol from tissues and transports it back to the liver for excretion or reutilization. It is known as "good cholesterol" for its role in reducing cardiovascular risk. Lipid Metabolism Page 23 Lipid Utilization and Storage Fatty Acid Oxidation: ○ In the Mitochondria: Fatty acids are broken down through β-oxidation to produce acetyl- CoA, which enters the TCA cycle to generate ATP. Ketogenesis: ○ In the Liver: Excess acetyl-CoA, particularly during fasting or carbohydrate restriction, is converted into ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) that serve as an alternative energy source for tissues like the brain. Fat Storage: ○ Adipose Tissue: Excess fatty acids and triglycerides are stored in adipocytes. Lipogenesis (the synthesis of fatty acids) occurs in the liver and adipose tissue, with excess glucose and dietary fats converted into triglycerides for long-term storage. Regulation of Lipid Metabolism Hormonal Regulation: ○ Insulin: Promotes lipogenesis and triglyceride storage while inhibiting lipolysis. ○ Glucagon and Epinephrine: Stimulate lipolysis, promoting the release of free fatty acids from adipose tissue for energy. Enzyme Regulation: ○ Lipoprotein Lipase (LPL): Activity is regulated by hormonal signals and nutritional state. ○ Hormone-Sensitive Lipase (HSL): Regulates lipolysis in adipose tissue in response to hormonal signals. Clinical Relevance Dyslipidemia: Abnormal levels of lipoproteins (e.g., high LDL, low HDL) are associated with increased risk of cardiovascular diseases. Obesity: Excessive accumulation of triglycerides in adipose tissue can lead to obesity, which is a risk factor for metabolic syndrome and type 2 diabetes. Fatty Liver Disease: Accumulation of lipids in the liver, often due to insulin resistance or excessive alcohol consumption, can lead to non-alcoholic fatty liver disease (NAFLD) or alcoholic liver disease. Integration with Other Metabolic Pathways Lipid metabolism is interconnected with carbohydrate and protein metabolism. The balance between these pathways influences overall energy homeostasis and metabolic health. Energy Balance: The storage and mobilization of lipids are regulated to maintain energy balance, especially during periods of fasting and feeding. Lipid Metabolism Page 24 Thursday, September 5, 2024 12:36 Fatty Acid, Triacylglycerol, and Ketone Body Metabolism: Fatty Acid Metabolism Synthesis of Fatty Acids Location: Occurs primarily in the cytoplasm of liver and adipose tissues. Starting Material: Acetyl-CoA, which is derived from carbohydrates and proteins. Key Enzyme: Acetyl-CoA Carboxylase (ACC) catalyzes the conversion of acetyl-CoA to malonyl- CoA, the first step in fatty acid synthesis. Fatty Acid Synthase Complex: A multi-enzyme complex that facilitates the elongation of the fatty acid chain by adding two-carbon units derived from malonyl-CoA. Process: 1. Initiation: Acetyl-CoA and malonyl-CoA are loaded onto the fatty acid synthase. 2. Elongation: The fatty acid chain is extended by successive addition of two-carbon units. 3. Termination: The process continues until the chain reaches 16 carbons (palmitate), which is then released. Oxidation of Fatty Acids Location: Occurs in the mitochondria of cells. Process: Fatty acids are broken down into acetyl-CoA units through a series of reactions known as β-oxidation. Steps: 1. Activation: Fatty acids are activated to fatty acyl-CoA by the enzyme acyl-CoA synthetase. 2. Transport: Fatty acyl-CoA is transported into the mitochondria via the carnitine shuttle. 3. β-Oxidation: In the mitochondria, fatty acyl-CoA undergoes a series of four reactions— oxidation, hydration, oxidation, and cleavage—to produce acetyl-CoA, NADH, and FADH2. Outcome: Each round of β-oxidation shortens the fatty acid chain by two carbons, producing one acetyl-CoA molecule per cycle. Triacylglycerol (Triglyceride) Metabolism Synthesis of Triacylglycerols Location: Occurs mainly in the liver and adipose tissue. Process: 1. Glycerol-3-Phosphate Formation: Derived from glucose through glycolysis (in the liver) or from glycerol (in adipose tissue). 