A 6-month-old male infant presents with lethargy, poor feeding, and vomiting. Serum ammonia is markedly elevated at 180 µmol/L (normal <50). Plasma amino acid analysis shows elevated glutamine and alanine. Genetic testing reveals a homozygous loss-of-function mutation in NAG synthase. The structure marked **A** in the urea cycle diagram is the rate-limiting enzyme of ammonia detoxification. Which of the following best explains why NAG synthase deficiency presents clinically identical to CPS-1 deficiency despite normal CPS-1 protein expression?

Explanation

## Why "N-acetylglutamate is the obligate allosteric activator of CPS-1..." is right N-acetylglutamate (NAG) is the OBLIGATE allosteric activator of carbamoyl phosphate synthetase I (CPS-1), the rate-limiting enzyme of the urea cycle. Without NAG binding to CPS-1, the enzyme is inactive and cannot catalyze the condensation of ammonia and carbamoyl phosphate, even if the CPS-1 protein itself is structurally normal. NAG synthase deficiency therefore results in NAG depletion, rendering CPS-1 functionally inactive and producing hyperammonemia clinically indistinguishable from primary CPS-1 deficiency. This is a classic example of allosteric regulation controlling enzyme function independent of protein expression. (Harper 32e, Ch 28) ## Why each distractor is wrong - **"NAG synthase catalyzes the first committed step..."**: NAG synthase does not directly participate in the urea cycle; it is a separate enzyme that synthesizes the allosteric activator. Its deficiency does not prevent substrate availability but rather removes the activating signal for CPS-1. - **"CPS-1 requires NAG as a cofactor..."**: NAG is an allosteric activator, not a cofactor. Cofactors are stoichiometric substrates (e.g., ATP, Mg²⁺); allosteric activators bind to regulatory sites and modulate enzyme activity without being consumed. - **"NAG is a competitive inhibitor of arginase..."**: NAG is not an inhibitor of arginase; it is the activator of CPS-1. Arginase catalyzes the final step of the urea cycle (arginine → urea + ornithine) and is not directly regulated by NAG. **High-Yield:** CPS-1 deficiency and NAG synthase deficiency are clinically indistinguishable because NAG is the obligate allosteric activator of CPS-1—without it, the rate-limiting enzyme is "locked off" despite normal protein expression. [cite: Harper 32e, Ch 28]