Reaction of Lactobacillus histidine decarboxylase with L-histidine methyl ester

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Abstract
L-Histidine methyl ester inactivates histidine decarboxylase in time-dependent manner. The possibility was considered that an irreversible reaction between enzyme and inhibitor occurs [Recsei, P. A., and Snell, E.E. (1970) Biochemistry 9, 1492-1497]. We have confirmed time-dependent inactivation by histidine methyl ester and have investigated the structure of the enzyme-inhibitor complex. Upon exposure to either 8 M guanidinium chloride or 6% trichloroacetic acid, unchanged histidine methyl ester is recovered. Formation of the complex involves Schiff base formation, most likely with the active site pyruvyl residue [Huynh, Q. K., and Snell, E.E. (1986) J. Biol. Chem. 261, 4389-4394], but does not involve additional irreversible covalent interaction between inhibitor and enzyme. Complex formation is a two-step process involving rapidly reversible formation of a loose complex and essentially irreversible formation of a tight complex. For the formation of the tight complex, Ki = 80 nM and koff = 2.5 .times. 10-4 min-1. Time-dependent inhibition was also observed with L-histidine ethyl ester, L-histidinamide, and DL-3-amino-4-(4-imidazolyl)-2-butanone. No inactivation was observed with glycine methyl ester or histamine. We propose that in the catalytic reaction the carboxyl group of the substrate is in a hydrophobic region. The unfavorable interaction between the carboxylate group and the hydrophobic region facilitates decarboxylation [Crosby, J., Stone, R., and Liehard, G.E. (1970) J. Am. Chem. Soc. 92, 2891-2900]. With histidine methyl ester this unfavorable interaction is no longer present; hence, there is tight binding. The effect of substrate ester analogues on the activity of other decarboxylases was examined to determine whether this approach to inhibition has general applicability. Enzymes investigated were (with substrate analogues in parentheses) acetoacetate decarboxylase from Clostridium acetobutylicum (methyl acetoacetate), thiamin pyrophosphate dependent pyruvate decarboxylase from brewers'' yeast (methyl pyruvate), pyridoxal phosphate dependent tryosine/phenylalanine decarboxylase from Streptococcus faecalis (L-tyrosine methyl ester, L-phenylalanine methyl ester, DL-2-amino-4-phenyl-2-butanone), and arginine and ornithine decarboxylases from Escherichia coli (L-arginine methyl ester, D-arginine methyl ester). No time-dependent inhibition was observed in those cases.