2. Acylation: Fatty acids are esterified to glycerol-3-phosphate to form triacylglycerols. Key Enzyme: Diacylglycerol acyltransferase (DGAT) catalyzes the final step of triacylglycerol synthesis. Breakdown of Triacylglycerols Location: Occurs in adipose tissue. Process: 1. Lipolysis: The breakdown of triacylglycerols into glycerol and free fatty acids by the enzyme hormone-sensitive lipase (HSL). 2. Release: Free fatty acids are released into the bloodstream and transported to various tissues for oxidation. Glycerol is transported to the liver for gluconeogenesis or glycolysis. Lipid Metabolism Page 25 Ketone Body Metabolism Production of Ketone Bodies Location: Occurs in the liver mitochondria. Process: 1. Ketogenesis: During periods of fasting or low carbohydrate intake, fatty acids are mobilized and converted into ketone bodies (acetoacetate, β-hydroxybutyrate, and acetone) in the liver. 2. Enzymes Involved: Acetoacetyl-CoA thiolase, HMG-CoA synthase, and HMG-CoA lyase are key enzymes in ketone body production. Utilization of Ketone Bodies Transport: Ketone bodies are released into the bloodstream and transported to peripheral tissues. Oxidation: In tissues like muscle and brain, ketone bodies are converted back into acetyl-CoA by the enzymes acetoacetate decarboxylase and β-hydroxybutyrate dehydrogenase, which then enter the TCA cycle for energy production. Regulation of Lipid Metabolism Insulin: Promotes the synthesis of fatty acids and triacylglycerols while inhibiting lipolysis and β- oxidation. Glucagon and Epinephrine: Stimulate lipolysis, leading to increased fatty acid release and oxidation. They also promote ketogenesis during fasting or low carbohydrate intake. AMPK (AMP-Activated Protein Kinase): Activated during energy stress, AMPK inhibits fatty acid synthesis and promotes β-oxidation. Clinical Relevance Metabolic Disorders: Conditions such as type 2 diabetes and obesity can affect lipid metabolism, leading to altered fatty acid and triglyceride levels. Ketosis: Can occur during prolonged fasting or ketogenic diets. While mild ketosis is normal, excessive ketosis can lead to ketoacidosis, particularly in diabetic individuals. Fatty Liver Disease: Excessive accumulation of lipids in the liver, often due to insulin resistance or high alcohol intake, can lead to non-alcoholic fatty liver disease (NAFLD). Integration with Other Metabolic Pathways Lipid metabolism is interconnected with carbohydrate and protein metabolism. The regulation of lipid metabolism impacts overall energy homeostasis and metabolic health, especially during different nutritional states such as fasting and feeding. Lipid Metabolism Page 26 Thursday, September 5, 2024 12:41 Phospholipid, Glycosphingolipid, and Eicosanoid Metabolism Phospholipid Metabolism Synthesis of Phospholipids Phosphatidylcholine: ○ Synthesis: Involves the transfer of choline to a diacylglycerol molecule. Key enzymes include phosphatidylcholine synthase and choline kinase. ○ Function: Major component of cell membranes, involved in membrane fluidity and signaling. Phosphatidylinositol: ○ Synthesis: Derived from inositol and diacylglycerol. Key enzyme: phosphatidylinositol synthase. ○ Function: Plays a crucial role in cell signaling pathways, particularly in the phosphatidylinositol 4,5-bisphosphate (PIP2) pathway. Phosphatidylethanolamine: ○ Synthesis: Formed by the methylation of phosphatidylethanolamine to form phosphatidylcholine. ○ Function: Important for membrane structure and fusion processes. Degradation of Phospholipids Phospholipases: Enzymes that hydrolyze phospholipids into their constituent fatty acids and other molecules. ○ Phospholipase A2 (PLA2): Releases fatty acids (e.g., arachidonic acid) from the sn-2 position of phospholipids. ○ Phospholipase C (PLC): Hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol trisphosphate (IP3) and diacylglycerol (DAG), which are involved in signaling pathways. ○ Phospholipase D (PLD): Converts phosphatidylcholine to phosphatidic acid and choline. Glycosphingolipid Metabolism Synthesis of Glycosphingolipids Sphingolipid Backbone: ○ Sphingolipids are based on a sphingosine backbone, which is acylated with a fatty acid. Glycosphingolipids: ○ Ceramide: The basic