Conformational changes and the regulation of glutamate-dehydrogenase activity

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Abstract
The effect of reduced nicotinamide adenine dinucleotide (NADH) and the non-competitive inhibitor guanosine triphosphate (GTP) on the optical-rotatory-dispersion properties of glutamate dehydrogenase has been studied. Analysis of the data in terms of the ao and bo parameters of the Moffitt-Yang equation indicates that a conformational change is induced either by NADH or by GTP in the presence of small amounts of NADH. Sedimentation measurements under comparable conditions showed that the enzyme reversibly dissociates into sub-units but that this dissociation is only secondary to the conformational changes. Fluorescence measurements showed that the binding constant of NADH and the number of binding sites on the enzyme increased in the presence of GTP. This is confirmed by studies of fluorescence polarization, which in addition showed that the movement of NADH on the enzyme surface is more restricted in the presence of GTP. The relation of these results to possible regulatory mechanisms is discussed.