Activation of the coenzyme at the early step of the catalytic cycle of tryptophanase.

Abstract

Tryptophanase is catalytically more competent in alkaline pH even though the coenzyme exists as an inactive aldamine structure in this pH region. The binding of a substrate analog, 3-indolepropionate to the enzyme shifts the equilibrium from the substituted aldamine to the ketoenamine form in the entire pH region studied. The resultant ketoenamine form is favorable for transaldimination with the substrate amino group, a prerequisite for subsequent catalysis. This implies that the binding of tryptophan in alkaline pH, where the enzyme shows maximum activity, converts the inactive aldamine form of the coenzyme to the active ketoenamine form, which is favorable for undergoing the next step of the catalytic process.