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The NAD-dependent D-(-)-lactate dehydrogenase (D-LDH) from Lactobacillus delbrueckii subsp. bulgaricus (in short, L. bulgaricus) has been modified at position 175 by site-directed mutagenesis, changing a conserved aspartate residue into an alanine. The D175A mutant enzyme displays a 40-fold shift in coenzyme preference from NADH to NADPH. This demonstrates that D175 truly belongs to the amino acid consensus GXGXXGX(17)D (where X represents any residue) which is the signature of the coenzyme binding site of most NAD-dependent dehydrogenases.

The malate dehydrogenase from Escherichia coli has been specifically altered at a single amino acid residue by using site-directed mutagenesis. The conserved Arg residue at amino acid position 102 in the putative substrate binding site was replaced with a Gln residue. The result was the loss of the high degree of specificity for oxaloacetate. The difference in relative binding energy for oxaloacetate amounted to about 7 kcal/mol and a difference in specificity between oxaloacetate and pyruvate of 8 orders of magnitude between the wild-type and mutant enzymes. These differences may be explained by the large hydration potential of Arg and the formation of a salt bridge with a carboxylate group of oxaloacetate.

Using site-directed mutagenesis, the NADH-linked lactate dehydrogenase from Bacillus stearothermophilus has been specifically altered at a single residue to shift the coenzyme specificity towards NADPH. The single change is at position 53 in the amino acid sequence where a conserved aspartate has been replaced by a serine. This substitution was made to reduce steric hindrance on binding of the extra phosphate group of NADPH and to remove the negative charge of the aspartate group. The resultant mutant enzyme is 20 times more catalytically efficient than the wild-type enzyme with NADPH.

A general technique for monitoring the intramolecular motion of a protein is described. Genetic engineering is used to replace all the natural tryptophan residues with tyrosine. A single tryptophan residue is then inserted at a specific site within the protein where motion is then detected from the fluorescence characteristics of this fluorophore. This technique has been used in B. stearothermophilus lactate dehydrogenase mutant (W80Y, W150Y, W203Y, G106W) to correlate the slow closure of a surface loop of polypeptide (residues 98-110) with the maximum catalytic velocity of the enzyme.

The rational design of enzyme catalysts for chiral chemistry and of drugs which bind to proteins would be facilitated if rules for the recognition of one partner by the other could be formulated. This communication suggests and tests one generalization: arginine forms a tighter ion pair with a carboxylate group than does lysine and is always used for ion-pairs which are not broken during turnover in naturally-occurring enzymes.

A variant of lactate dehydrogenase from Bacillus stearothermophilus has been engineered by site-directed mutagenesis in which an active-site arginine residue at position 171 in the protein sequence is replaced by lysine. Replacement of this arginine by lysine has no effect on co-enzyme binding, a relatively small effect on the rate of turnover of the enzyme, but causes a 2000-fold increase in the Michaelis constant for pyruvate, a 6000-fold increase in the dissociation constant for oxamate and results in a Michaelis constant for lactate which is too high to measure. The decrease in binding energy for these carboxylate-containing substrates caused by this mutation is very large, around 5.5 kcal.mol-1 and in part, is explained by the small increase in the distance of a lysine-substrate carboxylate interaction at this site and the absence of the additional hydrogen bond from a two-point arginine-carboxylate interaction. Consistent with this last observation, the ability of this mutant enzyme to stabilize an NAD+-sulphite compound in its active site (an alternative enzyme-substrate complex which does not involve bifurcated bonding to arginine) is only reduced 14-fold.

The function of the amino acid Thr246 in L-lactate dehydrogenase from Bacillus stearothermophilus has been investigated by site-directed replacement with glycine. Kinetic experiments with a number of 2-oxo acids showed strongly reduced activity for the mutated enzyme. However, the mutant enzyme shows a relative preference for the large hydrophobic sidechains of alpha-keto acids and an even higher specific activity than the wild-type lactate dehydrogenase for the polar oxaloacetate substrate. Graphic analyses indicate that the loss of one hydrogen bond, or intrusion of water into the active site, might be responsible for the reduced activity. The kinetic results suggest that the binding modes of bulky hydrophobic or polar substrates compensate to some degree for the partially disrupted active site.