Question

Which one is component of ornithine cycle?

• A. Ornithine, citrulline and alanine
• B. Ornithine, citrulline and arginine
• C. Amino acid are not used
• D. Ornithine, citrulline and fumaric acid

Answer 1

Answer: (B)

Ornithine, citrulline and arginine

Ornithine cycle converts harmful ammonia into urea though a series of four reactions with amino acid intermediates. This makes option C incorrect. The cycle starts with formation of citrulline from ornithine and carbamoyl phosphate, the entry of the first amino group, followed by transfer of citrulline into the cytosol. Second reaction includes formation of argininosuccinate through a citrullyl-AMP intermediate i.e., the entry of the second amino group. It is followed by formation of arginine from argininosuccinate and release of fumarate, which then enters the kerb cycle. Last step is hydrolysis of arginine into ornithine and urea.