Structural studies on the synchronization of catalytic centers in glutamate synthase

The complex iron-sulfur flavoprotein glutamate synthase (GltS) plays a prominent role in ammonia assimilation in bacteria, yeasts, and plants. GltS catalyzes the formation of two molecules of (L)-glutamate from 2-oxoglutarate and L- glutamine via intramolecular channeling of ammonia. GltS has the impressive ability of synchronizing its distinct catalytic centers to avoid wasteful consumption of (L)-glutamine. We have determined the crystal structure of the ferredoxin-dependent GUS in several ligation and redox states. The structures reveal the crucial elements in the synchronization between the glutaminase site and the 2-iminoglutarate reduction site. The structural data combined with the catalytic properties of GUS indicate that binding of ferredoxin and 2-oxoglutarate to the FMN-binding domain of GUS induce a conformational change in the loop connecting the two catalytic centers. The rearrangement induces a shift in the catalytic elements of the amidotransferase domain, such that it becomes activated. This machinery, over a distance of more than 30 Angstrom. controls the ability of the enzyme to bind and hydrolyze the ammonia-donating substrate L-glutamine.