Glutamate synthase: structural, mechanistic and regulatory properties, and role in the amino acid metabolism

Abstract

Ammonium ion assimilation constitutes a central metabolic pathway in many organisms, and glutamate synthase, in concert with glutamine synthetase (GS, EC 6.3.1.2), plays the primary role of ammonium ion incorporation into glutamine and glutamate. Glutamate synthase occurs in three forms that can be distinguished based on whether they use NADPH (NADPH-GOGAT, EC 1.4.1.13), NADH (NADH-GOGAT, EC 1.4.1.14) or reduced ferredoxin (Fd-GOGAT, EC 1.4.7.1) as the electron donor for the (two-electron) conversion of L-glutamine plus 2-oxoglutarate to L-glutamate. The distribution of these three forms of glutamate synthase in different tissues is quite specific to the organism in question. Gene structures have been determined for Fd-, NADH- and NADPH-dependent glutamate synthases from different organisms, as shown by searches in nucleic acid sequence data banks. Fd-glutamate synthase contains two electron-carrying prosthetic groups, the redox properties of which are discussed. A description of the ferredoxin binding by Fd-glutamate synthase is also presented. In plants, including nitrogen-fixing legumes, Fd-glutamate synthase and NADH-glutamate synthase supply glutamate during the nitrogen assimilation and translocation. The biological functions of Fd-glutamate synthase and NADH-glutamate synthase, which show a highly tissue-specific distribution pattern, are tightly related to the regulation by the light and metabolite sensing systems. Analysis of mutants and transgenic studies have provided insights into the primary individual functions of Fd-glutamate synthase and NADH-glutamate synthase. These studies also provided evidence that glutamate dehydrogenase (NADH-GDH, EC 1.4.1.2) does not represent a significant alternate route for glutamate formation in plants. Taken together, biochemical analysis and genetic and molecular data imply that Fd-glutamate synthase incorporates photorespiratory and non-photorespiratory ammonium and provides nitrogen for transport to maintain nitrogen status in plants. Fd-glutamate synthase also plays a role that is redundant, in several important aspects, to that played by NADH-glutamate synthase in ammonium assimilation and nitrogen transport.

Photosynthetic carbon and nitrogen metabolism and the relationship between their metabolites and lipid peroxidation in dwarf bamboo (Fargesia rufa Yi) during drought and subsequent recovery

“…Differently, GDH activity in dwarf bamboo was not affected by drought stress (Fig. 4c), as previously observed by Suzuki and Knaff (2005). Meanwhile, this also indicates that incorporation of NH 4…”

“…will seriously damage plant cells (Thomas and Hilker 2000), thus it needs to be assimilated immediately through the GS/GOGAT cycle and/or the GDH pathway (Suzuki and Knaff 2005;Robredo et al 2011). However, short-term drought depressed GS/ GOGAT activities in dwarf bamboo (Fig.…”

Molecular Evolution of Aminoacyl tRNA Synthetase Proteins in the Early History of Life

“…Amino acid biosynthesis pathways are all closely tied to central metabolism, and frequently their products are an intrinsic part of this metabolism. For example, glutamate and glutamine are essential components in the GS-GOGAT system for nitrogen assimilation (Suzuki and Knaff 2005), serine and glycine are essential in single-carbon metabolism (Newman & Magasanik, 1963), cysteine and methionine are central to sulfur uptake and metabolism (Wirtz and Droux 2005); many amino acids are also involved in small molecule/cofactor synthesis (Fischer et al 2010;White, 2001) and even RNA modification (Grosjean and Benne 1998;Ikeuchi et al 2010;Suzuki and Miyauchi 2010). Therefore, we can reasonably separate the question of the functional chemical origin of many (if not most) amino acids from their incorporation in proteins via the semantics of a genetic code.…”

Inorganic nitrogen fertilizers induce changes in ammonium assimilation and gas exchange in Camellia sinensis L.

“…The utilization of N in leaves also helped confirm certain patterns that are closely related to gas exchange, where understanding of the GS/GOGAT cycle is critical for successful application of inorganic N. This cycle provides organic N for transport so as to maintain N levels in plants (Suzuki and Knaff, 2005). We found that GS activity significantly correlated with Pn and Ci, with measured correlation coefficients of 0.40 (P < 0.01) and -0.39 (P < 0.01), respectively.…”