Glutamine glutamate synthetase pathway

The short-lived radionuclide, has been extremely useful in studies of N2 fixation and NH4 assimilation. Wolk and his collaborators have demonstrated that the glutamine synthetase/glutamate synthase pathway is the major assimilatory route for NH4 in N2-fixing cultures of Anabaena and many other cyanobacteria. They have extended these studies to show that the labeling pattern observed after exposure of detached or attached soybean root nodules to [ N]N2 was similar to that observed in Anabaena. On this basis Meeks et al. (19) concluded that the primary pathway for assimilating NH4" derived from N2 in soybean nodules involves the sequential assimilatory activities of glutamine synthetase and glutamate synthase. 

Figure 20.9 The positions in the pathway for de novo pyrimidine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. Glucose forms ribose 5-phosphate, via the pentose phosphate pathway (see chapter 6), which enters the pathway, after phosphorylation, as 5-phospho-ribosyl 1-pyrophosphate. Glutamine provides the nitrogen atom to synthesise carbamoylphos-phate (with formation of glutamate), and also to form cytidine triphosphate (CTP) from uridine triphosphate (UTP), catalysed by the enzyme CTP synthetase. It is the amide nitrogen of glutamine that is the nitrogen atom that is provided in these reactions.

Fig. 1. The nitrate assimilation pathway in higher plants. The pathway of nitrate assimilation in the tobacco leaf is illustrated. In some other species an additional cytosolic GS is found in the leaf. The pathway in plant roots is more poorly documented and more variable GS in roots is mostly cytosolic, and some enzymes such as GOGAT are found as isoforms utilising alternate reducing substrates. T, expected nitrate carrier NR, nitrate reductase NiR, nitrite reductase GS, glutamine synthetase GOGAT, glutamate synthase Fd, ferredoxin Gin, glutamine Glu, glutamate.

Figure 11 Nitrogen sources and metabolic pathways in marine phytoplankton. Solid circles are transporters. Boxes are the catalytic enzymes and open circles are metals associated with each enzyme. Ur, Urease NR, Nitrate Reductase NiR, Nitrite Reductase AAOx, amino acid oxidase AmOx, amine oxidase GS-GOGAT, Glutamine Synthetase- Glutamate oxy-glutarate aminotransferase (or glutamate synthase).

Some bacteria employ an unusual pathway in which glutamine is incorporated into gln-tRNA without a glutamine-specific aminoacyl tRNA-synthetase. In this case, the synthetase for glutamate is used and glutamine is created by transamination, as follows ... 

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