Involvement of Ionizable Groups in Catalysis of Human Liver Glycolate Oxidase

Abstract

Glycolate oxidase is a flavin-dependent, peroxisomal enzyme that oxidizes ␣-hydroxy acids to the corresponding ␣-keto acids, with reduction of oxygen to H 2 O 2 . 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 presteady-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 (k cat ) 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 (k cat /K glycolate ) and the K d(app) for the formation of the enzyme-substrate complex suggested the presence of a protonated group with apparent pK a of 8.5 participating in substrate binding. The k cat /K oxygen values were an order of magnitude faster when a group with pK a of 6.8 was unprotonated. These results are discussed in the context of the available three-dimensional structure of GOX.Glycolate oxidase (EC 1.1.3.15; glycolate:oxygen oxidoreductase; GOX) 2 catalyzes the FMN-mediated oxidation of glycolate to glyoxylate with reduction of oxygen to hydrogen peroxide (Scheme 1) (1). The enzyme has been identified in plants and mammals and contains tightly but not covalently linked FMN. GOX has been grouped in the superfamily of the ␣-hydroxy acid oxidases, which includes among others long-chain hydroxy acid oxidase, lactate oxidase, mandelate dehydrogenase, and the flavin-binding domain of yeast flavocytochrome b 2 (2-6). In plants, GOX is localized in the glyoxysome of photosynthetic tissues, where it participates in photorespiration (7,8). In humans and other vertebrates, such as pigs, the enzyme is found in the peroxisomes of liver and kidney and is involved in the production of oxalate (9 -11). The latter finding recently prompted considerable interest in the study of the mechanistic and structural properties of human GOX for the potential development of therapeutic agents targeting the treatment of primary hyperoxaluria (12), a genetic disorder in which overproduction of oxalate results in large deposits of calcium oxalate.To date, the GOX that is best characterized in its structural, kinetic, and biochemical properties is the one from spinach leaves (13-19). Recently, the structure of the enzyme from human liver has been solved in complex with a number of ligands to resolutions of Յ1.95 Å (20). In the active site of the human enzyme (Fig. 1) has been proposed to initiate catalysis by acting as the proton acceptor for the deprotonation of the hydroxyl group or the ␣-carbon of the substrate (19,2...

Experimental Evidence for a Hydride Transfer Mechanism in Plant Glycolate Oxidase Catalysis

“…In this work, recombinant His-tagged GOX enzymes from A. thaliana (C 3 species) and Z. mays (C 4 species) were compared ( Table 1). The obtained kinetic parameters agreed with previous K m(glycolate) values from plant-purified GOXs (0.3 mM in pumpkin cotyledons (13) and P. sativum leaves (14)) and recombinant mammalian GOX (0.32 mM in human (20) for purified and recombinant human liver GOXs (11,21). Therefore, our GOX proteins appear to be fully functional.…”

“…We nevertheless recognize that other effects may have contributed to the observed SIE such as an effect of solvent viscosity, which is indeed higher in heavy water (36). In fact, recombinant human liver GOX has lower k cat and k cat /K m values in a more viscous solvent such as glycerol (21). Also, three water molecules have been reported to participate in the catalytic site architecture in recombinant spinach GOX, and their presumed role is to maintain the H-bond network around FMN in the absence of glycolate (6,37 (38).…”

Solvent-Slaved Motions in the Hydride Tunneling Reaction Catalyzed by Human Glycolate Oxidase

“…Recombinant pure HsGOX was obtained as published. 16 [2R-2 H]Glycolate and glycolate were synthesized in our laboratory as described in the Supporting Information (SI). All other chemicals were of the highest purity commercially available.…”

“…These results suggested that glycerol might act as a competitive inhibitor in the HsGOX reaction. This hypothesis was ruled out on the basis of the fact that the HsGOX K m value with glycolate (0.05 mM at pH 7.0, 30 °C, and atmospheric oxygen) does not change in the presence of 6, 14, 21, and 28% (m/m) glycerol (unpublished data obtained for ref 16). Alternatively, the interactions in the HsGOX-glycolate complex may be weaker due to the presence of glycerol since this compound can affect various solvent and enzyme properties besides viscosity.…”

Small Molecule-Based Enzyme Inhibitors in the Treatment of Primary Hyperoxalurias

“…In the first half-reaction, glycolate is oxidized to glyoxylate and the flavin is reduced. In the second half reaction, the cofactor FMN is reoxidized by the action of O 2 , resulting in the formation of H 2 O 2 [ 85 , 96 ].…”