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

Step 1: Understanding the urea cycle.
The urea cycle is a metabolic pathway that eliminates excess nitrogen by converting ammonia into urea. The first amino group entering the cycle is derived from ammonia and is incorporated into carbomyl phosphate.
Step 2: Analyzing the options.
(1) Carbomyl phosphate: Correct — Carbomyl phosphate is the first intermediate in the urea cycle, where ammonia combines with CO2 to form this compound.
(2) Ornithine: Ornithine is an important intermediate in the cycle but does not carry the first amino group.
(3) Citrulline: Citrulline is formed later in the cycle but is not the first amino group carrier.
(4) Argininosuccinate: Argininosuccinate is formed after citrulline and is not the first intermediate.
Step 3: Conclusion.
The correct answer is (1) Carbomyl phosphate, as it carries the first amino group entering the urea cycle.

Answer 2

The urea cycle disposes of two nitrogen atoms per turn, and they enter at different points, so tracking the order of entry identifies the answer.

1. Carbomyl phosphate: Free ammonia combines with bicarbonate and is activated by carbamoyl phosphate synthetase I to form carbamoyl phosphate -- this is the very first step of the cycle, meaning the first nitrogen atom enters the cycle carried on this molecule.
2. Ornithine: Ornithine is the carrier molecule that accepts the carbamoyl group from carbamoyl phosphate to form citrulline; it is the acceptor onto which the already-formed carbamoyl group is transferred, not the first carrier of the incoming amino group.
3. Citrulline: Citrulline is formed only after carbamoyl phosphate reacts with ornithine, so any nitrogen it carries has already entered the cycle earlier -- it does not mark the point of first entry.
4. Argininosuccinate: Argininosuccinate forms later still, when citrulline reacts with aspartate to bring in the cycle's second nitrogen atom -- this is the second amino group entering, not the first.

Since ammonia is fixed into carbamoyl phosphate before any other step of the cycle occurs, this is where the first amino group enters.

Therefore, the correct answer is carbomyl phosphate.