Amino Acids and Protein Metabolism

Amino Acid Pool

• Comprises free amino acids distributed throughout the body.
• Amino acids are released by hydrolysis of dietary or tissue proteins.
• Essential amino acids cannot be synthesized and include:
  • Histidine
  • Lysine
  • Threonine
  • Isoleucine
  • Methionine
  • Tryptophan
  • Leucine
  • Phenylalanine
  • Valine
  • Arginine

Endogenous Protein Proteolysis

• Continuous breakdown of body proteins results in free amino acids.
• Degradation rates differ among individual proteins:
  • Liver enzymes have half-lives of a few hours.
  • Red blood cells (hemoglobin) last approximately 120 days.
  • Structural proteins like collagen have long life spans.

Inputs for Amino Acids

• Derived from dietary protein and proteolysis of cellular proteins.
• Factors influencing protein degradation rates:
  • Proteins serve as long-term energy storage and provide gluconeogenic precursors.
  • Degradation removes abnormal or damaged proteins to maintain cell function.
  • Metabolic activity regulation requires the degradation and replacement of specific regulatory proteins.

Protein Turnover

• Rapidly degraded proteins function outside the cell, with half-lives from hours to days (e.g., pancreatic enzymes).
• Degradation process includes:
  • Stomach pepsin acting on large polypeptides.
  • Pancreatic proteases further breaking down peptides into oligopeptides and amino acids.

Release and Activation of Zymogens

• Zymogens are activated by enzymes such as enteropeptidase.
• Key hormones like cholecystokinin and secretin influence pancreatic acinar cells for enzyme secretion.
• Trypsinogen is converted into trypsin by the cleavage of a hexapeptide.

Proteolytic Enzymes and Degradation Systems

• The 26S proteasome degrades endogenous proteins.
• The 19S cap recognizes and unfolds ubiquitinated proteins for degradation.
• Lysosomal enzymes target extracellular proteins for breakdown.

Signals for Protein Turnover

• Preferential degradation of chemically altered proteins, particularly those oxidized.
• Proteins with PEST sequences are targeted for degradation due to their composition of proline, glutamic acid, serine, and threonine.
• Acidic environments enhance pepsin activity for protein digestion.

Transport Mechanisms for Amino Acids

• Active transport systems create a gradient, moving amino acids from low to high concentration spaces.
• COAL system facilitates uptake of cystine and dibasic amino acids such as ornithine, arginine, and lysine.
• Na+-dependent luminal transport and Na+-independent contraluminal transport methods.

α-Amino Groups and Nitrogen Disposal

• Protect amino acids from oxidative breakdown by keeping them locked within the structure.
• Transamination is the first step in amino acid catabolism, transferring an amino group to an α-keto acid, generating a new amino acid.
• Oxidative deamination occurs primarily in the liver and kidney, releasing ammonia as a waste product.

This quiz explores the concepts related to the amino acid pool, including the distribution of free amino acids and the importance of essential amino acids that cannot be synthesized by the body. Additionally, it covers endogenous protein proteolysis and its role in amino acid release. Test your knowledge on these critical biochemical processes.