Adventitious reactions of alkene monooxygenase reveal common reaction pathways and component interactions among...

Alkene monooxygenase (AMO) from Rhodococcus rhodochrous (formerly Nocardia corallina) B-276 belongs to a family of multicomponent nonheme binuclear iron-centre oxygenases that includes the soluble methane monooxygenases (sMMOs) found in some methane-oxidizing bacteria. The enzymes catalyse the insertion of oxygen into organic substrates (mostly hydrocarbons) at the expense of O2 and NAD(P)H. AMO is remarkable in its ability to oxidize low molecular-mass alkenes to their corresponding epoxides with high enantiomeric excess. sMMO and other well-characterized homologues of AMO exhibit two adventitious activities: (1) turnover-dependent inhibition by alkynes and (2) activation by hydrogen peroxide in lieu of oxygen and NAD(P)H (the peroxide shunt reaction). Previous studies of the AMO had failed to detect these activities and opened the possibility that the mechanism of AMO might be fundamentally different from that of its homologues. Thanks to improvements in the protocols for cultivation of R. rhodochrous B-276 and purification and assay of AMO, it has been possible to detect and characterize turnover-dependent inhibition of AMO by propyne and ethyne and activation of the enzyme by hydrogen peroxide. These results indicate a similar mechanism to that found in sMMO and also, unexpectedly, that the enantiomeric excess of the chiral epoxypropane product is significantly reduced during the peroxide shunt reaction. Inhibition of the oxygen/NADH-activated reaction, but not the peroxide shunt, by covalent modification of positively charged groups revealed an additional similarity to sMMO and may indicate very similar patterns of intersubunit interactions and/or electron transfer in both enzyme complexes.