Enzyme generating urea is

UREA CYCLE: STEP 1 :One molecule of ammonia condenses with CO2 in presence of two molecules of ATP to form carbamoyl phosphate. STEP 2 :Formation of Citrulline: The carbamoyl group is transferred to the NH2 group of ornithine by ornithine transcarbamoylase. STEP 3:Formation of ARGININOSUCCINATE One molecule of aspaic acid adds to citrulline forming a carbon to nitrogen bond, which provides the 2nd nitrogen atom of urea.Argininosuccinate synthetase catalyzes the reaction.In this two ATP' s are utilized. STEP 4 : Formation of Arginine Argininosuccinate is cleaved by argininosuccinate lyase to arginine and fumarate. STEP 5: Formation of Urea. The finsl reaction of the cycle is the hydrolysis of argine to urea and ornithine by arginase REF:DM VASUDEVAN TEXTBOOK :EIGHTH EDITION, Page no:258 & 259. more details