The transition-state (TS) structure for the hydride transfer in lactate dehydrogenase (LDH) enzyme has been calculated with analytical gradients at MNDO, AM1 and PM3 semiempirical levels. The TS is a first-order saddle point on the hypersurface. The geometrical arrangement of the TS results in optimal frontier orbital interaction. Information extracted from these structures is used to calculate theoretical kinetic isotope effects (KIE). The results are independent of the semiempirical procedure. The transition vector suggests that primary and secondary KIEs must be strongly coupled. The relative orientation imposed by the active-site constraints on the reactants in LDH (endo configuration) is optimal for the hydride transfer and situates the system in the neighbourhood of the saddle point.

By comparison of this TS with other first-order saddle points associated with the hydride-transfer process in another related enzyme's active site, the structure of the moieties and dynamical fluctuations in the TS appear to be fairly transferable and invariant for the dehydrogenase enzyme family.

When the reactants are moulded into the quadratic zone of the TS structure, the KIE can be rendered in fairly good qualitative agreement. This result suggests that the active site moulds the reactants into the structure and fluctuation patterns of the calculated TS.