The urea cycle participants arginine and citrulline are both nonstandard amino acids.
a.All three statements are true.
b.Two of the three statements are true.
c.Only one of the statements is true.
d.None of the statements is true.
Correct Answer: (B) Two of the three statements are true.
Explanation:
The urea cycle, also known as the ornithine cycle, plays a crucial role in removing excess nitrogen from the body by converting ammonia into urea, which is then excreted in urine. This cycle involves several intermediates, including arginine and citrulline.
- “The urea cycle participants arginine and citrulline” – True
- Arginine and citrulline are indeed intermediates in the urea cycle. Arginine is a key amino acid that, when cleaved by the enzyme arginase, produces urea and ornithine. Citrulline is another key intermediate formed when ornithine combines with carbamoyl phosphate.
- “Arginine is a nonstandard amino acid” – False
- Arginine is a standard (proteinogenic) amino acid, meaning it is one of the 20 amino acids commonly found in proteins.
- “Citrulline is a nonstandard amino acid” – True
- Citrulline is nonstandard because it is not directly used for protein synthesis. Instead, it plays a metabolic role in the urea cycle and nitric oxide production.
Since two of the three statements are true (statements 1 and 3), the correct answer is option (B).
