Wl assimilation in crop Indeed as will be N plants high conc

Wl assimilation in crop Indeed, as will be N plants high concentrations. often generates nase reaction in animals most in the highly g utamate phate synthesis by kJ/mol) Metabolite Regulation f Glutamine Syntheta of Glutamine synthetase is best characterized in bacteria, exists as a tein complex containing 12 identical subunits, as shown in i of glutamine sy use is tightly controlled by a form of a Te 17.12 fic regulation Figure 17.11 The glutamate dehydrogenase reaction can NAD(P) H H NAD(P) Hzo assimilate NH into glutamate under conditions of very high CH NH H,CT Glutamate dehydrogenase NH concentrations, as in heavily fertilized agricultural fields. Glutamate dehydrogen Ketoglutarate Glutamate can use either NADH or NADPH as the reductant. AG nd th the al an overa in protein ial glutamin modifica the enzyme s the adenyl activation activity of (Figure 17 tase activi regulator transfera with a m uridine ly lation the Ty ferase

Solution

Answer: (The discussion involves the mass action ratio under physiological conditions)

The equilibrium constant for the overall glutamate dehydrogenase reaction at 38°, pH 7 and at intracellular ionic strength, is about 4x10^-6 M. In the liver cell where the concentration of ammonia is about 5x10^-4 M and the value for mitochondrial [NAD(P)⁺]/[NAD(P)H] is roughly 10, the reaction is at equilibrium when [glutamate]/[oxoglutarate] is about 12. It thus seems possible that in the liver the reaction is close to equilibrium since glutamate is normally fairly abundant.

Not all cells of course are animal, and in plants and micro-organisms the operation of the reaction from right to left was once thought to have greater physiological significance. However in plants it is unlikely that ammonia is present at a concentration as high as 5x10^-4 M and the rather high Km of the dehydrogenase towards ammonias normally mitigates against the importance of the reaction for nitrogen assimilation.

An ATP-consuming reaction of glutamine synthetase, also present in animal cells, constitutes a more efficient means of NH₃ assimilation, and use of energy determines direction.

The coupling of glutamine synthetase with glutamate synthase has the net effect of reversing the glutamate dehydrogenase reaction (using NADPH) but the overall sequence is also driven by cleavage of ATP.