Multiple choice Which among the following plays a crucial role in the third step of the urea cycle? TMC1 END FXN GALT ASS Reveal answer Fill a bubble to check yourself E Correct answer Explanation Argininosuccinate synthase (ASS) is an enzyme that catalyzes the synthesis of argininosuccinate from citrulline and aspartate. ASS is responsible for the third step of the urea cycle and one of the reactions of the citrulline-NO cycle.