Degradation of aminoacids

• Nonpolar Aliphatic R Groups

  • Glycine
  • Alanine
  • Valine
  • Leucine
  • Isoleucine
  • Methionine
  • Proline
• Aromatic R Groups


  • Phenylalanine
  • Tyrosine
  • Tryptophan
• Polar Uncharged R Groups

  • Serine
  • Threonine
  • Cysteine
  • Asparagine
  • Glutamine
• Positively Charged R Groups

  • Lysine
  • Arginine
  • Histidine
• Negatively Charged R Groups

  • Aspartate
  • Glutamate
• Branched Amino Acids


  • Valine
  • Leucine
  • Isoleucine
• Mixed Amino Acids


  Can undergo deamination or transamination depending on the pathway and energy needs
• Transaminases (Aminotransferases)


  1. Catalyze the transfer of an amino group from an amino acid to an α-keto acid

  2. Cofactor: Pyridoxal phosphate (PLP) is the prosthetic group

  3. Mechanism: Involves the formation of a Schiff base (internal aldimine) with the enzyme, which then forms an external aldimine with the substrate amino acid
• Glutamate-pyruvate transaminase


  Transfers the amino group from glutamate to pyruvate, forming α-ketoglutarate and alanine
• Glutamate Dehydrogenase


  1. Catalyzes the oxidative deamination of glutamate to α-ketoglutarate and ammonia (NH3)

  2. Regulation: Allosterically activated by ADP/GDP and inhibited by ATP/GTP, reflecting the cell's energy state
• Urea Cycle


  1. Converts toxic ammonia to urea for excretion

  2. Carbamoyl Phosphate Synthetase I: Converts NH3 and CO2 to carbamoyl phosphate

  3. Ornithine Transcarbamylase: Forms citrulline from carbamoyl phosphate and ornithine

  4. Argininosuccinate Synthetase: Combines citrulline with aspartate to form argininosuccinate

  5. Argininosuccinate Lyase: Splits argininosuccinate into arginine and fumarate

  6. Arginase: Hydrolyzes arginine to urea and ornithine, completing the cycle
• Ketogenic Amino Acids


  • Leucine
  • Lysine
• Glucogenic Amino Acids


  • Alanine
  • Glutamine
• Mixed Amino Acids


  • Isoleucine
  • Phenylalanine
  • Tyrosine
• Transaminases


  1. Role: Key enzymes in amino acid catabolism and nitrogen metabolism

  2. Reaction: Amino group transfer from an amino acid to an α-keto acid
• Glutamine Dehydrogenase


  1. Reaction: Converts glutamate to α-ketoglutarate and NH3

  2. Importance: Plays a crucial role in nitrogen metabolism
• Urea Cycle


  Reaction Pathway: Converts ammonia to urea for safe excretion
• Epinephrine (Adrenaline)


  Derived from tyrosine, acts as a hormone and neurotransmitter
• GABA (Gamma-Aminobutyric Acid)


  Derived from glutamate, functions as an inhibitory neurotransmitter
• Glutathione


  Composed of glutamate, cysteine, and glycine, acts as an antioxidant
• Nitric Oxide (NO)


  Produced from arginine, functions as a signaling molecule
• Histamine


  Derived from histidine, involved in immune responses and gastric acid secretion
• Serotonin


  Derived from tryptophan, acts as a neurotransmitter and regulator of mood
• Phenylalanine Metabolism


  1. Initial Reaction: Conversion to tyrosine via phenylalanine hydroxylase

  2. Further Reactions: Tyrosine is further metabolized to fumarate and acetoacetate
• Branched Amino Acids


  Example: Isoleucine, leucine, and valine are initially transaminated and then undergo oxidative decarboxylation
• Asparagine and Glutamine Metabolism

  1. Asparagine: Hydrolyzed by asparaginase to aspartate and NH4+

  2. Glutamine: Hydrolyzed by glutaminase to glutamate and NH4+
• Alanine, Aspartate, and Glutamate Metabolism

  Transamination: Conversion to pyruvate, oxaloacetate, and α-ketoglutarate, respectively
• Nitric Oxide Synthase (NOS)


  Converts arginine to nitric oxide (NO) and citrulline
• Catabolic Pathways for Specific Amino Acids

  1. Alanine, Glycine, Serine, Cysteine, Tryptophan, and Threonine: Each amino acid follows a specific pathway leading to key metabolic intermediates like pyruvate, oxaloacetate, and α-ketoglutarate

  2. Color-Coding of Carbon Atoms: Used to trace the fate of carbon atoms through these pathways
• Central Reactions: Involving transaminases, glutamine dehydrogenase, and the urea cycle
• Fate of Degradation: Ketogenic, glucogenic, or mixed pathways
• Pathways of Degradation: Normal degradation pathways of amino acids
• Active Molecules: Examples include epinephrine, GABA, glutathione, nitric oxide, histamine, and serotonin
• Ammonia Excretion: Via the urea cycle in mammals