Lecture 7: Protein and Lipid Metabolism PDF

Lecture 7 Protein and Lipid Metabolism Dr. Sophie SHI Ling [email protected] Department of Applied Science School of Science and Technology 1 Class of Biomolecules for Energy Ø Step 1: Production of acetyl-CoA q Glucose To pyr...

Lecture 7 Protein and Lipid Metabolism Dr. Sophie SHI Ling [email protected] Department of Applied Science School of Science and Technology 1 Class of Biomolecules for Energy Ø Step 1: Production of acetyl-CoA q Glucose To pyruvate via glycolysis To acetyl-CoA by pyruvate dehydrogenase q Fatty acid To acetyl-CoA via β-oxidation q Amino acid To acetyl-CoA via oxidation Ø Step 2: Oxidation of acetyl-CoA via TCA cycle Generation of NADH and FADH2 Ø Step 3: ATP production from NADH and FADH2 via oxidative phosphorylation 2 Part I: Protein Metabolism q Transamination and Deamination q Fate of Amino Group —— Urea Cycle q Fate of Carbon Skeletons —— Glucogenic or Ketogenic 3 Overview of protein metabolism q Feature of amino acids All contain amino groups q Key step Lose amino group (deamination) Form α-keto acid q Amino group Reused or excreted q α-keto acid Oxidized or recycled 4  Amino acid catabolism Most amino acids are metabolized in liver Amino group is removed as ammonia (NH4+) ü Some ammonia is recycled and used in biosynthesis ü Excess ammonia is excreted Amino acids are deaminated via transamination Generate glutamate and α-ketoacid α-ketoacid enters gluconeogenic or TCA cycle pathways, depending on glucogenic or ketogenic amino acids Glutamate is deaminated Generate ammonia and α-ketoglutarate Ammonia enters the urea cycle for clearance from the body ü AA amino acid, ü α-KG α-ketoglutarate α-ketoglutarate enters TCA cycle for ü Glu glutamate energy production ü α-KA α-ketoacid ü GDH glutamate dehydrogenase 5  Transamination Removal of amino groups from amino acids and transfer to keto acids Catalyzed by aminotransferase or transaminase Requires coenzyme pyridoxal phosphate (PLP, the active form of vitamin B6) Found in high concentrations in the liver The original alpha-keto acid loses its keto group and gains an amino group, becoming a nonessential AA α-ketoglutarate is a general acceptor of amino group, glutamate is a temporary storage of amino group carbon skeletons amino group carbon skeletons amino group 6 Transamination Examples TCA cycle TCA cycle TCA cycle Glycolysis, Gluconeogenesis 7 Oxidative deamination Glutamate formed by transamination is deaminated Catalyzed by glutamate dehydrogenase Produce α-ketoglutarate and ammonia (NH4+) Ammonia (NH4+) processed to urea for excretion Urea cycle TCA cycle 8 Part I: Protein Metabolism q Transamination and Deamination q Fate of Amino Group —— Urea Cycle q Fate of Carbon Skeletons —— Glucogenic or Ketogenic 9 Excreted fate of amino group —— Urea cycle The nitrogen of amino acids, converted to ammonia, is toxic to the body Thus, it is converted to urea and detoxified Urea is synthesized in liver and transported to kidneys for excretion in urine Urea has two amino (NH2) groups, one derived from NH3 and the other from aspartate Carbon atom is supplied by CO2 Urea synthesis is a five-step cyclic process, with five distinct enzymes The first two reactions take place in mitochondria, while the rest occur in cytosol 10 Excreted fate of amino group —— Urea cycle 1. Synthesis of carbamoyl phosphate Enzyme: Carbamoyl Phosphate Synthase I (CPS I) Rate-limiting step Consume ATP 2. Formation of citrulline Enzyme: Ornithine Transcarbamoylase 3. Synthesis of arginosuccinate Enzyme: Arginosuccinate Synthase Consume ATP 4. Cleavage of arginosuccinate Enzyme: Arginosuccinase Product: Arginine & Fumarate Fumarate link with TCA cycle, gluconeogenesis 5. Formation of urea Enzyme: Arginase Product: Urea and Ornithine Ornithine enters mitochondria and reused in urea cycle 11 Significance