Involvement of ionizable groups in catalysis of human liver glycolate oxidase

Abstract

Glycolate oxidase is a flavin-dependent, peroxisomal enzyme that oxidizes α-hydroxy acids to the corresponding α-keto acids,with reduction of oxygen to H2O2. In plants, the enzyme participates in photorespiration. In humans, it is a potential drug target for treatment of primary hyperoxaluria, a genetic disorder where overproduction of oxalate results in the formation of kidney stones. In this study, steady-state and pre-steady-state kinetic approaches have been used to determine how pH affects the kinetic steps of the catalytic mechanism of human glycolate oxidase. The enzyme showed a Ping-Pong Bi-Bi kinetic mechanism between pH 6.0 and 10.0. Both the overall turnover of the enzyme (kcat) and the rate constant for anaerobic substrate reduction of the flavin were pH-independent at pH values above 7.0 and decreased slightly at lower pH, suggesting the involvement of an unprotonated group acting as a base in the chemical step of glycolate oxidation. The second-order rate constant for capture of glycolate (kcat/Kglycolate) and the Kd(app) for the formation of the enzyme-substrate complex suggested the presence of a protonated group with apparent pKa of 8.5 participating in substrate binding. The kcat/Koxygen values were an order of magnitude faster when a group with pKa of 6.8 was unprotonated. These results are discussed in the context of the available three-dimensional structure of GOX. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.