In urea synthesis, the amino acid functioning solely as an enzyme activator:
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
N-acetyl glutamate
Ornithine
Citrulline
Arginine