Chemistry and Biology of Proteins - F. Haurowitz 1953

Proteins with Enzymatic Properties
Functions of Apoenzymes

The apoenzyme molecule evidently performs two main Functions: 1) activation of the prosthetic group, and 2) binding of the enzyme to the substrate. To explain The Mechanism of activation of the prosthetic group (coenzyme), a concept has been proposed according to which the apoenzyme fixes the prosthetic group on its surface in a position most favorable for interaction with the substrate. It is easy to see how this facilitates the possibility of collisions between substrate molecules and the active group of the enzyme.

On the other hand, apoenzymes probably can combine not only with the coenzyme, but also form temporary associations with the substrate. Such is, for example, the MECHANISM OF ACTION of peptidases containing metal atoms. These atoms form the coordinating center of a complex in which both the apoenzyme and the substrate are bound to the metal [16]. Due to The formation of the substrate–coenzyme–apoenzyme complex, the contact between the substrate and the apoenzyme becomes so intimate that the course of the reaction is greatly facilitated.

At present, we can as yet say nothing regarding the Chemical Nature of those enzyme groupings that take part in binding the coenzyme or substrate; meanwhile, the resolution of this question is a prerequisite for solving the entire problem of the Mechanism of Enzymatic action. The most probable assumption is that the bond between the apoenzyme and the coenzyme or substrate is realized by polar groups of the apoenzyme. One can imagine that the shape of these groups acts as a sort of negative imprint of the corresponding groups of the coenzyme or substrate, thereby easily giving rise to dipole bonds between the closely fitted surfaces of the apoenzyme and the coenzyme or substrate [16, 19, 20] (see p. 219).