Amino Acid Catabolism and Nitrogen Disposal Summary

Sources and Transport of Ammonia

• Major Sources: Amino acid catabolism is the primary source; minor sources include gastrointestinal tract (GIT) bacterial activity and the skeletal muscle (SKM) purine nucleotide cycle.

• Storage and Transport:

  • Glutamate: The primary storage form of ammonia in all tissues.

  • Glutamine: The major transport form of ammonia from most extrahepatic tissues (brain, kidney, intestine) to the liver.

  • Alanine: The major transport form of ammonia from skeletal muscle to the liver via the Alanine-Glucose cycle (Cahill cycle).

• Amino Acid Loss: Defects in amino acid transporters (e.g., Hartnup disease and Cystinuria) lead to loss in feces and urine.

Transamination and Enzyme Markers

• Mechanism: Transfer of an $\alpha-NH_2$ group to $\alpha-Ketoglutarate$ ($AKG$) to form Glutamate. This requires the coenzyme Pyridoxal phosphate ($PLP$).

• Exclusions: $Lys$, $Thr$, and $Pro$ do not undergo transamination.

• Clinical Biomarkers:

  • Aspartate transaminase ($AST$): Marker for liver and other tissue injuries (Cirrhosis, Hepatitis).

  • Alanine transaminase ($ALT$): Primarily liver-specific marker for liver injury.

Oxidative Deamination and Tissue Specificity

• Glutamate Dehydrogenase ($GDH$): A reversible liver mitochondrial enzyme that liberates free $NH_4^+$ for the urea cycle.

  • Uses $NAD^+$ for catabolic energy needs and $NADP^+$ for anabolic synthesis.

• Extrahepatic Roles of GDH:

  • Kidneys: Liberates $NH_4^+$ to regulate pH (increases during acidosis).

  • Brain: Regulates ammonia and glutamate (neurotransmitter) concentrations to avoid toxicity.

The Urea Cycle (Krebs-Henseleit Cycle)

• Overview: A five-reaction cycle occurring exclusively in the liver to detoxify toxic ammonia into water-soluble, non-toxic urea.

• Subcellular Location: Two reactions occur in the mitochondria; three occur in the cytoplasm.

• Rate-Limiting Step: Carbamoyl phosphate synthetase-I ($CPS-I$).

• Energetics: Consumes $4\,ATP$ and generates one $NADH$ ($3\,ATP$) via the Krebs cycle, resulting in a net consumption of $1\,ATP$.

• Transporters: Utilizes the Ornithine transporter ($OT$) and Citrulline transporter ($CT$).

• Urea Composition: Carbonyl group from $CO_2$ and $H_2O$; one amino group from $NH_3$; second amino group from Aspartate.

Regulation and Metabolic Connections

• Allosteric Regulation: $CPS-I$ is activated by N-acetylglutamate ($NAG$), which is synthesized by NAG synthase ($NAGS$). $NAGS$ is regulated by Arginine.

• Aspartate-Argininosuccinate Shunt: Reciprocal interaction where the Krebs cycle provides Aspartate to the urea cycle, and the urea cycle returns Fumarate to the Krebs cycle.

Urea Cycle Enzyme Deficiencies

• Hyperammonemia-I: deficiency in $CPS-I$ (Autosomal recessive).

• Hyperammonemia-II: deficiency in Ornithine transcarbamoylase ($OTC$) (X-linked recessive). Note: Treated via gene therapy; the case of Jesse Gelsinger (1999) involved a fatal immune response in a trial.

• Hypercitrullinemia: deficiency in AS synthetase (Autosomal recessive).

• Hyperargininosuccinate aciduria: deficiency in AS lyase (Autosomal recessive).

• Hyperarginemia: deficiency in Arginase (Autosomal recessive).

Questions & Discussion

• What is the major source of ammonia in human cells? Amino acid catabolism.

• What is a major storage form of ammonia? Glutamate.

• What are the major transport forms of ammonia? Glutamine and Alanine.

• Where is glutamate dehydrogenase present and what are its coenzymes? Present in liver mitochondria; uses $NAD^+$ or $NADP^+$ as coenzymes.

• How are TCA and urea cycles linked via argininosuccinate shunt pathway? TCA supplies Aspartate to the urea cycle; urea cycle returns Fumarate to TCA.

• What cycle produces ammonia in skeletal muscle? Purine nucleotide cycle.

• What is alanine-glucose cycle and its significance? Transport of ammonia from SKM to liver; allows liver to produce glucose for muscles.

• Why is gluconeogenesis important during intense exercise? To provide glucose to muscles when glycogen is depleted.

• Which enzyme catalyses the rate limiting step of urea cycle? Carbamoyl phosphate synthetase-I ($CPS-I$).

• Which amino acid does TCA supply to urea cycle? Aspartate.

• What is the consequence of CPS-1 and OTC deficiencies? Hyperammonemia-I and Hyperammonemia-II.

• Which urea cycle enzyme deficiency is an X-linked recessive condition? OTC deficiency.