Several intermediates in the reaction of 2-methylglutamate with glutamate decarboxylase from Escherichia coli were detected by stopped-flow spectrophotometry and by rapid-scanning spectrophotometry after conventional mixing. Structures were assigned to intermediates on the basis of kinetic and spectral evidence. In the early stages of the reaction an intermediate with the properties expected of a geminal diamine accumulated significantly. Changes consistent with the conversion of this species into the external aldimine were also observed. The course of product formation was determined and linked with spectral changes taking place in the bound coenzyme. The effect of the minor decarboxylation-dependent transamination that accompanies the major reaction was analysed.

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