Catalysis through induced intramolecularity: What can be learned by mimicking enzymes with carbonyl compounds that covalently bind substrates?

Protein engineering and the use of catalytic antibodies have given substance to the possibility of making artificial enzymes, whereas chemical design has provided functional, though not yet useful, enzyme mimics. This article is devoted to a class of mimics that uses covalent binding to substrates at a site different from the reaction center and that exhibits turnover. These molecular devices are capable of converting the bimolecular reaction of a substrate into a fast intramolecular one and may be defined by the concept of catalysis by induced intramolecularity. As examples of such systems, several reactions of bifunctional substrates that involve catalysis by carbonyl compounds are brought together in this review. Their ability to catalyze difficult reactions such as the hydrolysis of unactivated amides by mimicking enzyme-substrate complementarity is the result of a uniform stabilization of bound species, including both the transition state and also encounter complexes, which is the specific feature of these systems. They allow quantitative analysis of differences between intramolecular and bimolecular processes and support the explanation based on the entropic disadvantage of bimolecular reactions that can be compensated by binding energy as in enzymes. Three-dimensional energy diagrams are proposed as valuable tools to describe these systems, as well as enzymatic mechanisms and other catalytic mechanisms that take advantage of induced intramolecularity. Unexpectedly, these diagrams are appropriate to a wide field of applications, such as the description of intramolecular and bimolecular reactions, resulting in a dynamic description of the notion of "tight" binding. In addition, the high efficiency of catalysts that covalently bind substrates raises the question of what their contribution may have been in early enzyme evolution. Their potential association with primitive enzymes as catalytic cofactors is considered. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).