Protein Synthesis and Amino Acid Catabolism

Protein Synthesis

• Process by which cells create proteins
• Occurs in the ribosomes
• Involves three stages:
  1. Transcription: mRNA synthesis from DNA template
  2. Translation: mRNA translation into polypeptide chain
  3. Post-translational modification: folding, modification, and targeting of nascent protein

Amino Acid Catabolism

• Breakdown of amino acids into simpler molecules
• Occurs in the liver and kidneys
• Involves three stages:
  1. Transamination: conversion of amino acids to alpha-keto acids
  2. Deamination: removal of amino group (-NH2) from alpha-keto acids
  3. Decarboxylation: conversion of alpha-keto acids to acetyl-CoA or other metabolites

Peptide Hormones

• Hormones composed of two or more amino acids
• Examples:
  • Insulin
  • Glucagon
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
• Functions:
  • Regulate various physiological processes
  • Produced by endocrine glands
  • Secreted into the bloodstream

Urea Cycle

• Process by which ammonia is converted to urea
• Occurs in the liver
• Involves five enzyme-catalyzed reactions:
  1. Citrulline synthesis: conversion of ammonia and ornithine to citrulline
  2. Arginosuccinate synthesis: conversion of citrulline and aspartate to arginosuccinate
  3. Arginosuccinate lyase: conversion of arginosuccinate to arginine and fumarate
  4. Arginase: conversion of arginine to ornithine and urea
  5. Urea excretion: excretion of urea in the urine

Protein Degradation

• Breakdown of proteins into smaller peptides and amino acids
• Occurs in the lysosomes and proteasomes
• Mechanisms:
  • Lysosomal proteolysis: breakdown of proteins by lysosomal enzymes
  • Ubiquitin-proteasome pathway: breakdown of proteins by proteasomes
• Functions:
  • Regulate protein levels
  • Remove damaged or misfolded proteins
  • Recycle amino acids for new protein synthesis

Protein Synthesis

• Protein synthesis occurs in ribosomes and involves three stages
• Transcription: synthesis of mRNA from DNA template
• Translation: translation of mRNA into polypeptide chain
• Post-translational modification: folding, modification, and targeting of nascent protein

Amino Acid Catabolism

• Breakdown of amino acids into simpler molecules occurs in liver and kidneys
• Involves three stages: transamination, deamination, and decarboxylation
• Amino acids are converted to alpha-keto acids, then to acetyl-CoA or other metabolites

Peptide Hormones

• Hormones composed of two or more amino acids, produced by endocrine glands
• Examples: insulin, glucagon, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH)
• Regulate various physiological processes, secreted into the bloodstream

Urea Cycle

• Process by which ammonia is converted to urea in the liver
• Involves five enzyme-catalyzed reactions: citrulline synthesis, arginosuccinate synthesis, arginosuccinate lyase, arginase, and urea excretion

Protein Degradation

• Breakdown of proteins into smaller peptides and amino acids in lysosomes and proteasomes
• Mechanisms: lysosomal proteolysis and ubiquitin-proteasome pathway
• Functions: regulate protein levels, remove damaged or misfolded proteins, and recycle amino acids for new protein synthesis