Lecture on Amino Acid Breakdown and Deamination

Introduction

• Fate of amino acids: Once proteins are degraded, amino acids can:
  • Be used in biosynthetic processes (e.g., building proteins, nucleotide bases).
  • Be broken down if in excess, as they cannot be stored like glucose or fatty acids.

Amino Acid Breakdown

• Main site: Liver.
• Other sites: Muscle cells (e.g., breakdown of branched-chain amino acids like leucine, isoleucine, and valine).

Two-Step Breakdown Process

1. Transamination

• Goal: Remove the amino group from amino acids.
• Enzyme: Aminotransferase (also known as transaminase).
• Coenzyme: Pyridoxal phosphate (Vitamin B6 derivative).
• Process:
  • Transfer amino group onto alpha-keto acid.
  • Form glutamate and an alpha-keto acid.

2. Oxidative Deamination

• Purpose: Deaminate glutamate.
• Enzyme: Glutamate dehydrogenase (uses NAD+ or NADP+).
• Location: Mitochondria (to prevent cell damage from toxic ammonium).
• Process:
  • Dehydrogenation and hydrolysis reactions.
  • Formation of ammonium and alpha-ketoglutarate.

Examples of Transamination and Deamination

• Alanine aminotransferase:
  • Starts with alanine, forms pyruvate.
• Aspartate aminotransferase:
  • Starts with aspartate, forms oxaloacetate.
• Glutamate undergoes oxidative deamination to release ammonium.

Reversibility and Importance

• Reactions can proceed in both directions, allowing for breakdown and synthesis of amino acids.
• Driving Forces: Urea cycle uses up ammonium, pushing reactions forward.

Alternative Deamination Pathways

• Serine and Threonine: Single-step deamination via dehydratases.
  • Serine: Converts to pyruvate, releases ammonium.
  • Threonine: Converts to alpha-ketobutyrate, releases ammonium.

Conclusion

• Outcome: Deamination forms carbon skeletons for energy and ammonium for the urea cycle.
• Pathway Variations: Two-step vs. single-step processes for different amino acids.