10- BCC AA Metabolism and Synthesis Lecture Handout

Learning Objectives

  • Explain conditions under which amino acids are used as an energy source.

  • Define the importance of glutamate, glutamine, alanine, and aspartate in amino acid catabolism.

  • Describe reactions catalyzed by key enzymes in amino acid catabolism:

    • Aminotransferases

    • Glutamate dehydrogenase

    • Glutamine synthetase

    • Glutaminase

  • Discuss disease conditions when aminotransferases are detected in plasma.

  • Explain alanine's role in NH4+ group removal in muscle.

  • Understand the urea cycle's key enzymatic steps.

  • Describe the linkage between the urea cycle and the citric acid cycle.

Sources of Amino Acids

  • Dietary Protein

  • Biosynthesis

  • Tissue Breakdown

Fates of Amino Acids

  • Excretion

  • Energy Metabolism

  • Biosynthesis of Small Molecules

Conditions for Amino Acid Energy Usage

  • Breakdown occurs when:

    • New amino acid synthesis is not required.

    • During protein turnover (e.g., misfolded or denatured proteins).

    • In scarcity of carbohydrates (e.g., starvation, diabetes mellitus).

Overview of Amino Group Disposal

  • Oxidative degradation releases NH4+:

    • Used in various biosynthetic pathways; excess eliminated via urea cycle.

    • Carbon skeletons oxidized through the citric acid cycle, generating metabolic energy.

Key Amino Acids in Catabolism

  • Amino acids readily converted to citric acid cycle intermediates:

    • Glutamate & Glutamine ➜ α-Ketoglutarate

    • Alanine ➜ Pyruvate

    • Aspartate ➜ Oxaloacetate

  • Roles:

    • Glutamate & Glutamine: Collection points for amino groups.

    • Alanine: Removal of amino groups in muscle.

    • Aspartate: Important in the urea cycle.

Key Enzymes in Ammonium Ion Removal

  1. Aminotransferases

  2. Glutamate dehydrogenase (GDH)

  3. Glutamine synthetase

  4. Glutaminase

  5. Urea cycle enzymes

Aminotransferases

  • Catalyze the first catabolic step of L-amino acids, removing amino groups.

  • Freely reversible reaction transferring α-amino groups to α-ketoglutarate, forming glutamate.

  • Also involved in amino acid biosynthesis (reverse direction).

  • Utilize PLP (Pyridoxal phosphate) as a prosthetic group.

Pyridoxal Phosphate (PLP)

  • Derived from Vitamin B6, acts as an intermediate carrier of amino groups.

  • Forms covalent Schiff-base intermediates with amino acid substrates.

  • Involved in transamination, racemization, and decarboxylation reactions.

Diagnostic Function of Aminotransferases

  • Blood levels indicate liver damage (e.g., viral hepatitis, alcohol consumption, acetaminophen reaction).

  • Elevated aminotransferases may also indicate non-hepatic diseases (e.g., myocardial infarction).

Glutamate Dehydrogenase (GDH)

  • Catalyzes oxidative deamination for NH4+ release.

  • Occurs mainly in liver and kidney; provides α-keto acids for energy metabolism or glucose synthesis.

  • GDH is allosterically regulated by ADP, GDP (promote degradation) and GTP, ATP (promote synthesis).

Transdeamination

  • Combines functions of aminotransferases & GDH to facilitate amino acid degradation.

Role of Glutamine in Ammonia Removal

  • NH4+ released on oxidative deamination by GDH; glutamine synthetase converts NH4+ into non-toxic glutamine in liver.

  • Glutamine serves as a nitrogen donor in various biosynthetic pathways.

Glutaminase Action

  • Excess glutamine deaminated in the intestine and kidney, contributing NH4+ back to urea cycle.

Glucose-Alanine Cycle in Muscle

  1. α-amino groups are transferred to glutamate.

  2. Amino group transferred to pyruvate, forming alanine.

  3. Alanine taken up by liver, converted back to pyruvate.

  4. Pyruvate used in gluconeogenesis; amino group ends up as urea.

Urea Cycle Overview

  • Converts ammonium ion from amino acids to urea (discovered by Krebs & Hensel in 1932).

  • Begins in liver mitochondria; involves combining NH4+ from GDH and aspartate to form urea.

Urea Cycle Steps

  1. Formation of carbamoyl phosphate (ATP required).

  2. Condensation of ornithine and carbamoyl phosphate to form citrulline.

  3. Citrulline condenses with aspartate to form argininosuccinate.

  4. Breakdown of argininosuccinate to arginine and fumarate.

  5. Arginase conversion of arginine to generate ornithine and urea.

Fumarate and Urea Cycle Link

  • Fumarate generated can convert to malate, which enters citric acid cycle, and can be metabolically linked, generating NADH and energy.

Overall Energetic Cost of Urea Cycle

  • Requires:

    • 2 ATP for carbamoyl phosphate synthesis.

    • 1 ATP for argininosuccinate synthesis.

  • Generates ATP through conversion of malate to oxaloacetate.

Regulation of Urea Cycle

  • Energy-dependent, regulated by substrate availability and conditions promoting protein metabolism.

  • Allosteric activation of CPSI by N-acetyl glutamate.

Defects in Urea Cycle

  • Result in accumulation of intermediates, increased glutamine levels, and decreased α-ketoglutarate, affecting ammonium ion fixation.

Ammonia Toxicity

  • High levels cross the blood-brain barrier, leading to neuronal loss and metabolic defects.

  • Glutamate depletion may affect neurotransmitter availability; high glutamine increases brain water absorption.

Carbon Skeletons of Amino Acids

  • Degraded to major metabolic intermediates:

    • Ketogenic amino acids: Convert to acetyl CoA or acetoacetyl CoA (fats/ketone bodies).

    • Glucogenic amino acids: Synthesize glucose; only leucine and lysine are strictly ketogenic.

Cofactors and Coenzymes in Amino Acid Catabolism

  • Tetrahydrofolate (THF): Accepts/donates one-carbon groups.

  • S-adenosylmethionine (SAM): Preferred methyl transfer cofactor.

  • Vitamin B12, Biotin, Tetrahydrobiopterin: Involved in amino acid metabolism.

Amino Acid Metabolism Defects and Diseases

  • Albinism: Defect in tyrosine metabolism leading to melanin deficiency.

  • Phenylketonuria (PKU): Caused by deficient phenylalanine hydroxylase; elevated phenylalanine levels.

  • Maple Syrup Urine Disease: Defect in branched-chain amino acid degradation.

Significance of Nitrogen Cycle

  1. Role of aminotransferases in amino acid biosynthesis.

  2. Various modes of regulation of glutamine synthetase.

  3. Pathway intermediates as precursors for amino acid synthesis.

  4. Biosynthesis of Serine and Glycine: From 3-phosphoglycerate involves oxidation and transamination steps.

  5. Cysteine Synthesis: From Methionine.

  6. Biosynthesis of Neurotransmitters: Involves decarboxylation reactions.

  7. Creatine Synthesis: From Glycine and Arginine.

  8. Biosynthesis of Glutathione: Key reducing agent in cells.

  9. Biosynthesis of Nitric Oxide: Important for blood pressure regulation and derived from arginine.