Question

The first amino group entering into the urea cycle is

• A. Carbomyl phosphate
• B. Ornithine
• C. Citrulline
• D. Argininosuccinate

Hint:

Carbomyl phosphate is synthesized in the mitochondria and plays a critical role in the entry of nitrogen into the urea cycle.

Answer 1

The Correct Option is A

The urea cycle takes in nitrogen at two separate points. The first comes from free ammonia, which is fixed together with bicarbonate into carbamoyl phosphate in the very first step of the cycle.

Ornithine only receives this already-formed carbamoyl group afterward to make citrulline, citrulline forms only after that transfer, and argininosuccinate forms later still when the cycle's second nitrogen enters -- so none of these three mark the first point of nitrogen entry.

The first amino group entering the urea cycle is carried by carbomyl phosphate.

Answer 2

Mapping out the cycle step by step in order clarifies which intermediate corresponds to "first":

1. Carbomyl phosphate: Formed in step one of the cycle, directly from ammonia and bicarbonate -- the earliest point at which a nitrogen atom is committed to the pathway.
2. Ornithine: Present at the start as the carrier molecule but does not itself introduce a new nitrogen atom into the cycle; it receives the carbamoyl group in step two.
3. Citrulline: The product of step two, so any nitrogen present in citrulline was already incorporated one step earlier.
4. Argininosuccinate: Formed in a later step when aspartate donates the second nitrogen atom of the cycle, making this the entry point of the second amino group, not the first.

Ordering the steps confirms that nitrogen first enters the cycle at the carbamoyl phosphate formation step.

So, the correct answer is carbomyl phosphate.