A source for the special catalytic power of enzymes: orbital steering.

The velocities of acid catalyzed esterification and gamma-lactonizations were studied to test the sensitivity of a chemical reaction to the orientation of the reacting atoms. Variation in orientation of the attacking oxygen atom relative to the carbon atom of the carboxylic acid was achieved by using bicyclic ring systems to limit the conformational mobility of the gamma-hydroxy acids. Factors as high as 2 x 10(4) were observed for the acceleration of a reaction due to this "orientation factor" even after corrections for proximity and torsional strain have been made. The orientation factor is related to the shape of the electron orbitals and must have an angular preference far greater than previously estimated. Such sensitivity to orientation would provide factors large enough to explain the gap in our understanding of enzyme catalysis. It is suggested that the catalytic efficiency of enzymes depends on their ability not only to juxtapose the reacting atoms but also to "steer" their orbitals along a path which takes advantage of this strong directional preference.