Uncoupling and isotope effects in gamma-butyrobetaine hydroxylation

Abstract

Replacement of unlabeled gamma-butyrobetaine with gamma-[2,3,4-2H6]butyrobetaine has a profound effect on the stoichiometry between decarboxylation of 2-oxoglutarate and hydroxylation in the reaction catalyzed by human gamma-butyrobetaine hydroxylase. The ratios between decarboxylation and hydroxylation are 1.16 with unlabeled and 7.48 with deuterated gamma-butyrobetaine as substrate. From these ratios an internal isotope effect of 41 has been calculated. DV in the overall reaction measured as 2- oxoglutarate decarboxylation is 2.5 and DV/K is 1.0. For gamma-butyrobetaine hydroxylase from Pseudomonas sp. AK 1, 2-oxoglutarate decarboxylation exceeds hydroxylation with 10% when deuterated gamma-butyrobetaine is used. No excess was found with unlabeled substrate and no internal isotope effect could be calculated. DV for the bacterial enzyme is 6.