REMOVAL OF SUBSTRATE-INHIBITION IN A LACTATE-DEHYDROGENASE FROM HUMANMUSCLE BY A SINGLE RESIDUE CHANGE

High concentrations of ketoacid substrates inhibit most natural hydrox yacid dehydrogenases due to the formation of an abortive enzyme-NAD(+) -ketoacid complex. It was postulated that this substrate inhibition co uld be eliminated from lactate dehydrogenases if the rate of NAD(+) di ssociation could be increased, An analysis of the crystal structure of mammalian LDHs showed that the amide of the nicotinamide cofactor for med a water-bridged hydrogen bond to S163. The LDH of Plasmodium falci parum 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 fr om a weakening of the interaction of pyruvate with the enzyme-coenzyme complex.