Glutamine Synthetase (GS) Activation

• Conditions for activity:

  • Activated by hydroxychloroquine.
  • Covalent regulation occurs through the actions of GS adenylyltransferase.
  • Active form: GS is activated with addition of adenosine monophosphate (AMP).

• Key reactions:

  • Converts glutamate to glutamine using ammonium ions:
    \text{Glutamate} + \text{NH}_4^+ \rightarrow \text{Glutamine}

• Function and context:

  • Essential for synthesizing amino acids and removing excess nitrogen, as nitrogen cannot be stored in the body and leads to toxicity.

Feedback Regulation of GS

• Adenosine Monophosphate (AMP):

  • Acts as a negative feedback inhibitor for GS, linking allosteric and covalent regulation.
  • The regulation by AMP allows for a robust control mechanism accustomed to cellular nitrogen levels.

• Negative feedback inhibitors:

  • Tryptophan and histidine function similarly, linked to the regulation of amino acid balance in metabolism.

Protein Degradation & Ubiquitination

• Importance of protein turnover:

  • Proteins must be degraded to regulate responses to cellular environment, and turnover is essential for changing gene expression.

• Degradation pathways: There are generally two pathways for protein degradation:

  1. Ubiquitination pathway: ATP-dependent process where proteins are tagged for degradation.
  • Requires ubiquitin molecules coupled with E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase).
  1. Lysosomal degradation: More nonspecific protein breakdown.

Ubiquitin Pathway Mechanism

• Stages of ubiquitination:
  1. Activation: ATP combines with ubiquitin on E1.
  2. Conjugation: Ubiquitin transferred to E2.
  3. Ligation: E3 binds to E2 and substrate, facilitating ubiquitin transfer to substrate protein.
• Multiple ubiquitin molecules (usually 4) signal for degradation.
• Lysines in proteins are the target for ubiquitination.

Regulation of Ubiquitination Pathway

• Selectivity:
  • E3 specificity allows for precise degradation control, essential for maintaining cellular homeostasis without excessive degradation.
• Enzymatic regulation:
  • E1 is limited, E2 is more abundant, but E3 is the most due to the requirement for specificity for different substrates.

Amino Acid Synthesis & Degradation

• Sources of amino acids:
  • Dietary intake of proteins (60g/day recommended) and degradation of cellular proteins.
  • Nitrogen excretion via urea cycle post-deamination of amino acids.
• Nitrogen Cycle: Excess nitrogen must be excreted as ammonium ions (toxic) via urea in the liver.

Gastrointestinal Protein Digestion Sequence

1. Ingestion: Protein is released through the esophagus into the stomach.
2. Acidic environment: Hydrochloric acid and pepsinogen activated to pepsin, denaturing proteins into peptides.
3. Small intestine digestive enzymes:
   • Pancreatic enzymes like trypsinogen activated, leading to peptide degradation into amino acids.
4. Absorption: Amino acids absorbed into bloodstream through intestinal epithelial cells.

Takeaways

• Interdependence of pathways: Understanding the regulatory mechanisms of GS through nitrogen balance exemplifies the complex regulatory environments in metabolic pathways.
• Dynamic protein turnover: Both dietary and cellular proteins undergo continual synthesis and degradation to ensure cellular processes remain functional, highlighting the significance of enzymatic regulation in metabolism.