structure, composed of sphingosine and a fatty acid. ○ Glycosylation: Ceramide is modified by the addition of sugar residues to form various glycosphingolipids: ▪ Cerebrosides: Contain a single sugar (glucose or galactose). ▪ Gangliosides: Complex glycosphingolipids with sialic acid-containing oligosaccharide chains. ○ Function: Glycosphingolipids are involved in cell recognition, signaling, and maintaining membrane structure. Degradation of Glycosphingolipids Lysosomal Enzymes: Degradation occurs in lysosomes via specific hydrolases: Lipid Metabolism Page 27  Lysosomal Enzymes: Degradation occurs in lysosomes via specific hydrolases: ○ β-Galactosidase: Degrades cerebrosides. ○ Sphingomyelinase: Degrades sphingomyelin to ceramide. ○ Hexosaminidase: Degrades GM2 gangliosides. Disorders: Genetic defects in lysosomal enzymes can lead to conditions such as Tay-Sachs disease, Gaucher disease, and Niemann-Pick disease. Eicosanoid Metabolism Synthesis of Eicosanoids Eicosanoids: Bioactive lipids derived from arachidonic acid (a 20-carbon fatty acid). Key Enzymes: ○ Cyclooxygenases (COX): Convert arachidonic acid to prostaglandins and thromboxanes. ▪ COX-1 and COX-2: Isozymes with distinct roles in inflammation and homeostasis. ○ Lipoxygenases (LOX): Convert arachidonic acid to leukotrienes and lipoxins. ▪ 5-LOX: Produces leukotrienes involved in inflammation and immune response. ▪ 15-LOX: Produces lipoxins that resolve inflammation. Prostaglandins: Involved in inflammation, pain, fever, and regulation of blood flow. Thromboxanes: Involved in platelet aggregation and vasoconstriction. Leukotrienes: Involved in allergic reactions and inflammation. Regulation and Function of Eicosanoids Inflammatory Response: Eicosanoids play key roles in the initiation and resolution of inflammation. Cardiovascular System: Influence blood clotting and vascular tone. For example, thromboxane promotes platelet aggregation, while prostacyclin inhibits it. Immune Response: Leukotrienes are involved in the recruitment of immune cells to sites of infection or injury. Clinical Relevance Nonsteroidal Anti-Inflammatory Drugs (NSAIDs): Inhibit COX enzymes, reducing the production of prostaglandins and thus alleviating pain and inflammation. Aspirin: Irreversibly inhibits COX-1 and COX-2, reducing thromboxane production and thereby preventing platelet aggregation. Leukotriene Inhibitors: Used to manage asthma and allergic rhinitis by inhibiting leukotriene synthesis. Integration with Other Metabolic Pathways Phospholipids, glycosphingolipids, and eicosanoids interact with other metabolic pathways involved in cell signaling, inflammation, and membrane dynamics. Their metabolism is tightly regulated to maintain cellular and systemic homeostasis. Lipid Metabolism Page 28 Thursday, September 5, 2024 12:43 Cholesterol, Lipoprotein, and Steroid Metabolism: Cholesterol Metabolism Synthesis of Cholesterol Location: Primarily in the liver and, to a lesser extent, in other tissues. Process: 1. Acetyl-CoA to Mevalonate: Acetyl-CoA is converted to mevalonate via the enzyme HMG- CoA reductase. This is the rate-limiting step in cholesterol synthesis. 2. Mevalonate to Isoprenoid Units: Mevalonate is further converted into isoprenoid units through a series of reactions. 3. Formation of Squalene: Isoprenoid units are assembled into squalene. 4. Cyclization: Squalene is cyclized to form lanosterol, which is then converted into cholesterol through multiple steps. Regulation: ○ HMG-CoA Reductase: Inhibited by cholesterol and its derivatives. Statins, a class of cholesterol-lowering drugs, inhibit this enzyme. ○ Transcriptional Regulation: Sterol regulatory element-binding proteins (SREBPs) regulate the expression of genes involved in cholesterol synthesis. Absorption and Transport of Cholesterol Dietary Absorption: Cholesterol from the diet is absorbed in the intestine and incorporated into chylomicrons. Lipoproteins: ○ Chylomicrons: Transport dietary cholesterol and triglycerides from the intestines to peripheral tissues. ○ Low-Density Lipoprotein (LDL): Transports cholesterol from the liver to peripheral tissues. High levels are associated with an increased