Biomolecules Energy: Protein and Lipid Metabolism

Lecture 7: Protein and Lipid Metabolism

• Lecture presented by Dr. Sophie Shi Ling
• Department of Applied Science, School of Science and Technology, Hong Kong Metropolitan University

Class of Biomolecules for Energy

• Step 1: Production of acetyl-CoA
  • Glucose: Converted to pyruvate via glycolysis, then to acetyl-CoA by pyruvate dehydrogenase
  • Fatty acids: Converted to acetyl-CoA via β-oxidation
  • Amino acids: Converted to acetyl-CoA via oxidation
• Step 2: Oxidation of acetyl-CoA via TCA cycle
  • Generates NADH and FADH2
• Step 3: ATP production from NADH and FADH2 via oxidative phosphorylation

Part I: Protein Metabolism

• Transamination and Deamination
• Fate of Amino Group → Urea Cycle
• Fate of Carbon Skeletons → Glucogenic or Ketogenic

Overview of Protein Metabolism

• Feature of amino acids: All contain amino groups.
• Key step: Lose amino group (deamination), form α-keto acid
• Amino group: Reused or excreted.
• α-keto acid: Oxidized or recycled.

Amino Acid Catabolism

• Most amino acids are metabolized in the liver.
• Amino group is removed as ammonia (NH4+).
• Some ammonia is recycled and used in biosynthesis; excess is excreted.
• Amino acids are deaminated via transamination to generate glutamate and a-ketoacids.
• α-ketoacids enter gluconeogenic or TCA cycle pathways.
• Glutamate is deaminated, producing ammonia and α-ketoglutarate.
• Ammonia enters the urea cycle for clearance from the body.
• α-ketoglutarate enters the TCA cycle for energy production.

Transamination

• Removal of amino groups from amino acids and transfer to keto acids.
• Catalyzed by aminotransferase or transaminase.
• Requires coenzyme pyridoxal phosphate (PLP).
• Found in high concentrations in the liver.
• α-ketoglutarate is a general acceptor of amino group; glutamate is a temporary storage of amino group.

Oxidative Deamination

• Glutamate formed by transamination is deaminated.
• Catalyzed by glutamate dehydrogenase.
• Produce α-ketoglutarate and ammonia (NH4+).
• Ammonia (NH4+) is processed to urea for excretion.

Excreted Fate of Amino Group → Urea Cycle

• Nitrogen of amino acids converted to ammonia (toxic)
• Converted to urea for detoxification.
• Urea synthesized in the liver and transported to kidneys for excretion in urine.
• Urea has two amino groups (NH2).
• Carbon atoms supplied by CO2.
• Urea synthesis is a five-step cyclic process.

Excreted Fate of Amino Group → Urea Cycle - Detailed steps

• Synthesis of carbamoyl phosphate.
• Formation of citrulline.
• Synthesis of arginosuccinate.
• Cleavage of arginosuccinate.
• Formation of Urea.

Significance of Urea Cycle

• Converts toxic NH4+ into harmless urea.
• Maintains nitrogen homeostasis.
• Recycles TCA cycle intermediates.
• Recycles amino acids and keto acids.
• Ornithine is a precursor of prolines and polyamines.

Regulation of Urea Cycle

• The first reaction catalyzed by carbamoyl phosphate synthase I is the rate-limiting reaction.
• CPS I is activated by N-acetylglutamate (NAG).
• The rate of urea synthesis in the liver is correlated with the concentration of N-acetylglutamate.
• High concentrations of arginine increase NAG.
• The urea cycle is regulated in part by the availability of substrates.

Urea Cycle Disorders

• Hyperammonemia: Raised ammonia levels leading to toxicity.
• Caused by metabolic defects associated with the five enzymes.

Reused Fate of Amino Group → Biosynthesis

• Amino acids and nitrogenous compounds
• Urea, proline, etc

Fate of Carbon Skeletons → Gluconeogenesis

• Amino group is removed by transamination.
• Remaining carbon skeleton (α-keto acid) is catabolized by a pathway unique to that acid.
• Glucogenic amino acids: Form intermediates of carbohydrate metabolism, converted to glucose via gluconeogenesis.
• Metabolized to pyruvate or TCA cycle metabolites.
• 13 amino acids are exclusively glucogenic.

