Biochemistry Proteins and Amino Acids cc97330c Key enzyme in urea synthesis is: A Urease B Carbamoyl Synthetase C Arginase D Ornithine High-Yield Explanation Ans. B. Carbamoyl SynthetaseCarbamoyl synthetase catalyses the first step in urea cycle or Krebs Henseleit cycle and it is a rate limiting step in urea synthesis.