risk of atherosclerosis. ○ High-Density Lipoprotein (HDL): Collects excess cholesterol from tissues and transports it back to the liver for excretion. Known as "good cholesterol" for its protective cardiovascular effects. Lipoprotein Metabolism Lipoprotein Classes Chylomicrons: ○ Function: Transport dietary lipids from the intestines to peripheral tissues. ○ Key Enzyme: Lipoprotein lipase (LPL) hydrolyzes triglycerides in chylomicrons to free fatty acids and glycerol. Very Low-Density Lipoproteins (VLDL): ○ Function: Transport endogenous triglycerides from the liver to peripheral tissues. ○ Transformation: VLDL is converted to intermediate-density lipoprotein (IDL) and then to LDL as triglycerides are removed. Low-Density Lipoproteins (LDL): ○ Function: Delivers cholesterol to cells. High levels are linked to cardiovascular disease. ○ Receptor-Mediated Endocytosis: LDL cholesterol is taken up by cells via LDL receptors. High-Density Lipoproteins (HDL): Function: Scavenges excess cholesterol from tissues and transports it to the liver. Lipid Metabolism Page 29  ○ Function: Scavenges excess cholesterol from tissues and transports it to the liver. ○ Reverse Cholesterol Transport: HDL plays a crucial role in this process, which is protective against atherosclerosis. Steroid Hormone Synthesis Synthesis of Steroid Hormones Location: Occurs mainly in the adrenal glands and gonads (testes and ovaries). Steps: 1. Cholesterol to Pregnenolone: Cholesterol is converted to pregnenolone in the mitochondria by cholesterol side-chain cleavage enzyme. 2. Pregnenolone to Various Steroids: ▪ Mineralocorticoids: E.g., aldosterone, produced in the adrenal cortex. Regulates sodium and potassium balance. ▪ Glucocorticoids: E.g., cortisol, also produced in the adrenal cortex. Regulates metabolism and stress response. ▪ Sex Steroids: E.g., estrogen, progesterone, and testosterone, produced in the gonads. Regulate reproductive functions and secondary sexual characteristics. Regulation of Steroid Hormone Production Hypothalamic-Pituitary-Adrenal (HPA) Axis: Regulates the release of adrenal steroids through feedback mechanisms. Hormonal Regulation: ACTH stimulates the production of adrenal steroids, while luteinizing hormone (LH) and follicle-stimulating hormone (FSH) regulate sex steroid production. Clinical Relevance Cholesterol Disorders Hypercholesterolemia: Elevated levels of cholesterol in the blood, increasing the risk of cardiovascular disease. Managed with lifestyle changes and medications such as statins. Familial Hypercholesterolemia: A genetic disorder leading to extremely high LDL cholesterol levels and early onset of atherosclerosis. Lipoprotein Disorders Dyslipidemia: Abnormal levels of lipoproteins, including high LDL and low HDL levels, contribute to cardiovascular risk. Apolipoprotein Deficiencies: Genetic defects affecting apolipoproteins can lead to dyslipidemia. Steroid Disorders Adrenal Insufficiency: Reduced production of adrenal steroids, leading to symptoms such as fatigue and weight loss. Cushing’s Syndrome: Excess cortisol production, causing symptoms like obesity, hypertension, and diabetes. Androgen Insensitivity Syndrome: A genetic condition where the body does not respond to androgens, affecting sexual development. Integration with Other Metabolic Pathways Cholesterol and Steroid Metabolism: Both processes are interconnected, with cholesterol serving as the precursor for steroid hormone synthesis. Lipoprotein Metabolism: Influences overall lipid and cholesterol balance in the body, impacting cardiovascular health. Lipid Metabolism Page 30 Saturday, October 26, 2024 14:25 Amino Acids: Nitrogen Disposal: Amino Acid Catabolism and the Role of Nitrogen Transamination Reaction: ○ Amino acid catabolism often begins with transamination, where the amino group from an amino acid is transferred to α-ketoglutarate, converting it to glutamate. This reaction is catalyzed by enzymes called aminotransferases (or transaminases). ○ Each amino acid has a specific aminotransferase, such as alanine aminotransferase (ALT) for alanine or aspartate aminotransferase (AST) for aspartate. ○ Transamination is reversible, and