A neonate presents with lethargy, vomiting, and seizures. Labs show elevated blood ammonia, low citrulline, and elevated urinary orotic acid. Which enzyme deficiency is most consistent with this presentation?
ACarbamoyl phosphate synthetase I (CPS I)
BOrnithine transcarbamoylase (OTC)
CArgininosuccinate synthetase
DArginase
OTC deficiency (the most common inherited urea cycle defect, X-linked) causes carbamoyl phosphate to accumulate in the mitochondria. This excess carbamoyl phosphate spills into the cytoplasm and enters the pyrimidine synthesis pathway, producing elevated urinary orotic acid — the diagnostic signature. Because OTC converts ornithine + carbamoyl phosphate into citrulline, citrulline levels are low. CPS I deficiency (option A) would also cause low citrulline but would NOT produce elevated orotic acid, because without CPS I there is no carbamoyl phosphate to overflow. Argininosuccinate synthetase deficiency (option C) causes elevated citrulline. Arginase deficiency (option D) causes elevated arginine.
Question 2 Multiple Choice
Why is N-acetylglutamate (NAG) an essential allosteric activator of CPS I? What is the physiological logic of this regulation?
ANAG provides the nitrogen atom that CPS I incorporates into carbamoyl phosphate
BHigh amino acid levels stimulate NAG synthesis, signaling that nitrogen disposal is urgently needed
CNAG prevents feedback inhibition of CPS I by urea, keeping the cycle running continuously
DNAG protects CPS I from proteolytic degradation in the mitochondrial matrix
NAG is synthesized from acetyl-CoA and glutamate by NAG synthase. Glutamate is the major nitrogen carrier from amino acid degradation via transamination. When amino acid catabolism is high, glutamate levels rise, NAG synthesis increases, CPS I is activated, and the urea cycle runs faster — precisely when nitrogen disposal is most needed. This is elegant feedforward regulation: the signal for increased nitrogen load directly activates the rate-limiting disposal step. NAG does not provide a nitrogen atom to CPS I (option A); urea does not inhibit CPS I (option C); and NAG is not a protease inhibitor (option D).
Question 3 True / False
Both nitrogen atoms incorporated into urea originate from free ammonia (NH₄⁺) produced by amino acid degradation.
TTrue
FFalse
Answer: False
One nitrogen atom enters urea from free ammonia via carbamoyl phosphate synthetase I (the first reaction). The second nitrogen atom enters from aspartate via argininosuccinate synthetase (the third reaction). Aspartate is produced by transamination of oxaloacetate with glutamate. This dual-source design connects the urea cycle to both mitochondrial ammonia production and cytoplasmic amino acid metabolism, making it a hub that integrates nitrogen disposal from multiple pathways simultaneously.
Question 4 True / False
Fumarate released by argininosuccinate lyase in the urea cycle can re-enter the citric acid cycle, creating a metabolic connection sometimes called the 'bicycle.'
TTrue
FFalse
Answer: True
Argininosuccinate lyase cleaves argininosuccinate into arginine and fumarate. Fumarate is a citric acid cycle intermediate — it can be hydrated to malate, then oxidized to oxaloacetate, which can be transaminated back to aspartate. Aspartate then re-enters the urea cycle as the second nitrogen donor at argininosuccinate synthetase. This aspartate-argininosuccinate shunt connects the two cycles at fumarate, justifying the 'bicycle' metaphor and illustrating why urea cycle activity is coupled to citric acid cycle flux.
Question 5 Short Answer
Why does the urea cycle span two cellular compartments, and what functional constraint does this impose?
Think about your answer, then reveal below.
Model answer: The first two reactions occur in the mitochondrial matrix — where free ammonia is generated and where CPS I combines it with CO₂ to form carbamoyl phosphate, then OTC transfers the carbamoyl group to ornithine to produce citrulline. The remaining three reactions occur in the cytoplasm. Citrulline must be exported from mitochondria and ornithine must be imported back in, requiring specific mitochondrial carrier proteins. The functional constraint is that the cycle depends on intact intracellular transport: if these carriers are defective, the cycle fails even when all five enzymes are normal.
The compartmental design reflects metabolic logic: ammonia is generated in the mitochondria, so it makes sense to begin neutralizing it there. The cytoplasm is where the subsequent condensation and cleavage reactions occur, connecting the cycle to cytoplasmic aspartate metabolism. But this split creates a dependency on transport systems that is clinically significant — transporter defects can cause hyperammonemia indistinguishable from enzymatic defects without detailed metabolite profiling.