X-ray crystallography predicts hydrogen-bonding interactions between the side chains of Thr198 and two other amino acid residues, Glu194 (adjacent to the catalytic His195) and Ser318 (on the alpha-H helix which rearranges on substrate binding). In order to investigate the contribution of this conserved amino acid residue, Thr198, two mutants of Bacillus stearothermophilus lactate dehydrogenase were created (Val198 and Ile198). The steady-state kinetic parameters for both mutant enzymes were very similar with increased substrate Km and reduced kcat when compared with the wild-type enzyme. The mutation Val198 allowed non-productive binding of pyruvate to the unprotonated form of His195. Steady-state kinetic parameters determined for the Val198 mutant enzyme in high solvent viscosity suggested both an altered rate-limiting step in catalysis and implicated Thr198 in allosteric activation by the effector fructose 1,6-bisphosphate (Fru1,6P2). A shift in the Fru1,6P2 activation constant for the Val198 mutant enzyme suggested that Thr198 stabilises the catalytically competent (Fru1,6P2-activated) form of the enzyme by 6.6 kJ/mol. However, Thr198 was not important for maintaining the thermal stability of the Fru1,6P2-activated form. Equilibrium unfolding in guanidinium chloride indicated that Thr198 contributes 17.2 kJ/mol subunits towards the tertiary structural stability. The results emphasise the importance of the side chain-hydroxyl group of Thr198 which is required for (a) productive substrate binding, (b) allosteric activation and (c) protein conformational stability. The characteristics of the B. stearothermophilus lactate dehydrogenase mutations reported here were significantly different from those of the same mutations made in the corresponding position of the analogous enzyme Thermus flavus malate dehydrogenase [Nishiyama, M., Shimada, K., Horinouchi, S., & Beppu, T. (1991) J. Biol. Chem. 266, 14294-14299].