Removal of substrate inhibition in a lactate dehydrogenase from human muscle by a single residue change

High concentrations of ketoacid substrates inhibit most natural hydroxyacid dehydrogenases due to the formation of an abortive enzyme-NAD(+)-ketoacid complex. It was postulated that this substrate inhibition could be eliminated from lactate dehydrogenases if the rate of NAD(+) dissociation could be increased, An analysis of the crystal structure of mammalian LDHs showed that the amide of the nicotinamide cofactor formed a water-bridged hydrogen bond to S163. The LDH of Plasmodium falciparum is not inhibited by its substrate and, uniquely, in this enzyme the serine is replaced by a leucine, In the S163L mutant of human LDH-M(4) pyruvate inhibition is, indeed, abolished and the enzyme retains high activity, However, the major contribution to this effect comes from a weakening of the interaction of pyruvate with the enzyme-coenzyme complex.