| Type: | Family | Name: | Glutamate synthase, large subunit domain 3 stand-alone protein |
| Description: | The large (alpha, GltB, ) subunit of bacterial glutamate synthase (GOGAT) consists of three domains: N-terminal domain (amidotransferase domain) or related (in archaeal GOGAT), central domain and the FMN-binding domain, and C-terminal domain. This family represents a stand-alone form of the C-terminal domain. The stand-alone form occurs in the archaeal type of GOGAT, where the large subunit is represented by three separate proteins, corresponding to the three domains of the "standard" bacterial enzyme []. Similar organisation of GOGAT with stand-alone domains has been found in some bacteria (e.g., members from Sinorhizobium meliloti, Thermotoga maritima), but its function is not clear in those organisms where the "standard" bacterial form is also present (e.g., Sinorhizobium meliloti). This domain is also present in a stand-alone form in subunit C of the formylmethanofuran dehydrogenase (, closely related to this group).This domain is also called the GXGXG structural domain, containing repeated sequence motif G-XX-G-XXX-G). It has a right-handed beta-helix topology composing seven beta-helical turns. It 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 [, ].Originally, only the ORF encoding the central domain of GOGAT has been recognised and annotated as GltB in archaea, and the rest of the large subunit was thought to be missing, which may lead to some miss-annotations []. This has led to speculations that the archaeal form of the GOGAT large subunit is the ancestral minimum form of the enzyme. Later analysis showed, however, that in all archaea where the large subunit has been found, its entire sequence is represented by three separate ORFs [].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 []. GOGAT is a multifunctional enzyme that performs L-glutamine hydrolysis, conversion of 2-oxoglutarate into L-glutamate, and electron uptake from an electron donor [].There are four classes of GOGAT [, ]: 1. Bacterial NADPH-dependent GOGAT (NADPH-GOGAT, ). This standard bacterial NADPH-GOGAT is composed of a large (alpha, GltB) subunit (, subfamily , which in turn contains three domains) and a small (beta, GltD) subunit ().2. Ferredoxin-dependent form in cyanobacteria and plants (Fd-GOGAT, ) displays a single-subunit structure corresponding to the large bacterial subunit (, subfamily ).3. Pyridine-linked form in both photosynthetic and nonphotosynthetic eukaryotes (eukaryotic GOGAT or NADH-GOGAT, ) displays a single-subunit structure corresponding to the fusion of the small and the large bacterial subunits ().4. The archaeal type with stand-alone proteins corresponding to the N-terminal, FMN-binding, and the C-terminal domains of the large subunit [, ] (, , and this family), and to the small subunit.For more information on GOGAT and its domains, please see .For additional information please see [, ]. | Short Name: | Glu_synth_lsu_3 |
