Glutamine Synthetase: Diverse Regulation and Functions of an Ancient Enzyme

Abstract

Glutamine synthetase (GS) is a ubiquitous enzyme central to nitrogen metabolism, catalyzing the ATP-dependent formation of glutamine from glutamate and ammonia. Positioned at the intersection of nitrogen metabolism with carbon metabolism, the activity of GS is subject to sophisticated regulation. While the intricate regulatory pathways that govern Escherichia coli GS were established long ago, recent work has demonstrated that homologues are controlled by multiple distinct regulatory patterns, such as the metabolite induced oligomeric state formation in archaeal GS by 2-oxoglutarate. Such work was enabled in large part by advances in cryo-electron microscopy (cryoEM) that allowed greater structural access to this large enzyme complex, such as assessment of the large heterogeneous oligomeric states of GS and protein-interactor-GS complexes. This perspective highlights recent advances in understanding GS regulation, focusing on the dynamic interplay between its oligomeric state, metabolite binding, and protein interactors. These interactions modulate GS activity, influencing cellular processes such as nitrogen assimilation, carbon metabolism, and stress responses. Furthermore, we explore the emerging concept of GS "moonlighting" functions, revealing its roles in palmitoylation, cell cycle regulation, and ion channel modulation. These diverse functions highlight a newfound versatility of GS beyond its primary catalytic role and suggest complex roles in health and disease that warrant further study.

Figure 1

Architecture of glutamine synthetases. Crystal structures of a representative Type I GS (Bacillus subtilis, PDB 4LNI; top), Type II GS (Homo sapiens, PDB 2QC8; middle), and Type III GS (Bacteroides fragilis, PDB 3O6X fit into EMD 1204; bottom) with individually colored subunits. The active site bifunnel is highlighted in the side view (left). At right is an overlay of a single active site bifunnel for each GS model shown at left. Substrate binding site is highlighted in green. ATP or ADP nucleotides are in stick representation with active site residues demonstrating conserved architecture. Bottom right: overlay of extant structures of GS (from PDB on October 31, 2024 that contained E.C. 6.3.1.2) demonstrates conformational flexibility about the E-flap and adenylation loop.

Figure 2

Complex regulation of glutamine synthetase and quaternary state remodeling. Outlined are the protein interactors, cofactors, and metabolites that promote different oligomeric states of GS including monomers, pentamers, hexamers, octamers, decamers, dodecamers, tetradecamers, and higher order filaments.

Figure 3

Moonlighting functions of human glutamine synthetase. Top left: PDB 2QC8 colored by subunit and fit into surface representation. Top right: chemical reaction of palmitoylation activity of human GS. Bottom left: GS binding BEST2 ion channel conferring presumed ion concentration and activity modulation. Bottom right: GS interaction with NUP88 can outcompete NUP88 binding to CDC20 thus allowing CDC20 binding to the anaphase promoting complex (APC/C) to promote progression through mitosis.