it is a crucial step for amino acid degradation, especially in the liver and muscles, as it helps transfer nitrogen safely without producing free ammonia initially. Oxidative Deamination: ○ After transamination, glutamate often undergoes oxidative deamination, mainly in the liver and kidneys. This reaction, catalyzed by glutamate dehydrogenase, converts glutamate back to α-ketoglutarate while releasing ammonia (NH₃). ○ Oxidative deamination produces free ammonia, which must be immediately managed due to its toxicity. This reaction is essential for the balance between amino acids and α-keto acids and plays a significant role in energy metabolism, especially under fasting conditions. 2. Transport and Detoxification of Ammonia Ammonia Transport Mechanisms: ○ Glutamine Formation: Peripheral tissues, like muscles, convert ammonia to glutamine by combining it with glutamate in a reaction catalyzed by glutamine synthetase. This process safely transports ammonia in the bloodstream as glutamine, a non-toxic carrier. ○ Alanine Cycle: In muscle tissue, especially during protein breakdown or exercise, pyruvate (from glycolysis) combines with ammonia to form alanine. The alanine is then transported to the liver, where it can release its amino group to form urea, while the remaining carbon skeleton is used to regenerate glucose in a process called the glucose-alanine cycle. Detoxification in the Liver: ○ The liver plays a central role in converting ammonia into urea through the urea cycle, an efficient method to handle the potential toxicity of ammonia. 3. The Urea Cycle (Ornithine Cycle) Overview: ○ The urea cycle is a series of biochemical reactions occurring primarily in liver cells, where ammonia is detoxified by converting it into urea, which can then be excreted by the kidneys. Steps of the Urea Cycle: ○ Formation of Carbamoyl Phosphate: This initial step combines ammonia (NH₃) with CO₂ to form carbamoyl phosphate, catalyzed by carbamoyl phosphate synthetase I. This enzyme requires N-acetylglutamate as an activator. ○ Ornithine and Citrulline Production: Carbamoyl phosphate donates its carbamoyl group to ornithine, forming citrulline. Citrulline is then transported out of the mitochondria into the cytosol. ○ Argininosuccinate Formation: In the cytosol, citrulline combines with aspartate (which provides an additional nitrogen atom), forming argininosuccinate. Nitrogen Metabolism Page 31  provides an additional nitrogen atom), forming argininosuccinate. ○ Cleavage to Arginine and Fumarate: Argininosuccinate is then cleaved into arginine and fumarate. Fumarate can be utilized in the citric acid cycle, linking nitrogen metabolism to energy production. ○ Formation of Urea: Finally, arginine is hydrolyzed by arginase to release urea and regenerate ornithine, which can re-enter the urea cycle. Regulation of the Urea Cycle: ○ The cycle is regulated by the availability of its substrates and by the concentration of N- acetylglutamate, which is necessary for carbamoyl phosphate synthetase I activation. ○ In conditions where amino acid breakdown increases (like fasting or high-protein intake), the urea cycle activity also increases to handle the excess nitrogen. 4. Clinical Implications and Disorders Hyperammonemia: ○ Hyperammonemia is a condition characterized by elevated ammonia levels in the blood, leading to symptoms such as confusion, tremors, and in severe cases, coma. It’s especially toxic to the brain because ammonia interferes with the energy metabolism in neurons. ○ Causes include liver disease (like cirrhosis, where the urea cycle is compromised) and genetic defects in any urea cycle enzyme, leading to an inability to detoxify ammonia properly Inherited Urea Cycle Disorders: ○ Genetic defects can occur in any of the enzymes of the urea cycle, resulting in specific urea cycle disorders. Common examples include ornithine transcarbamylase deficiency and carbamoyl phosphate synthetase I deficiency. ○ Treatment involves reducing protein intake, using medications to help remove excess ammonia, and in some cases, supplementing with compounds that bypass defective steps in the cycle. Nitrogen Metabolism Page 32 