Quantifying the inactivation rate constants for the molecular species comprising the catalytic cycle of Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase

Abstract

When an unstable enzyme is incubated with its substrate(s), catalysis may cease before chemical equilibrium is attained. The residual substrate concentrations depend on their initial concentrations, the initial enzymic activity, and the inactivation rate constants for each molecular species that comprise the catalytic cycle. The underlying theory has been elaborated previously for single-substrate reactions and here it is extended to bi-substrate reactions. The theory is illustrated by application to glucose 6-phosphate dehydrogenase, which is unstable when exposed to a low concentration of sodium dodecyl sulphate. It is shown that the ternary complex containing both substrates is resistant to inactivation while each of the remaining complexes undergoes first-order decay. Rate constants for the inactivation of each complex are calculated.

Keywords::

• Bi-substrate reaction
• enzyme inactivation
• glucose 6-phosphate dehydrogenase
• rate constant
• residual substrate
• unstable enzyme

Acknowledgements

Some of the experiments reported here were performed by Stephen Pike.