One of the most common, and yet least well understood, enzymatic transformations is proton abstraction from activated carbon acids such as carbonyls. Understanding the mechanism for these proton abstractions is basic to a good understanding of enzyme function. Significant controversy has arisen over the means by which a given enzyme might facilitate these deprotonations. Creating small molecule mimics of enzymes and physical organic studies that model enzymes are good approaches to probing mechanistic enzymology. This Account details a number of molecular recognition and physical organic studies, both from our laboratory and others, dealing with the elucidation of this quandary. Our analysis launches from an examination of the active sites and proposed mechanism of several enzyme-catalyzed deprotonations of carbon acids. This analysis highlights the geometries of the hydrogen bonds found at the enzyme active sites. We find evidence to support pi-oriented hydrogen bonding, rather than lone pair oriented hydrogen bonding. Our observations prompted us to study the stereochemistry of hydrogen bonding that activates carbonyl alpha-carbons to deprotonation. The results from our own thermodynamic, kinetics, and computational studies, all of which are reviewed herein, suggest that an unanticipated level of intermediate stabilization occurs via an electrophilic interaction through the pi-molecular orbital as opposed to traditional lone pair directed coordination. We do not postulate that hydrogen bonding to pi-systems is intrinsically stronger than to lone pairs, but rather that there is a greater change in bond strength during deprotonation when the hydrogen bonds are oriented at the pi-system. Through these studies, we conclude that many enzymes preferentially activate their carbon acid substrates through an electrophilic coordination directed towards the pi-bond of the carbonyl rather than the conventional lone pair directed model.
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