title: Cyanolysis and azidolysis of epoxides by haloalcohol dehalogenase: theoretical study of the reaction mechanism and origins of regioselectivity.
abstract: Haloalcohol dehalogenase HheC catalyzes the reversible dehalogenation of vicinal haloalcohols to form epoxides and free halides. In addition, HheC is able to catalyze the irreversible and highly regioselective ring-opening of epoxides with nonhalide nucleophiles, such as CN (-) and N 3 (-). For azidolysis of aromatic epoxides, the regioselectivity observed with HheC is opposite to the regioselectivity of the nonenzymatic epoxide-opening. This, together with a relatively broad substrate specificity, makes HheC a promising tool for biocatalytic applications. We have designed large quantum chemical models of the HheC active site and used density functional theory to study the reaction mechanism of the HheC-catalyzed ring-opening of ( R)-styrene oxide with the nucleophiles CN (-) and N 3 (-). Both the cyanolysis and the azidolysis reactions are shown to take place in a single concerted step. The results support the suggested role of the putative Ser132-Tyr145-Arg149 catalytic triad, where Tyr145 acts as a general acid, donating a proton to the substrate, and Arg149 interacts with Tyr145 and facilitates proton abstraction, while Ser132 positions the substrate and reduces the barrier for epoxide opening through interaction with the emerging oxyanion of the substrate. We have also studied the regioselectivity of ( R)-styrene oxide opening for both the cyanolysis and the azidolysis reactions. The employed active site model was shown to be able to reproduce the experimentally observed beta-regioselectivity of HheC. In silico mutations of various groups in the HheC active site model were performed to elucidate the important factors governing the regioselectivity.
authors.affiliation: Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, AlbaNova University Center, SE-106 91 Stockholm, Sweden.
authors.forename: Kathrin H
authors.initials: KH
authors.lastname: Hopmann
authors.forename: Fahmi
authors.initials: F
authors.lastname: Himo
mesh_terms: D001386:Azides / Q000378:metabolism*; D001665:Binding Sites; D002384:Catalysis; D019295:Computational Biology; D003486:Cyanides / Q000378:metabolism*; D004852:Epoxy Compounds / Q000737:chemistry* / Q000378:metabolism*; D006867:Hydrolases / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D009154:Mutation; D011789:Quantum Theory; D013237:Stereoisomerism; D013379:Substrate Specificity
chemical_list: D001386:Azides; D003486:Cyanides; D004852:Epoxy Compounds; C013690:styrene oxide; D006867:Hydrolases; C113022:haloalcohol dehalogenase