Fate of Carbon Skeletons → Ketogenesis

• Ketogenic amino acids: Can be converted to acetoacetyl-CoA or acetyl-CoA; used for the synthesis of ketone bodies, not glucose.
• Enter 1 of 3 metabolic pathways:
  • Enter the TCA cycle to produce ATP/energy.
  • Ketogenesis (production of ketone bodies).
  • Synthesis of fatty acids or cholesterol.
• Two amino acids are exclusively ketogenic.

Glucogenic and Ketogenic Amino Acids

• Metabolized to intermediates of both gluconeogenesis and ketogenesis pathways.
• 5 amino acids are both glucogenic and ketogenic.

Part II: Lipid Metabolism

• Fatty acid oxidation (β-oxidation)
• Metabolism of ketone bodies

Overview of Lipid Metabolism

• Triacylglycerols → Fatty Acids → β-oxidation → Acetyl-CoA → Citric Acid Cycle → Oxidative Phosphorylation → ATP
• Fatty acid synthesis → Membrane lipids → Cholesterol

Fatty Acid Oxidation β-Oxidation

• Definition: Metabolic process breaking down fatty acids to acetyl-CoA, NADH, and FADH2.
• Location: Primarily in mitochondria of liver and muscle tissues.
• Products: Each cycle shortens the fatty acid chain by two carbon atoms; produces one acetyl-CoA, one NADH, and one FADH2 for each cycle.
• Function: Provides significant energy source, especially during fasting, low-carb status, or prolonged exercise.

Process of β-Oxidation

• Activation of Fatty Acids: Fatty acids are converted to fatty acyl-CoA in the cytoplasm using ATP.
• Transport into Mitochondria: Fatty acyl-CoA is transported into mitochondria via the carnitine shuttle.
• Four Main Steps of Beta Oxidation: Oxidation (dehydrogenation), hydration, second oxidation (dehydrogenation), and thiolysis.

Regulation of β-Oxidation

• Substrate Availability: Higher concentrations of free fatty acids promote beta-oxidation.
• Hormonal Regulation: Insulin inhibits beta-oxidation; glucagon stimulates beta-oxidation.
• Enzyme Regulation: CPT I inhibited by malonyl-CoA; controls entry of fatty acids into mitochondria; acyl-CoA synthetase regulated by substrate availability and energy status.
• Energy Status: High NADH/FADH2 inhibits beta-oxidation; high ATP levels inhibit beta-oxidation.
• Nutritional Status: Fasting upregulates beta-oxidation, fed state downregulates.

Ketogenesis Pathway

• Condensation: 2 acetyl-CoA → acetoacetyl-CoA, catalyzed by thiolase.
• Formation of HMG-CoA: Acetoacetyl-CoA → HMG-CoA, catalyzed by HMG-CoA synthase.
• Production of Ketone Bodies: HMG-CoA → acetoacetate; acetoacetate → beta-hydroxybutyrate; acetoacetate → acetone, spontaneously.
• Export and Utilization: Release into bloodstream. Tissues convert ketone bodies back to Acetyl-CoA for energy.

Regulation of Ketogenesis

• Substrate availability: Free fatty acid levels, increased acetyl-CoA from beta-oxidation.
• Hormonal regulation: Insulin inhibits ketogenesis, glucagon stimulates.
• Enzyme regulation: CPT I inhibited by malonyl-CoA, HMG-CoA Synthase is key regulatory enzyme.
• Energy status: High NADH/NAD+ ratio favors beta-hydroxybutyrate conversion. Low ATP/ADP ratio stimulates ketogenesis.
• Nutritional status: fasting upregulates; low-carb diets promotes ketogenesis.

Disorders Associated with Lipid Metabolism

• Hyperlipidemia: Elevated cholesterol and triglycerides in the blood. Primary (genetic) or secondary (related to conditions like diabetes).
• Fatty Liver Disease: Fat accumulation in the liver (NAFLD, unrelated to alcohol; or alcoholic).
• Ketoacidosis: High ketone bodies in the blood often seen in uncontrolled diabetes.
• Lipid Storage Disorders: Gaucher or Fabry disease: accumulation of certain lipids due to enzyme deficiencies.

Lipid Metabolism and Diabetes

• Insulin promotes lipogenesis and inhibits lipolysis.
• Insulin resistance increases lipolysis leading to elevated free fatty acids in the bloodstream.
• Elevated FFAs impair insulin signaling.
• Insulin resistance disrupts the balance between lipid and glucose metabolism.
• Increased risk of NAFLD and diabetic ketoacidosis.