On the molecular basis of inhibition by excess substrate in wild-type and Y143F flavocytochrome b2

Purification and characterization of recombinant human liver glycolate oxidase

Glycolate oxidase, an FMN-dependent peroxisomal oxidase, plays an important role in plants, related to photorespiration, and in animals, where it can contribute to the production of oxalate with formation of kidney stones. The best studied plant glycolate oxidase is that of spinach; it has been expressed as a recombinant enzyme, and its crystal structure is known. With respect to animals, the enzyme purified from pig liver has been characterized in detail in terms of activity and inhibition, the enzyme from human liver in less detail. We describe here the purification and initial characterization of the recombinant human glycolate oxidase. Its substrate specificity and the inhibitory effects of a number of anions are in agreement with the properties expected from previous work on glycolate oxidases from diverse sources. The recombinant enzyme presents an inhibition by excess glycolate and by excess DCIP, which has not been documented before. These inhibitions suggest that glycolate binds to the active site of the reduced enzyme, and that DCIP also has affinity for the oxidized enzyme. Glycolate oxidase belongs to a family of l-2-hydroxy-acid-oxidizing flavoenzymes, with strongly conserved active-site residues. A comparison of some of the present results with studies dealing with other family members suggests that residues outside the active site influence the binding of a number of ligands, in particular sulfite.

Functional properties of the histidine-aspartate ion pair of flavocytochrome b2 (L-lactate dehydrogenase): substitution of Asp282 with asparagine

The FMN prosthetic group of flavocytochrome b2 or L-lactate dehydrogenase oxidizes lactate to pyruvate. The reducing equivalents are then transferred one by one, intramolecularly, to heme b2 and then to external acceptors. Substrate oxidation is thought to begin with abstraction of the substrate alpha-hydrogen as a proton by an enzyme base. It has been proposed that this role is played by His373, which lies close to the flavin in the crystal structure and interacts with Asp282. It has also been shown before, using hydrogen exchange measurements, that the pKa of His373 is substantially increased in the wild-type reduced enzyme compared to that in the oxidized state. We report here the enzymatic properties of the D282N mutant flavocytochrome b2. Steady-state rate measurements with [2-1H]lactate and [2-2H]-lactate indicate that, as predicted, the Michaelis complex stability is hardly affected, whereas the transition state for proton abstraction increases in energy by 2.8 kcal/mol. Steady-state inhibition studies were conducted with a number of active-site ligands: sulfite, D-lactate, pyruvate, and oxalate. Binding was found to be most affected for oxalate, but kinetic patterns indicated oxalate and pyruvate were still capable of binding to the enzyme both at the oxidized and semiquinone stages, whereas inhibition by excess substrate, due to lactate binding at the semiquinone stage, was lost. Finally, analysis of the intermolecular hydrogen transfer catalyzed by the enzyme between [2-3H]lactate and fluoropyruvate indicated that the substitution with asparagine facilitates exchange of the histidine-bound proton and hence induces a decrease in the pKa value of H373 in the reduced enzyme of about 1.4 pH units. Nevertheless, the rate constant value for exchange with the solvent of the enzyme-bound substrate alpha-proton indicates that H373 is still protonated in the reduced mutant enzyme at neutral pH. Thus, the D282N mutation destabilizes the transition state for proton abstraction and decreases the pKa of H373 in the reduced enzyme but is insufficient to bring it back to a normal value.