Oxoglutarate, glutamate synthase

Glutamate synthase (NADPH) [EC 1.4.1.13], an iron-sulfur flavoprotein, catalyzes the reaction of L-glutamine with a-ketoglutarate (or, 2-oxoglutarate) and NADPH to produce NADP+ and two glutamate molecules. Ammonia can act as the nitrogen donor substrate instead of L-glutamine, albeit weaker. 

Glutamate synthase (GltS) catalyzes the conversion of 2-oxoglutarate into L-glutamate with L-glutamine serving as the nitrogen source for the reaction (Equation (26)). 

The reductant can be NADH, NADPH (in yeast and bacteria), or reduced ferredoxin (in plants). The enzyme that catalyzes this reaction is glutamate synthase it is also known as glutamate oxoglutarate aminotransferase (GOGAT). A GS/GOGAT complex exists in plants and allows them to cope with conditions of limited nitrogen availability. Enzymes that catalyze transamination reactions require pyridoxal phosphate as a coenzyme (Figure 23.8). We discussed this compound in Section 7.8 as a typical example of a coenzyme, and here we can see its mode of action in context. 

Figure 11.4 Metabolic reactions related to glutamic acid biosynthesis in C. glutamicum. GS, glutamine synthetase GOGAT, glutamate synthase (glutamate-oxoglutarate aminotransferase) and GDH, glutamate dehydrogenase.

Mutant lines of A. thaliana (Somerville and Ogren, 1983) and barley (Walls-grove et al, 1986) have been isolated and shown to have properties very similar to that of a mutant lacking glutamate synthase. By a detailed analysis of the chloroplast envelope, the specific protein missing in the 2-oxoglutarate uptake mutant of A. thaliana has been identified (Somerville and Somerville, 1985). 

The 2-oxoglutarate produced is recycled for transamination or may enter the TCA cycle. The ammonia liberated by oxidative deamination is used to form glutamine (from glutamate, catalysed by glutamine synthase) prior to export from the muscle cell ... 

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