<prompt> In urea synthesis, the amino acid functioning solely as an enzyme activator:N-acetyl glutamateOrnithineCitrullineArginine  <response>: The major regulatory step is catalyzed by CPS-I where the positive effector is N-acetyl glutamate NAG. It is formed from glutamate and acetyl CoA. Arginine is an activator of NAG synthase.Reference: Vasudevan 7th ed, pg 205 