<p>Glutamate synthase (GltS)1 is a key enzyme in the early stages of the assimilation of ammonia in bacteria, yeasts, and plants. In bacteria, L-glutamate is involved in osmoregulation, is the precursor for other amino acids, and can be the precursor for haem biosynthesis. In plants, GltS is especially essential in the reassimilation of ammonia released by photorespiration. On the basis of the amino acid sequence and the nature of the electron donor, three different classes of GltS can de defined as follows: 1) ferredoxin-dependent GltS (Fd-GltS), 2) NADPH-dependent GltS (NADPH-GltS), and 3) NADH-dependent GltS (properties of the three classes have been reviewed extensively [<cite idref="PUB00009386"/>]). The enzyme is a complex iron-sulphur flavoprotein catalysing the reductive transfer of the amido nitrogen from L-glutamine to 2-oxoglutarate to form two molecules of L-glutamate via intramolecular channelling of ammonia from the amidotransferase domain to the FMN-binding domain.</p><p>Reaction of amidotransferase domain:</p><reaction> L-glutamine + H2O = L-glutamate + NH3</reaction><p>Reactions of FMN-binding domain:</p><reaction> 2-oxoglutarate + NH3 = 2-iminoglutarate + H2O</reaction> <reaction>2e + FMNox = FMNred </reaction> <reaction>2-iminoglutarate + FMNred = L-glutamate + FMNox </reaction>The central domain of glutamate synthase connects the N-terminal amidotransferase domain with the FMN-binding domain and has an alpha/beta overall topology [<cite idref="PUB00008698"/>].