Amino Acid Metabolism and Carbon Skeletons

Removal of Alpha Amino Group

• During amino acid breakdown in the liver:
  • Alpha amino group is removed.
  • Results in a carbon skeleton.

Fate of Carbon Skeletons

• Carbon skeletons from 20 amino acids lead to 7 different molecules.
• These molecules are intermediates in metabolic pathways for ATP production.

Review of Metabolic Pathways

• Citric Acid Cycle
  • Generates oxaloacetate in liver.
  • Oxaloacetate: starting material for glucose production via gluconeogenesis.
  • Pyruvate can form oxaloacetate (enzyme: pyruvate carboxylase).
• Pyruvate Pathways
  • Pyruvate to acetyl-CoA (enzyme: pyruvate decarboxylase):
    • Acetyl-CoA generates fatty acids and ketone bodies.
    • Acetyl-CoA can't form glucose.

Glucogenic vs Ketogenic Amino Acids

• Glucogenic Amino Acids
  • Metabolized into intermediates leading to glucose.
  • Examples: Tryptophan (forms pyruvate), Aspartate (forms oxaloacetate).
• Ketogenic Amino Acids
  • Lead to formation of ketone bodies.
  • Solely ketogenic: Leucine and Lysine.

Dual Pathways - Both Glucogenic and Ketogenic

• Tryptophan, Isoleucine, Phenylalanine, Tyrosine.
  • Tryptophan can lead to both glucose and ketone bodies.

Memorization Tips

• Memorize the two solely ketogenic (Leucine, Lysine) and four dual-pathway amino acids.
• The rest are glucogenic.
• Possible variations include Threonine being classified as both.

Conclusion

• Majority of amino acids in the liver convert to glucose.
• Next lectures will explore pathways for each amino acid to these intermediates.