Saturday, October 26, 2024 14:29 Amino Acids: Degradation and Synthesis Overview of Amino Acid Classification and Catabolism Essential vs. Nonessential Amino Acids: Amino acids are categorized into essential (must be obtained from the diet) and nonessential (synthesized in the body). Some amino acids are conditionally essential, needed from the diet during periods of stress or growth. Glucogenic vs. Ketogenic: Amino acids are also classified based on their breakdown products: ○ Glucogenic Amino Acids: Degraded into pyruvate or citric acid cycle intermediates, which can be converted into glucose via gluconeogenesis. ○ Ketogenic Amino Acids: Broken down into acetyl-CoA or acetoacetate, which are precursors for ketone bodies. ○ Some amino acids are both glucogenic and ketogenic. 2. Key Pathways in Amino Acid Catabolism Transamination and Deamination: These reactions remove the amino group from amino acids, resulting in α-keto acids that can enter various metabolic pathways. Degradation of Specific Amino Acids: ○ Aromatic Amino Acids (Phenylalanine and Tyrosine): Phenylalanine is converted to tyrosine, which can further degrade into fumarate and acetoacetate. This pathway is significant because defects lead to disorders like phenylketonuria (PKU). ○ Branched-Chain Amino Acids (Valine, Leucine, and Isoleucine): These are primarily broken down in muscle tissues. Valine and isoleucine are glucogenic, while leucine is purely ketogenic. ○ Methionine and Homocysteine Metabolism: Methionine serves as a methyl group donor through S-adenosylmethionine (SAM). It converts into homocysteine, which can either be remethylated to methionine or converted to cysteine. Imbalances can increase cardiovascular disease risk. 3. Amino Acid Synthesis Nonessential Amino Acids: Synthesized in the body using intermediates from glycolysis or the citric acid cycle. ○ Glutamate and Glutamine: Derived from α-ketoglutarate, glutamate plays a central role in nitrogen transfer, while glutamine serves as a nitrogen donor in various biosynthetic reactions. ○ Aspartate and Asparagine: Synthesized from oxaloacetate, aspartate is critical in the urea cycle and nucleotide synthesis, and asparagine is formed by amidation of aspartate. ○ Serine, Glycine, and Cysteine: Derived from 3-phosphoglycerate, serine is a precursor to glycine and cysteine, both of which are involved in detoxification and antioxidant mechanisms. ○ Tyrosine: Synthesized from phenylalanine, tyrosine is essential for synthesizing neurotransmitters and hormones, such as dopamine, norepinephrine, and thyroid hormones. 4. Clinical Correlations and Genetic Disorders Phenylketonuria (PKU): Caused by mutations in the enzyme phenylalanine hydroxylase, which impairs the conversion of phenylalanine to tyrosine, leading to toxic phenylalanine buildup. Nitrogen Metabolism Page 33  impairs the conversion of phenylalanine to tyrosine, leading to toxic phenylalanine buildup. Untreated PKU can cause intellectual disability but is managed by a low-phenylalanine diet. Maple Syrup Urine Disease (MSUD): Deficiency in the enzymes degrading branched-chain amino acids results in toxic accumulation, leading to neurological damage if untreated. Homocystinuria: Impaired conversion of homocysteine to cysteine, causing homocysteine accumulation, which is associated with vascular and neurological issues. 5. Importance of Amino Acid Metabolism in Health and Disease Amino acid metabolism is not only vital for producing metabolic intermediates but also has roles in energy production, synthesizing biomolecules, and maintaining redox balance. Defects in these pathways lead to diverse metabolic disorders, highlighting the importance of regulated amino acid degradation and synthesis in health. Nitrogen Metabolism Page 34 Saturday, October 26, 2024 14:31 Amino Acids: Conversion to Specialized Products Overview of Amino Acids as Precursors for Specialized Compounds Amino acids are not only vital for protein synthesis but also serve as starting points for many bioactive molecules, each supporting a unique physiological function, such as neurotransmission, cellular signaling, and hormone production. 