of urea cycle Toxic NH4+ is converted into harmless urea via urea cycle Helps maintain nitrogen homeostasis in the body by managing excess nitrogen TCA cycle intermediates are recycled Amino acids and keto acids are recycled Ornithine is precursor of prolines and polyamines 12 Regulation of urea cycle Ø Enzyme activity The first reaction catalyzed by carbamoyl phosphate synthase I (CPS I) is rate-limiting reaction in urea cycle CPS I is activated by Nacetylglutamate (NAG) The rate of urea synthesis in liver is correlated with the concentration of N-acetylglutamate High concentrations of arginine increase NAG, the consumption of a protein-rich meal increases the level of NAG in liver, leading to enhanced urea synthesis Ø Substrate concentrations The urea cycle is regulated in part by the availability of substrates Large quantities of citrulline, arginine, or urea help prevent excessive urea formation High concentrations of ammonia and ornithine promote the advancement of the urea cycle 13 Urea cycle disorders Ø Hyperammonemia A metabolic condition characterized by the raised levels of ammonia, leading to toxicity Caused by metabolic defects associated with the five enzymes Ø Individual Disorders Defect Enzyme involved Hyperammonemia type I Carbomoyl phosphate synthase I Hyperammonemia type II Ornithine transcarbamoylase Citrullinemia Arginosuccinate synthase Arginosuccinic aciduria Arginosuccinase Hyperargininemia Arginase Ø Symptoms Lethargy, poor eating, vomiting, seizures, and developmental abnormalities are among the signs of these illnesses that frequently manifest 14 Reused fate of amino group —— biosynthesis q Amino acids q Nitrogenous compound 15 Part I: Protein Metabolism q Transamination and Deamination q Fate of Amino Group —— Urea Cycle q Fate of Carbon Skeletons —— Glucogenic or Ketogenic 16  Fate of carbon skeletons —— Gluconeogenesis Ø Carbon skeleton metabolism Amino group is removed by transamination Remaining carbon skeleton (α-keto acid) is catabolized by a pathway unique to that acid Ø Glucogenic amino acids Form any of the intermediates of carbohydrate metabolism Can subsequently be converted to glucose via gluconeogenesis Metabolized to pyruvate or metabolites of the TCA cycle: ü Pyruvate ü Succinyl-CoA ü Oxaloacetate ü α-Ketoglutarate ü Fumarate 13 amino acids are exclusively glucogenic 17  Fate of carbon skeletons —— Ketogenesis Ø Ketogenic amino acids Can be converted to acetoacetyl-CoA or acetyl-CoA Used for the synthesis of ketone bodies but not glucose Enter 1 of 3 metabolic pathways: ü Enter the TCA cycle to produce ATP/energy ü Ketogenesis (production of ketone bodies) ü Synthesis of fatty acids or cholesterol Only 2 amino acids are exclusively ketogenic 18 Glucogenic and Ketogenic Amino Acids Ø Glucogenic & Ketogenic Metabolized to intermediates of both gluconeogenesis and ketogenesis pathways 5 amino acids are both glucogenic and ketogenic 19 Glucogenic and Ketogenic Amino Acids 20 Summary 21 Part II: Lipid Metabolism q Fatty acid oxidation (β-oxidation) q Metabolism of ketone bodies 22 Overview of Lipid Metabolism 23 Fatty Acid Oxidation —— β-Oxidation Ø Definition β-oxidation is the metabolic process by which fatty acids are broken down to generate acetyl-CoA, NADH, and FADH₂ It specifically refers to the oxidation of the beta carbon atom of fatty acids Ø Location Occurs primarily in the mitochondria of cells, specifically in liver and muscle tissues Ø Products Each cycle of β-oxidation shortens the fatty acid chain by two carbon atoms Producing one acetyl-CoA, one NADH, and one FADH₂ for each cycle Acetyl-CoA enters the TCA cycle for further energy production or converted into ketone bodies Ø Function Provides a significant source of energy, especially during fasting, low carbohydrate status or prolonged exercise Plays a vital role in maintaining energy