Amino-Acid Metabolism & Nitrogen Disposal

Context & Scope of Today’s Lecture

• Focus exclusively on Amino-acid metabolism; other macromolecules (carbohydrates, lipids) discussed previously or in future sessions are outside today’s scope.
• Lecturer’s declared goal: keep session “high-yield,” problem-solving oriented and tightly aligned with the published learning objectives (LOs).
• Students are not expected to memorize every enzyme; instead, emphasis is on
  • Recognising breadcrumb trails in biochemical pathways.
  • Being able to deduce a diagnosis or solve board-style questions by following metabolic logic.

Difference Between Storage Forms of the Three Fuel Classes

• Carbohydrates → stored as glycogen (highly branched $\alpha(1\rightarrow4)$ backbone, $\alpha(1\rightarrow6)$ branches).
• Lipids → stored as triacylglycerol (TAG) droplets.
• Amino acids lack a dedicated storage polymer:
  • Incorporated into proteins/peptides (structural or enzymatic) or secreted as hormones & signalling molecules.
  • Surplus amino acids must be catabolised immediately; no “amino-glycogen” equivalent exists.
  • Consequence → systemic AA pool must remain in dynamic steady-state.

Free Amino-Acid Pool: Input vs Output Balance

• Input (3 sources)
  1. Protein turnover (major): continuous proteasomal & lysosomal degradation of endogenous proteins.
  2. Dietary intake: varies with lifestyle; deficiency or excess affects pool.
  3. De novo synthesis of non-essential AAs from metabolic precursors.
• Output
  1. New protein synthesis (growth, repair, signalling peptides, etc.).
  2. Catabolism → removal of amino-nitrogen & oxidation of carbon skeletons for energy or gluconeogenesis.
  3. Excretion (urea, NH₄⁺, creatinine, minor losses in feces, skin, hair).
• Homeostasis principle: $\text{Consumption} = \text{Excretion}$ in the long term.

The Nitrogen Problem

• C, H, O have robust metabolic clearance (CO₂, H₂O). N lacks an easily exhaled or diffusible form, mandating specialized disposal routes.
• Toxicity of Ammonia: even modest plasma [NH₃] elevation causes cerebral edema & encephalopathy.

Two-Step Universal Strategy for Removing Amino-Nitrogen

1. Transamination (reversible)
   • Definition: transfer of $\alpha\text{-}NH_2$ from an amino acid to an $\alpha$-keto acid.
   • General reaction: $\text{AA}<em>1 + \alpha\text{-ketoacid}</em>2 \rightleftharpoons \text{AA}<em>2 + \alpha\text{-ketoacid}</em>1$
   • Catalysed by aminotransferases; requires cofactor pyridoxal-5’-phosphate (PLP, vitamin B₆).
   • Physiological points: • Provides rapid interconversion (e.g.
     • Excess alanine → pyruvate + glutamate.
     • Deficient alanine ← pyruvate + glutamate.
       • Serves negative feedback buffering of specific AA concentrations.
2. Oxidative deamination / Urea cycle
   • Most transaminations funnel $NH_2$ to glutamate.
   • Glutamate dehydrogenase (GDH) in liver mitochondria releases free $NH_4^+$.
   • $NH_4^+$ then enters the urea cycle for detoxification.

Urea Cycle (Hepatic Mitochondria → Cytosol)

1. Carbamoyl phosphate synthetase I (CPS I)
2. Ornithine transcarbamylase (OTC)
3. Argininosuccinate synthetase (ASS1)
4. Argininosuccinate lyase (ASL)
5. Arginase (ARG1)
   Net equation (per turn): $2\,NH<em>3 + CO</em>2 + 3\,ATP + H<em>2O \rightarrow (NH</em>2)<em>2CO + 2\,ADP + AMP + 4\,P</em>i + 2\,H^+$

Key Regulatory Molecule & Enzyme Outside the Cycle

• N-acetylglutamate (NAG)
  • Essential allosteric activator of CPS I.
  • Synthesised by N-acetylglutamate synthase (NAGS) from glutamate + acetyl-CoA.

Clinical Correlation: Hyperammonemia

• Enzymes whose deficiency → elevated plasma NH₃ (“Urea-cycle disorders”)
  • NAGS (activator absent ⇒ CPS I inactive)
  • CPS I
  • OTC
  • ASS1
  • ASL
  • ARG1
• Presentation pearls (likely board vignette):
  • Neonate/child with vomiting, lethargy, seizures, or encephalopathy after high-protein feed.
  • Labs: ↑ NH₃, respiratory alkalosis, low BUN.
• Ancillary detail highlighted by lecturer: “not directly in the urea cycle itself” refers to NAG synthase-deficiency causing hyperammonemia.

Adaptive / Feedback Considerations

• Too much AA → transamination & catabolism accelerate.
• Deficiency of a given AA → reverse transamination replenishes, drawing from keto-acid pool.
• Trade-off: “stealing from Peter to pay Paul” → heavy reliance on protein turnover (major AA source) can deplete structural proteins if dietary supply inadequate.

Integration With Previous Topics

• Carbohydrate & lipid metabolism supply the carbon skeletons for non-essential AA synthesis (e.g. pyruvate → alanine; oxaloacetate → aspartate).
• During prolonged fasting, alanine cycle transfers nitrogen from muscle to liver while providing gluconeogenic substrate.

Example Question Framework (implicit from lecturer’s comments)

1. Identify enzyme deficiency by recognising accumulated intermediates + hyperammonemia.
2. Use transaminase logic to infer which AA ↔ which keto acid (e.g. ALT links alanine & pyruvate).

Practical / Ethical Implications

• Newborn screening for OTC & other urea-cycle defects prevents irreversible neurologic damage.
• Dietary management: controlled protein intake + essential AA mixtures; pharmacologic NH₃-scavengers (sodium benzoate, phenylbutyrate).

Key Numerical / Formulaic Points

• Urea cycle ATP cost: $3\text{ ATP}$ per urea.
• Normal plasma NH₃: $< 50\,\mu mol\,L^{-1}$ (adult); neurotoxic > $100\,\mu mol\,L^{-1}$.

Take-Home Messages

• Amino acids are functionally, not storage, molecules; surplus is dangerous.
• Transamination → oxidative deamination → urea cycle is the universal disposal workflow.
• Understanding pathway logic enables diagnosis & therapeutic decision-making without rote memorisation of every enzyme.