Enzymes achieve remarkable catalytic efficiency by several effects operating in concert. This is the result of evolution and natural selection over eons. One might wonder how primordial enzymes became catalytically active in the first place. To answer such a question it is important to know which of the effects operating in a modern enzyme is capable of triggering catalysis independently of the others. Catalytic antibodies, ['] which are designed by the experimenter, offer a unique opportunity to study such issues and to examine experimentally various hypotheses about primordial enzymes. It is much easier to dissect the individual parameters of catalysis in catalytic antibodies than in highly evolved enzymes in which all these parameters operate together. Indeed, Schultz et al.'s fundamental structural studies provide much insight into the evolution of catalytic capabilities in proteins.[*]
Along these lines and owing to the importance of carbohydrates in the early stages of evolution, the emergence of glycosidase activity is of particular interest. Herein we report on an antibody-catalyzed hydrolysis of unactivated cyclic ketals, a reaction that is closely related to the cleavage of the glycosidic bond.[32 These catalytic antibodies may therefore be considered as mechanistic analogs of the glycosidase enzymes. We show here that general acid catalysis and/or strain effects, which are of central importance in the activity of modern glyco~idases,[~~ have only minor significance in these catalytic antibodies. Conversely, simple charge complementarity proves to be the major factor in determining their catalytic activity.
Hydrolysis of ketal I involves protonation at one of the oxygen atoms, followed by heterolysis of a C -0 bond to generate the intermediate ion IV and ultimately the carbonyl compound V (Scheme 1). In the case of ketals and acetals with an activated L I II 111 IV V Scheme 1. General mechanism of ketdl hydrolysis under acidic conditions.
leaving group (e.g. R = aryl), the first step is the rate-limiting one, and their hydrolysis can be catalyzed by weak acids (general acid catalysis) . [5, 61 In contrast, with unactivated ketals (e.g. R = alkyl)['I the 0-protonated species I1 is a relatively stable intermediate formed in a preequilibrium. In such cases cleavage