Effect of substrates and coenzymes on the molecular symmetry-related properties of XXXX liver and @entity375 @entity167 dehydrogenases.

@entity2125 @entity1641 @entity167 @entity375 @entity1068 @entity1098
Thermal inactivation of @entity1641 liver @entity167 dehydrogenase (LADH) exhibits the following biphasic kinetics A = Afast.e-Kfast.t + Aslow.e-Kslow.t Where A is the per cent residual activity at time t,Afast and Aslow are amplitudes (expressed as % of initial activity) and kfast and kslow first-order rate constants of the fast and slow phases, respectively. For apoenzyme, Afast = Aslow = 50% of the initial activity under all conditions of temperature and pH. On the addition of a substrate or coenzyme ligand, there is a ligand concentration-dependent increase in per cent Aslow and a decrease in kslow. At sufficiently high ligand concentration, the entire time-course of inactivation can be described as a single exponential decay. The variations of per cent Aslow and of kslow with ligand concentration are consistent with the existence of two binding sites of different ligand affinities. Inactivation of LADH by excess @entity1098 also exhibits a similar biphasic kinetics with Afast = Aslow = 50% of the initial activity. Addition of @entity1068 or @entity2125 + brings about a concentration-dependent decrease in kfast and kslow without affecting amplitudes of the two phases. The @entity2125 concentration-dependence of this decrease is consistent with a single dissociation constant for the coenzyme. Inactivation of @entity375 @entity167 dehydrogenase by heat or excess @entity1098 can be represented as a single exponential decay of activity under all conditions of temperature and pH in the absence as well as presence of @entity1068 or @entity2125 +. Implications of these results for the molecular symmetry of the two oligomeric enzymes in solution are discussed.
