Enzymes that catalyse SN2 reaction mechanisms

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Abstract
Covering: up to the end of 2009 Enzymes have evolved to catalyse reactions, reducing the activation barrier by lowering transition state energy. Many reaction types are represented in enzymology; however, it is noticeable that SN2-type nucleophilic reactions at carbon are not common. The activation barrier of an SN2 reaction is generally high, as it progresses through a trigonal bipyramidal transition state, and this presents a challenge to efficient catalysis. This review summarises those enzyme reactions which almost certainly take place by a SN2 reaction mechanism, although it is recognised that the SN2 terminology, which derives from the bimolecular kinetics of a reaction in solution, is compromised to some extent in enzymes as they all display Michealis–Menten kinetics. Nonetheless, the SN2 terminology is used here to classify enzymes which catalyse nucleophilic reactions at sp3-hybridised carbon. There is a particular focus on highlighting the active-site residues involved in catalysis where known, information that comes primarily from a combination of structural and mutagenesis studies. Predictably, methyl transfer reactions are most widely represented; however, there are a number of enzymes that halogenate/dehalogenate, as well as epoxide hydrolases and inverting sulfatases. Although most of the enzymes have been known for some time, recent advances in structural biology are providing more details on how such enzymes function.

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