Nitrogen Disposal: Transamination and Deamination
| Process | Key Enzyme & Location | Co-factors / Partners | Primary Biochemical Goal |
|---|---|---|---|
| Transamination | Aminotransferases (ALT, AST) Cytosol / Mitochondria |
Pyridoxal Phosphate (PLP), α-Ketoglutarate / Glutamate |
Shuttle amino groups from various amino acids onto a common collector molecule (typically creating Glutamate). No net release of free ammonia. |
| Oxidative Deamination |
Glutamate Dehydrogenase (GDH) Mitochondria (mainly Liver) |
NAD+ (catabolism) NADP+ (anabolism) |
Liberate the collected amino group from Glutamate as **free toxic ammonia (NH3 / NH4+)** to feed into the Urea Cycle. Re-forms α-Ketoglutarate. |
High-Yield Core Realities:
- The Pyridoxal Phosphate (B6) Schiff Base Mechanism: All transamination reactions rely absolutely on Vitamin B6 derivative Pyridoxal Phosphate (PLP). The enzyme’s aldehyde group binds covalently with the amino group of the incoming amino acid, creating a temporary Schiff base intermediate. This transforms PLP into pyridoxamine phosphate, which subsequently hands off the amino group to an α-keto acid. Note: Threonine and Lysine are structural exceptions and cannot undergo transamination.
- The Funneling Strategy to Glutamate: Most peripheral amino acids dump their alpha-amino groups directly onto α-ketoglutarate via specific transaminases. This universally turns α-ketoglutarate into Glutamate. Glutamate serves as the ultimate metabolic funnel, acting as the singular nitrogen pool that carries ammonia into the liver mitochondria for final disposal.
- Diagnostic Enzyme Shifts (ALT vs. AST):
- ALT (Alanine Aminotransferase) shifts the amino group from Alanine to α-ketoglutarate, producing Pyruvate and Glutamate. ALT is highly specific to liver parenchyma.
- AST (Aspartate Aminotransferase) shifts the amino group from Glutamate to Oxaloacetate, producing Aspartate and α-ketoglutarate. Aspartate goes on to provide the second nitrogen atom required to build Urea. AST is found in both liver and cardiac/skeletal muscle mitochondria.
- Glutamate Dehydrogenase (GDH) Allosteric Tuning: Found inside the mitochondrial matrix, GDH catalyzes the reversible oxidative deamination of glutamate. It is strictly controlled by the cell’s energetic charge: ADP and GDP are allosteric activators (signaling low energy, stimulating amino acid carbon skeletons to enter the TCA cycle), while ATP and GTP are allosteric inhibitors.
- The Inter-Organ Alanine & Glutamine Safe-Transport Loops: Free ammonia is highly neurotoxic and cannot travel loose in plasma.
- Skeletal Muscle (Cahill/Alanine Cycle): Pyruvate produced during glycolysis is transaminated into **Alanine**, which traveling to the liver is converted back to pyruvate for gluconeogenesis while dropping its nitrogen.
- Peripheral Tissues / Brain (Glutamine Synthetase): Ammonia is fixed to glutamate to form non-toxic **Glutamine**, which migrates through the blood to the liver or kidneys where *Glutaminase* breaks it down to release free ammonia safely at its final clearance destination.