balance and metabolic homeostasis 24 Fatty Acid Oxidation —— β-Oxidation 25 Process of β-Oxidation Ø Activation of Fatty Acids Fatty acids are converted to fatty acyl-CoA in the cytoplasm using ATP Ø Transport into Mitochondria Fatty acyl-CoA is transported into mitochondria via the carnitine shuttle Ø Four Main Steps of Beta Oxidation 1. Oxidation (Dehydrogenation) 2. Hydration 3. Second Oxidation (Dehydrogenation) 4. Thiolysis Ø Energy sources Each cycle of β-oxidation produces: 1 molecule of Acetyl-CoA 1 molecule of FADH₂ 1 molecule of NADH 26  Step 1: Oxidation (Dehydration) by FAD Ø Substrate acyl-CoA, the activated form of the fatty acid Ø Enzyme The enzyme that catalyzes this reaction is acyl-CoA dehydrogenase Ø Reaction The acyl-CoA undergoes a dehydrogenation reaction where two hydrogen atoms are removed This process converts the acyl-CoA to trans-Δ²-enoyl-CoA The removed electrons are transferred to FAD, reducing it to FADH₂ Ø Significance This reaction introduces a double bond between the second and third carbon atoms of the fatty acid chain The formation of FADH₂ is important because it can enter the electron transport chain to generate ATP 27  Step 2: Hydration Ø Substrate trans-Δ²-enoyl-CoA, which was produced in the previous step Ø Enzyme The enzyme that catalyzes this reaction is enoyl-CoA hydratase Ø Reaction Water (H₂O) is added to the double bond of trans-Δ²-enoyl-CoA This hydration converts the double bond into a hydroxyl group Resulting in the formation of L-3-hydroxyacyl-CoA Ø Significance The addition of water adds a hydroxyl group (–OH) to the β-carbon of the fatty acid chain, preparing it for the next oxidation step L-3-hydroxyacyl-CoA is a crucial intermediate that will undergo further processing in the subsequent steps of β-oxidation 28  Step 3: Second Dehydration Ø Substrate 3-hydroxyacyl-CoA, which was produced in the previous step Ø Enzyme The enzyme that catalyzes this reaction is 3-hydroxyacyl-CoA dehydrogenase Ø Reaction The hydroxyl group (–OH) on the β-carbon of L-3-hydroxyacyl-CoA is oxidized to a carbonyl group (C=O) NAD⁺ is reduced to NADH Resulting in the formation of 3-ketoacyl-CoA Ø Significance This oxidation step is crucial for preparing the substrate for the next reaction in beta-oxidation The production of NADH is important for cellular energy metabolism, as NADH can enter the electron transport chain to generate ATP 29  Step 4: Thiolysis Ø Substrate 3-ketoacyl-CoA, which was produced in the previous step Ø Enzyme The enzyme that catalyzes this reaction is thiolase Ø Reaction 3-ketoacyl-CoA is cleaved by the addition of a CoA molecule The β-keto group is hydrolyzed Resulting in the production of acetyl-CoA and a new, shorter acyl-CoA Ø Significance Thiolysis is a critical step that not only produces acetyl-CoA, but also regenerates a fatty acyl-CoA that can continue to be oxidized This step allows for the continuous breakdown of fatty acids, ultimately leading to energy production 30 Regulation of β-Oxidation 1. Substrate Availability Free Fatty Acid Levels: Higher concentrations promote beta-oxidation 2. Hormonal Regulation Insulin: Inhibits beta-oxidation; promotes fatty acid storage Glucagon: Stimulates beta-oxidation during fasting; promotes lipolysis 3. Enzyme Regulation CPT I (Carnitine Palmitoyltransferase I) Inhibited by malonyl-CoA; controls entry of fatty acids into mitochondria Acyl-CoA Synthetase: Regulated by substrate availability and energy status 4. Energy Status NADH/FADH₂ Levels: High levels inhibit the electron transport chain ATP Levels: Elevated ATP signals sufficient energy, inhibiting oxidation 5. Nutritional Status Fasting State: Upregulates beta-oxidation for energy Fed State: Downregulates beta-oxidation; favors carbohydrate metabolism 31 Regulation of β-Oxidation 32 Part II: Lipid Metabolism q Fatty acid oxidation (β-oxidation) q Metabolism of ketone bodies 33 Introduction to Ketone Bodies Ø Definition Ketone bodies are water-soluble molecules produced in the liver from fatty acids during periods of low carbohydrate availability Three main types: acetoacetate, beta-hydroxybutyrate, and acetone Ketone bodies are formed in the liver from fatty acids through a process called ketogenesis Ø Production Occurs during fasting, prolonged exercise, or low-carbohydrate diets Fatty acids are converted to acetyl-CoA via β-oxidation, which is then transformed into ketone bodies Ø Function Serve as an alternative energy source for tissues, especially the brain, when glucose is scarce Help maintain energy balance and metabolic flexibility 34 Ketogenesis Pathway 1. Condensation 2 Acetyl-CoA → Acetoacetyl-CoA Catalyzed by thiolase 2. Formation of HMG-CoA Acetoacetyl-CoA → HMG-CoA Catalyzed by HMG-CoA synthase 3. Production of Ketone Bodies HMG-CoA → Acetoacetate by HMG-CoA lyase Acetoacetate → Beta-hydroxybutyrate by beta-hydroxybutyrate dehydrogenase Acetoacetate → Acetone (spontaneously) q Export and Utilization Release into Bloodstream: Acetoacetate and beta-hydroxybutyrate transported to tissues Energy Production: Tissues convert ketone bodies back to Acetyl-CoA for energy 35 Regulation of Ketogenesis 1. Substrate Availability Free Fatty Acid Levels: Higher concentrations enhance ketone body formation Acetyl-CoA Levels: Increased acetyl-CoA from beta-oxidation drives ketogenesis 2. Hormonal Regulation Insulin: Decreases during fasting; inhibits ketogenesis by suppressing fatty acid release and promoting glucose utilization Glucagon: Increases during fasting; stimulates ketogenesis by promoting lipolysis and fatty acid mobilization 3. Enzyme Regulation CPT I (Carnitine Palmitoyltransferase I) Inhibited by malonyl-CoA; controls entry of fatty acids into mitochondria HMG-CoA Synthase: Key regulatory enzyme in ketogenesis 4. Energy Status NADH/NAD⁺ Ratio: High NADH levels favor the conversion of acetoacetate to beta-hydroxybutyrate ATP/ADP Ratio: Low energy status (high ADP) stimulates ketogenesis 5. Nutritional Status Fasting State: Elevated ketogenesis due to increased fatty acid oxidation and high acetyl-CoA levels Low-Carbohydrate Diets: Promotes ketogenesis by reducing glucose availability and increasing fatty acid mobilization 36 Disorders associated with lipid metabolism 1. Hyperlipidemia Description: Elevated lipids (cholesterol and triglycerides) in the blood Types: ü Primary: Genetic disorders (e.g., familial hypercholesterolemia) ü Secondary: Related to conditions (e.g., diabetes, obesity) 2. Fatty Liver Disease Non-Alcoholic Fatty Liver Disease (NAFLD): Fat accumulation in the liver unrelated to alcohol Alcoholic Fatty Liver Disease: Caused by excessive alcohol consumption 3. Ketoacidosis Ketoacidosis: High ketone bodies in the blood, often seen in uncontrolled diabetes Symptoms: Nausea, vomiting, abdominal pain, confusion 4. Lipid Storage Disorders Gaucher Disease: Accumulation of glucocerebrosides due to enzyme deficiency Fabry Disease: Accumulation of globotriaosylceramide due to enzyme deficiency 37 Lipid metabolism and Diabetes 1. Insulin in Lipid Metabolism Promotes lipogenesis (fat storage) and inhibits lipolysis (fat breakdown) Facilitates conversion of excess glucose into triglycerides in adipose tissue 2. Insulin Resistance in Diabetes Cells become less responsive to insulin, impairing lipid regulation Increased lipolysis leads to elevated free fatty acids (FFAs) in the bloodstream 3. Impact on Glucose Metabolism Elevated FFAs impair insulin signaling, exacerbating hyperglycemia Insulin resistance disrupts the balance between lipid and glucose metabolism 4. Complications Increased risk of non-alcoholic fatty liver disease (NAFLD) Potential for diabetic ketoacidosis in uncontrolled type 1 diabetes 38