FUNCTIONAL-PROPERTIES OF THE HISTIDINE-ASPARTATE ION-PAIR OF FLAVOCYTOCHROME B(2) (L-LACTATE DEHYDROGENASE) - SUBSTITUTION OF ASP282 WITH ASPARAGINE

The FMN prosthetic group of flavocytochrome b(2) or L-lactate dehydrog enase oxidizes lactate to pyruvate. The reducing equivalents are then transferred one by one, intramolecularly, to heme b(2) and then to ext ernal accepters. Substrate oxidation is thought to begin with abstract ion 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 pK(a) of His373 is substantially increased in the wild-type reduc ed enzyme compared to that in the oxidized state. We report here the e nzymatic properties of the D282N mutant flavocytochrome b(2). Steady-s tate rate measurements with [2-H-1]lactate and [2-H-2]lactate indicate that, as predicted, the Michaelis complex stability is hardly affecte d, whereas the transition state for proton abstraction increases in en ergy by 2.8 kcal/mol, Steady-state inhibition studies were conducted w ith a number of active-site ligands: sulfite, D-lactate, pyruvate, and oxalate. Binding was found to be most affected for oxalate, but kinet ic patterns indicated oxalate and pyruvate were still capable of bindi ng to the enzyme both at the oxidized and semiquinone stages, whereas inhibition by excess substrate, due to lactate binding at the semiquin one stage, was lost. Finally, analysis of the intermolecular hydrogen transfer catalyzed by the enzyme between [2-H-3]lactate and fluoropyru vate indicated that the substitution with asparagine facilitates excha nge of the histidine-bound proton and hence induces a decrease in the pK(a) value of H373 in the reduced enzyme of about 1.4 pH units. Never theless, the rate constant value for exchange with the solvent of the enzyme-bound substrate alpha-proton indicates that H373 is still proto nated in the reduced mutant enzyme at neutral pH, Thus, the D282N muta tion destabilizes the transition state for proton abstraction and decr eases the pK(a) of H373 in the reduced enzyme but is insufficient to b ring it back to a normal value.