The trimethylammonium headgroup of choline is a major determinant for substrate binding and specificity in choline oxidase

Abstract

Choline oxidase catalyzes the oxidation of choline to glycine betaine via two sequential flavin-linked transfers of hydride equivalents to molecular oxygen and formation of a betaine aldehyde intermediate. In the present study, choline and glycine betaine analogs were used as substrates and inhibitors for the enzyme to investigate the structural determinants that are relevant for substrate recognition and specificity. Competitive inhibition patterns with respect to choline were determined for a number of substituted amines at pH 6.5 and 25 °C. The K is values for the carboxylate-containing ligands glycine betaine, N, N-dimethylglycine, and N-methylglycine increased monotonically with decreasing number of methyl groups, consistent with the trimethylammonium portion of the ligand being important for binding. In contrast, the acetate portion of glycine betaine did not contribute to binding, as suggested by lack of changes in the K is values upon substituting glycine betaine with inhibitors containing methyl, ethyl, allyl, and 2-amino-ethyl side chains. In agreement with the inhibition data, the specificity of the enzyme for the organic substrate ( k cat/ K m value) decreased when N, N-dimethylethanolamine, N-methylethanolamine, and the isosteric substrate 3,3-dimethyl-1-butanol were used as substrate instead of choline; a contribution of ∼7 kcal mol −1 toward substrate discrimination was estimated for the interaction of the trimethylammonium portion of the substrate with the active site of choline oxidase.