SUBSTITUTION OF THE AMINO-ACID AT POSITION-102 WITH POLAR AND AROMATIC RESIDUES INFLUENCES SUBSTRATE-SPECIFICITY OF LACTATE-DEHYDROGENASE

The Gln residue at amino acid position 102 of Bacillus stearothermophi lus lactate dehydrogenase was replaced with Ser, Thr, Tyr, or Phe to i nvestigate the effect on substrate recognition. The Q102S and Q102T mu tant enzymes were found to have a broader range of substrate specifici ty (measured by k(cat)/K(m)) than the wild-type enzyme. However, it is evident that either Ser or Thr at position 102 are of a size able to accommodate a wide variety of substrates in the active site and substr ate specificity appears to rely largely on size discrimination in thes e mutants. The Q102F and Q102Y mutant enzymes have low catalytic effic iency and do not show this relaxed substrate specificity. However, the ir activities are restored by the presence of an aromatic substrate. A ll of the enzymes have a very low catalytic efficiency with branched c hain aliphatic substrates.