2. Key Specialized Products and Their Synthesis Pathways Neurotransmitters and Hormones: ○ Catecholamines (Dopamine, Norepinephrine, and Epinephrine): ▪ These neurotransmitters are synthesized from the amino acid tyrosine. The pathway begins with tyrosine hydroxylation to form L-DOPA, which is then decarboxylated to produce dopamine. Dopamine can be converted into norepinephrine and further into epinephrine, both of which play crucial roles in the body’s stress response and in cardiovascular function. ▪ Dopamine regulates mood, cognition, and motor control; norepinephrine functions in attention and arousal; and epinephrine (also known as adrenaline) prepares the body for "fight-or-flight" situations. ○ Serotonin and Melatonin: ▪ Tryptophan is the precursor for serotonin, a neurotransmitter important for mood and sleep regulation. Tryptophan is first converted to 5-hydroxytryptophan and then to serotonin. In the pineal gland, serotonin undergoes further conversion into melatonin, a hormone that helps regulate the sleep-wake cycle. ○ Histamine: ▪ Derived from histidine through decarboxylation, histamine is a crucial mediator of immune responses, gastric acid secretion, and acts as a neurotransmitter within the central nervous system. It is stored in mast cells and released during allergic reactions, leading to symptoms such as itching and swelling. Creatine: ○ Synthesized from glycine, arginine, and methionine, creatine is essential in muscle tissue, where it stores and provides energy. In muscle cells, creatine is phosphorylated to form creatine phosphate, which acts as a rapidly mobilizable reserve of high-energy phosphate that regenerates ATP during muscle contractions, particularly useful for short bursts of intense activity. Melanin: ○ Tyrosine is also the precursor to melanin, the pigment responsible for skin, hair, and eye color. Tyrosine is converted to DOPA by tyrosinase, followed by a series of reactions leading to melanin production. Deficiencies in tyrosinase activity result in albinism, a condition characterized by a lack of melanin, leading to increased sensitivity to sunlight. 3. Roles in Detoxification and Antioxidant Defense Glutathione: ○ Made from glutamate, cysteine, and glycine, glutathione is a major antioxidant that protects cells by neutralizing reactive oxygen species (ROS). Glutathione exists in reduced (GSH) and oxidized (GSSG) forms, allowing it to act as a redox buffer. In the liver, it is essential for detoxifying harmful compounds through glutathione conjugation, preventing Nitrogen Metabolism Page 35  essential for detoxifying harmful compounds through glutathione conjugation, preventing cellular damage. Nitric Oxide (NO): ○ Synthesized from arginine via nitric oxide synthase (NOS), nitric oxide functions as a signaling molecule involved in blood vessel dilation (vasodilation), neurotransmission, and immune defense. Its vasodilatory effects are essential in regulating blood pressure and blood flow, while in immune cells, NO helps to neutralize pathogens. 4. Heme and Nucleotide Synthesis Heme Synthesis: ○ Heme is synthesized using glycine and succinyl-CoA. This pathway starts in the mitochondria, where aminolevulinic acid (ALA) is formed and continues partly in the cytoplasm. Heme is vital as an oxygen-carrying molecule in hemoglobin and myoglobin. Disruptions in this pathway lead to porphyrias, which are metabolic disorders where heme precursors accumulate, causing symptoms like light sensitivity and neurological issues. Nucleotide Bases: ○ Several amino acids contribute nitrogen and carbon atoms to the synthesis of purine and pyrimidine bases, which form DNA and RNA. Aspartate and glutamine provide nitrogen atoms for these bases, while glycine supplies both nitrogen and carbon atoms, playing a crucial role in genetic material synthesis. 