Role and regulation of glutamate synthases in higher plants

Nitrogen in higher plants is assimilated to amino acids primarily through the synthesis of glutamine and glutamate. The incorporation of ammonium nitrogen into glutamine and then to glutamate involves the sequential action of enzymes glutamine synthetase (GS, EC,6.3.1.2) and glutamate synthases (GOGAT, E.C. 1.4.1.13 and E.C.1.4.7.1), the latter often considered to be the rate limiting enzyme. The operation of this so called GS-GOGAT pathway in ammonium assimilation has been demonstrated by employing various biochemical techniques such as tracing the path of ¹⁵N (labeled NH₄Cl), by using specific inhibitors of GS and GOGAT, and by examining ammonium assimilation in mutants lacking GOGAT. Two molecular forms of GOGAT, namely Fd specific: and NADH: specific have been demonstrated to be active in almost all tissues of the plants. Both are monomeric proteins and contain FMN and 3Fe-4S cluster as prosthetic groups. The genes for both the species of GOGAT have been cloned and characterised. They are usually upregulated by light and by nitrogen supply and are also regulated by many other environmental and plant factors, for example, by seasonal variations, growth regulators, salinity, water stress, pollutants, plant age etc. Genetic manipulation of the expression of enzyme for qualitative improvement of crops through altered nitrogen assimilation efficiency in the near future, may be an important aspect of the enzymology of GOGAT.