<p>This group represents the eukaryotic type of glutamate synthase (NADH-GOGAT, <db_xref db="EC" dbkey="1.4.1.14"/>). This pyridine-linked form is found both photosynthetic and nonphotosynthetic eukaryotes. It displays a single-subunit structure corresponding to the fusion of the small and the large bacterial subunits.</p><p>Glutamate synthase (GOGAT, GltS) is a complex iron-sulphur flavoprotein that catalyses the reductive synthesis of L-glutamate from 2-oxoglutarate and L-glutamine via intramolecular channelling of ammonia, a reaction in the bacterial, yeast and plant pathways for ammonia assimilation [<cite idref="PUB00013982"/>]. GOGAT is a multifunctional enzyme that functions through three distinct active centres carrying out multiple reaction steps: L-glutamine hydrolysis, conversion of 2-oxoglutarate into L-glutamate, and electron uptake from an electron donor [<cite idref="PUB00008698"/>]. The small subunit functions as a FAD-dependent NADPH oxidoreductase, which serves to transfer reducing equivalents to the site of glutamate synthesis on the large subunit through the enzyme [3Fe-4S] cluster (on the large subunit) and at least one of its [4Fe-4S] centres [<cite idref="PUB00009386"/>, <cite idref="PUB00015747"/>]. The large subunit contains the GltS L-glutamine amidotransferase (GAT) site where L-Gln binds and is hydrolysed to yield L-Glu and ammonia. The latter is transferred through the intramolecular ammonia tunnel [<cite idref="PUB00013982"/>] to the glutamate synthase site where 2-OG binds, is converted to the iminoglutarate (2-IG) intermediate, and reduced to L-Glu by receiving reducing equivalents from the reduced FMN cofactor at this site [<cite idref="PUB00015837"/>].</p><p>There are four classes of GOGAT [<cite idref="PUB00009386"/>, <cite idref="PUB00015710"/>]: </p> <p>1. Bacterial NADPH-dependent GOGAT (NADPH-GOGAT, <db_xref db="EC" dbkey="1.4.1.13"/>). This standard bacterial NADPH-GOGAT is composed of a large (alpha, GltB) subunit (<db_xref db="PIRSF" dbkey="PIRSF000186"/>, subfamily <db_xref db="PIRSF" dbkey="PIRSF500059"/>, which in turn contains three domains) and a small (beta, GltD) subunit (<db_xref db="PIRSF" dbkey="PIRSF005836"/>).</p> <p>2. Ferredoxin-dependent form in cyanobacteria and plants (Fd-GOGAT from photosynthetic cells, <db_xref db="EC" dbkey="1.4.7.1"/>) displays a single-subunit structure corresponding to the large bacterial subunit (<db_xref db="PIRSF" dbkey="PIRSF000186"/>, subfamily <db_xref db="INTERPRO" dbkey="IPR014666"/>)</p> <p>3. Pyridine-linked form in both photosynthetic and nonphotosynthetic eukaryotes (eukaryotic GOGAT or NADH-GOGAT, <db_xref db="EC" dbkey="1.4.1.14"/>) displays a single-subunit structure corresponding to the fusion of the small and the large bacterial subunits.</p> <p>4. The archaeal type with stand-alone proteins corresponding to the N-terminal, FMN-binding, and the C-terminal domains of the large subunit [<cite idref="PUB00015710"/>, <cite idref="PUB00009386"/>] (<db_xref db="INTERPRO" dbkey="IPR012375"/>, <db_xref db="INTERPRO" dbkey="IPR012061"/>), and to the small subunit.</p> <p>The large subunit of GOGAT consists of three domains: N-terminal domain (amidotransferase domain <db_xref db="INTERPRO" dbkey="IPR000583"/>); central (consisting of <db_xref db="INTERPRO" dbkey="IPR006982"/> and the FMN-binding domain <db_xref db="INTERPRO" dbkey="IPR002932"/>), and the C-terminal domain (<db_xref db="INTERPRO" dbkey="IPR002489"/>). </p> <p>The N-terminal amidotransferase domain is characterised by a four layer alpha/beta/beta/alpha architecture and is similar to other Ntn-amidotransferases [<cite idref="PUB00008698"/>]. It contains the typical catalytic centre of Ntn-amidotransferases, and the N-terminal Cys-1 catalyses the hydrolysis of L-glutamine generating ammonia and the first molecule of L-glutamate [<cite idref="PUB00008698"/>]. </p> <p>The second (central) domain consists of <db_xref db="INTERPRO" dbkey="IPR006982"/> and <db_xref db="INTERPRO" dbkey="IPR002932"/>. <db_xref db="INTERPRO" dbkey="IPR006982"/> connects the amidotransferase domain with the FMN-binding domain and has an alpha/beta overall topology [<cite idref="PUB00008698"/>]. The FMN-binding domain (<db_xref db="INTERPRO" dbkey="IPR002932"/>) has a classic beta/alpha barrel topology. In this domain, the 2-iminoglutarate intermediate, formed upon the addition of ammonia onto 2-oxoglutarate, is reduced by the FMN cofactor producing the second molecule of L-glutamate [<cite idref="PUB00008698"/>]. This domain also contains the enzyme 3Fe-4S cluster [<cite idref="PUB00008698"/>]. </p> <p>The C-terminal, or GXGXG structural domain, has a right-handed beta-helix topology composing seven beta-helical turns. This domain does not have a direct function in glutamate synthase activity but rather a structural function through extensive interactions with the amidotransferase and FMN-binding domains [<cite idref="PUB00013982"/>, <cite idref="PUB00008698"/>].</p> <p>The structural data combined with the catalytic properties of GltS indicate that binding of ferredoxin and 2-oxoglutarate to the FMN-binding domain of GltS induce a conformational change in the loop connecting the two catalytic centres. The rearrangement induces a shift in the catalytic elements of the amidotransferase domain, such that it becomes activated [<cite idref="PUB00008698"/>].</p> <p>For additional information please see [<cite idref="PUB00015727"/>].</p> Glutamate synthase, eukaryotic