Question related to L-arginine biosynthesis
The synthesis of N-acetylglutamate is mediated by the enzyme N-acetylglutamate synthase. This enzyme has L-glutamate as its substrate and uses acetyl-coenzyme A as a co-enzyme acetyl donor. Acetylcoenzyme A (Acetyl-CoA) is generally abbreviated in structural formulas, because it is a relatively complex molecule. The only thing of relevance is the acetyl group which is transferred from a thiol group to L-glutamate. 

Note that ligands is a term reserved for molecules binding to receptors; substrates is the proper term for intermediates in enzymatic reactions.    


N-acetylglutamate synthesis. Source: The Medical Biochemistry page