The role of essential histidines in the mechanism of catalysis of the flavoenzyme, β-cyclopiazonate oxidocyclase

1. 1. A study of the pH-dependence of the binding of several indole derivatives to the flavoenzyme, β-cyclopiazonate oxidocyclase, indicates that groups with pK_a values of 6.0 to 7.0 are involved in the binding of these compounds to the active site. 2. 2. It is shown that the first order rate constant for the reduction of the enzyme by enzyme-bound substrate exhibits very little pH dependence. 3. 3. The inactivation of the enzyme by the carbethoxylation of histidyl residues and its reactivation by treatment with hydroxylamine is described. 4. 4. The spectral changes accompanying the binding of the substrate, β-cyclopiazonic acid, to the carbethoxylated enzyme are compared to those which characterize the formation of the enzyme-substrate intermediate complex during catalysis by the native enzyme. 5. 5. The significance of these results in relation to a general mechanism in which histidyl residues function to abstract a proton in flavo enzyme catalysis is briefly discussed.