The lactate dehydrogenase of Bacillus stearothermophilus exists in dimeric and tetrameric forms, the latter favored by the regulatory effector, fructose-1,6-bisphosphate. The kinetic behavior of the two forms is different, the tetramer showing activation relative to the dimer at low pyruvate concentrations, but pyruvate inhibition at high concentrations. Deuterium isotope effects at the transferring hydride site of NADH (primary isotope effect) and the methyl group of pyruvate (secondary isotope effects) allow estimation of the microscopic rate constants for individual processes for both enzyme forms. In the dimer, entrance and exit to and from the active site is slow for pyruvate nd lactate, rapid for NAD. In the tetramer, entrance and exit for pyruvate and lactate are fast while NAD exit is slow. Hydride transfer occurs at similar rates in both forms. This model explains the main features of the kinetics of both forms of the enzyme.
ASJC Scopus subject areas
- Chemical Engineering(all)