Rate acceleration by stereopopulation control: models for enzyme action.

As a result of alkyl substitution in both aromatic ring and side chain, the rate constant for acid-catalyzed lactonization of hydrocoumaric acid is increased by factors as high as 10(11) and, in comparison with the bimolecular esterification of phenol and acetic acid, by almost 10(16). In the most favorable case studied, the half-life of the phenolic acid (imidazole buffer, pH 7, 30 degrees C) is 6 sec, with 90% of the total rate being due to catalysis by the buffer species. The effect is attributed to a unique interlocking of methyl groups, which produces a severe conformational restriction of the side chain and increases greatly the population of the most productive conformer. This phenomenon is presented as a model for the conformational restraint imposed by an enzyme on its substrate. The magnitude of the effect suggests that the contribution of substrate "freezing" to total rate enhancement by an enzyme can be considerably greater than previously supposed and may in fact be sufficient to complete the justification for enzyme catalysis.