Symmetry breaking by enzyme-catalyzed epoxide hydrolysis

The strained epoxide ring is important in organic synthesis, because of its reactions with nucleophiles and ability to be converted to other functional groups of importance in the synthesis of biologically active compounds. Understanding the stereoselectivity of epoxide formation and ring opening is therefore of considerable interest. Work on the formation of chiral epoxides and their application was massively stimulated by the invention of the Sharpless asymmetric epoxidation reaction (Katsuki and Sharpless, 1980). However, detailed mechanistic studies on stereo- and regiochemical aspects of their ring opening are perhaps less well travelled. In a recent article in IUCrJ, Janfalk Carlsson et al. (2018) reveal studies on the factors regulating the enzyme-catalyzed ring opening of methylstyrene oxide by the epoxide hydrolase StEH1. They coupled mutagenesis and structural studies with detailed kinetic analyses, employing both steady-state and presteady- state methods, and modelling to give a detailed picture of the factors regulating the product selectivities obtained with the (S,S)- and (R,R)-methylstyrene oxide substrates. The mechanism of StEH1 proceeds via a covalent intermediate formed by reaction of an active-site aspartyl residue (Asp105) with one of the epoxide carbons to give an enzyme-bound alkoxide intermediate stabilized by hydrogen bonding with the phenol groups of tyrosine residues (Fig. 1). Subsequent ester hydrolysis yields the vicinal diol product. Thus, the reaction proceeds with stereochemical inversion at the epoxide carbon, which is attached by the nucleophilic Asp105. However, the reaction is complicated because attack can occur at either of the two epoxide carbons.