Electronic Rearrangement Induced by Substrate Analog Binding to the Enoyl-CoA Hydratase Active Site: Evidence for Substrate Activation

A series of α,β unsaturated CoA thiol esters have been characterized spectroscopically when they form noncovalent complexes at the active site of enoyl-CoA hydratase. The UV spectra of all of the thiol esters display significant red shifts when the esters are bound to the crotonase active site. The red shift increases with the ability of a para substituent of substituted cinnamoyl-CoA thiol esters to donate electrons by resonance. The affinity of the substituted cinnamoyl-CoA thiol esters is enhanced by electron-donating substituents, with the slope of the log of the ratio of the inhibition constants versus σ_p⁺ being near unity. Affinity is also increased by either para or meta electron-withdrawing substituents, suggesting that the enzyme stabilizes a partial positive charge at C-3. Binding to crotonase was shown to decrease the shielding of [3^-13C,3^-2H]cinnamoyl-CoA by +3.2 ppm, consistent with an increased partial positive charge at C-3. The Raman spectra of cinnamoyl-CoA bound at the crotonase active site similarly reflect the significant electronic ground state changes in the π electronic structure of the bound substrate. These data show that a major rearrangement of electrons occurs in the acryloyl portion of the cinnamoyl group upon binding, while only a minor perturbation occurs to the distribution of electrons in the phenyl ring. The strong electron polarization seen localized in the acryloyl moiety is consistent with a model in which the active site provides an electrophile at the C̳O oxygen (e.g., in the form of a positive charge or strong H-bonds) and a nucleophile near the C-3 carbon of the cinnamoyl moiety.