5. Clinical Significance and Disorders Related to Deficiencies in These Pathways Phenylketonuria (PKU) and Albinism: ○ PKU results from a deficiency in phenylalanine hydroxylase, leading to phenylalanine buildup and decreased synthesis of tyrosine-derived compounds like melanin and dopamine. Untreated PKU results in intellectual disability, but dietary management can prevent symptoms. ○ Albinism is primarily due to tyrosinase deficiency, leading to an inability to produce melanin, resulting in light sensitivity and pale pigmentation. Alkaptonuria: ○ This disorder is due to a defect in homogentisic acid oxidase, causing homogentisic acid to accumulate, which darkens connective tissues and urine. This condition can lead to arthritis due to the deposition of dark pigment in cartilage and other tissues. Porphyrias: ○ Defects in the heme biosynthesis pathway can lead to porphyrias, where intermediates like ALA and porphobilinogen build up. Symptoms vary but can include abdominal pain, skin sensitivity to sunlight, and neurological complications. Nitrogen Metabolism Page 36 Saturday, October 26, 2024 14:34 Nucleotide Metabolism 1. Overview of Nucleotide Structure and Types Nucleotide Components: Nucleotides consist of a nitrogenous base (purine or pyrimidine), a sugar (ribose or deoxyribose), and one or more phosphate groups. Types: Purines include adenine and guanine, while pyrimidines include cytosine, thymine (in DNA), and uracil (in RNA). 2. De Novo Synthesis of Purine Nucleotides Pathway: Purine synthesis starts with ribose-5-phosphate from the pentose phosphate pathway, which is converted to phosphoribosyl pyrophosphate (PRPP). PRPP acts as a scaffold for building the purine ring structure. IMP Formation: Through multiple steps, PRPP is converted to inosine monophosphate (IMP), which is a common precursor for AMP and GMP. IMP undergoes specific enzymatic modifications to form these purine nucleotides. Regulation: Feedback inhibition occurs at the PRPP amidotransferase step, where high levels of AMP or GMP decrease purine synthesis, maintaining a balance in nucleotide pools. 3. De Novo Synthesis of Pyrimidine Nucleotides Pathway: Unlike purines, the pyrimidine ring is synthesized before attachment to ribose. The pathway starts with carbamoyl phosphate and aspartate, forming orotate. UMP Formation: Orotate binds with PRPP to form orotidine monophosphate (OMP), which is then decarboxylated to produce uridine monophosphate (UMP), the precursor to other pyrimidines like CMP and TMP (thymidine monophosphate). Regulation: Carbamoyl phosphate synthetase II is the key regulatory enzyme, influenced by feedback from UTP, the end product of pyrimidine synthesis. 4. Salvage Pathways of Purine and Pyrimidine Nucleotides Purine Salvage: Important due to the energy cost of de novo synthesis, purine bases like adenine, hypoxanthine, and guanine can be recycled using the HGPRT (hypoxanthine-guanine phosphoribosyltransferase) enzyme to reform AMP, IMP, and GMP. Clinical Significance – Lesch-Nyhan Syndrome: A deficiency in HGPRT leads to this disorder, where hypoxanthine and guanine cannot be salvaged, leading to increased uric acid and neurological symptoms, including severe self-mutilating behaviors. Pyrimidine Salvage: Pyrimidines can also be salvaged but less commonly due to lower cellular demand. Uracil and thymine can be salvaged by respective kinases to form UMP and TMP. 5. Nucleotide Degradation Purine Degradation: Purines are degraded to uric acid, a process involving deamination, oxidation, and ring cleavage. High uric acid levels can crystallize and cause gout. Pyrimidine Degradation: Pyrimidines degrade to simpler compounds like beta-alanine and beta- aminoisobutyrate, which are excreted in urine or further metabolized. 6. Regulation of Nucleotide Synthesis and Therapeutic Relevance Regulatory Mechanisms: PRPP synthetase, PRPP amidotransferase, and ribonucleotide reductase Nitrogen Metabolism Page 37  Regulatory Mechanisms: PRPP synthetase, PRPP amidotransferase, and ribonucleotide reductase are key enzymes regulated by feedback inhibition. Maintaining the balance of nucleotide pools is crucial for DNA/RNA synthesis and repair. Pharmacological Agents: ○ Methotrexate: An anti-cancer drug, inhibits dihydrofolate reductase, reducing the availability of tetrahydrofolate needed for thymidylate synthesis. ○ Allopurinol: A treatment for gout, inhibits xanthine oxidase, decreasing uric acid production and thus preventing gout symptoms. Nitrogen Metabolism Page 38