Bioenergetics and Carbohydrate Overview PDF

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 t...

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 1  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 2 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 3 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 4 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 5 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 6 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 7  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 8 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. Bioenergetics and Carbohydrate Page 9 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 10 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 11 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 12 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 13  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 14 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 15  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 16