SEQUENCE-ALIGNMENT MODELING AND MOLECULAR DOCKING STUDIES OF THE EPOXYGENASE COMPONENT OF ALKENE MONOOXYGENASE FROM NOCARDIA-CORALLINA-B-276

Whole cells of Nocardia corallina B-276 catalyse the stereoselective e poxygenation of alkenes to chiral epoxides. The bacterium expresses an enzyme, alkene monooxygenase, which catalyses the epoxygenation react ion stereoselectively. The enzyme consists of a terminal oxygenase (ep oxygenase), an NADH-dependent reductase (reductase) and a regulatory c omponent (coupling protein). The epoxygenase component contains a brid ged diiron centre similar to that found in the hydroxylase component o f soluble methane monooxygenase. Sequence-alignment modelling, support ed by chemical modification and fluorescence probing, identified a hyd rophobic oxygen/substrate binding site within the epoxygenase. The dii ron centre was coordinated by the two His and two Glu residues from tw o conserved Glu-Xaa-Xaa-His sequences and by two further Glu residues. Molecular docking of substrates and products into the proposed active -site model of the epoxygenase suggested that Ala91 and Ala185 were re sponsible for the stereoselectivity exerted by AMO. It is proposed tha t these residues clamped the intermediate and/or product of the reacti on, thereby controlling the configuration of the epoxide produced. In soluble methane monooxygenase these residues are replaced by two Gly r esidues which do not provide sufficient steric hindrance to prevent ro tation of the intermediate in the active site and, therefore, the prod uct of the reaction catalysed by this enzyme is achiral.