Mechanism of allosteric inhibition of N-acetyl-L-glutamate synthase by L-arginine

Abstract

N-Acetylglutamate synthase (NAGS) catalyzes the first committed step in L-arginine biosynthesis in plants and micro-organisms and is subject to feedback inhibition by L-arginine. This study compares the crystal structures of NAGS from Neisseria gonorrhoeae (ngNAGS) in the inactive T-state with L-arginine bound and in the active R-state complexed with CoA and L-glutamate. Under all of the conditions examined, the enzyme consists of two stacked trimers. Each monomer has two domains: an amino acid kinase (AAK) domain with an AAK-like fold but lacking kinase activity and an N-acetyltransferase (NAT) domain homologous to other GCN5-related transferases. Binding of L-arginine to the AAK domain induces a global conformational change that increases the diameter of the hexamer by ∼10 Å and decreases its height by ∼20 Å. AAK dimers move 5 Å outward along their 2-fold axes, and their tilt relative to the plane of the hexamer decreases by ∼4°. The NAT domains rotate ∼109° relative to AAK domains enabling new interdomain interactions. Interactions between AAK and NAT domains on different subunits also change. Local motions of several loops at the L-arginine-binding site enable the protein to close around the bound ligand, whereas several loops at the NAT active site become disordered, markedly reducing enzymatic specific activity. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.