Effects of free Ca²⁺ on kinetic characteristics of holotransketolase.

Catalytic activity has been demonstrated for holotransketolase in the absence of free bivalent cations in the medium. The two active centers of the enzyme are equivalent in both the catalytic activity and the affinity for the substrates. In the presence of free Ca²⁺ (added to the medium from an external source), this equivalence is lost: negative cooperativity is induced on binding of either xylulose 5-phosphate (donor substrate) or ribose 5-phosphate (acceptor substrate), whereupon the catalytic conversion of the bound substrates causes the interaction between the centers to become positively cooperative. Moreover, the enzyme total activity increase is observed.