Glutamate synthase cycle

Baron, A. C., Tobin, T. H., WaHsgrove, R. M., and Tobin, A. K. (1994). AmetaboHc control analysis of the glutamine synthetase/glutamate synthase cycle in isolated barley Elordeum vulgare L.) chloroplasts. Plant Physiol. 105, 415-424. 

Fig. 1. The glutamate synthase cycle. Enzymes 1, glutamine synthetase 2, glutamate synthase.

The affinity of an enzyme for its substrate(s) is an important property for any enzyme. Since the affinity is closely related to the it is usually considered that any enzyme must have a low for a substrate likely to be present only in small concentrations if the reaction is to occur at a reasonable rate. In this regard the much lower of GS for ammonia compared to that of GDH for the same substrate has often been cited as evidence compatible with the glutamate synthase cycle. Whilst this is true it should also be remembered that there are other factors which can override such a conclusion—the measured may be wrong (cf. ribulose bisphosphate car-... 

Bauer et al. (1977) followed the effect of adding [ N]0 to pea leaves from plants previously grown on [ N]0. Their results showed that there was a rapid decrease, over the first 20 min, in ammonia and then glutamine followed by glutamate (cf. to the results of Yoneyama and Kumazawa, 1975, with rice roots). Some difficulties of interpretation arise because it was clear that multiple pools exist for all the compounds analysed but the results were generally consistent with the action of the glutamate synthase cycle. 

However, models which assume that ammonia is assimilated solely via GDH cannot account in quantitative terms for the observed rates of NH/ incorporation (Rhodes et al, 1980 Fentem et ai, 1983a). Rhodes et ai, (1989) modeled the kinetics of NH4 incorporation in tobacco cell cultures and suggest that up to 30% of the ammonia assimilated may enter via the GDH reaction. However, the relative sizes of the glutamate and glutamine pools made it difficult to distinguish between the simultaneous operation of alternative pathways and Rhodes et al. were unable to verify the model when inhibitors of GS and GOGAT were used to block the glutamate synthase cycle. 

The evidence, based on the isolation of GS and glutamate synthase mutants, for the role of the glutamate synthase cycle in the photorespiratory nitrogen cycle as discussed in Section III,B,3, mitochondrial GDH-deficient mutant of Zea mays has been isolated and is being investigated by Rhodes et al. (1989). In the absence of MSO, wild-type plants have a higher shoot/root ratio and show a... 

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