Alkene monooxygenase from Nocardia corallina B-276 is a member of the class of dinuclear iron proteins capable of stereospecific epoxygenation reactions.

Abstract

Nocardia corallina B-276 possesses a constitutive multi-component alkene monooxygenase which catalyses the epoxidation of terminal and sub-terminal alkenes. The epoxygenase component of this system has been purified with an overall yield of 35%. The electron paramagnetic resonance spectrum of the oxidised protein has a weak signal at g = 4.3, which we ascribe to rhombic iron and a free radical signal at g(ave) = 2.01. Upon partial reduction with dithionite using methyl viologen as a mediator, a signal at g(ave) = 1.9 appeared. Upon further reduction with excess dithionite a signal at g = 15 appeared with the concomitant disappearance of the g(ave) = 1.9 signal. These results indicate that the epoxygenase contains a bridged dinuclear iron centre similar to that found in a variety of proteins involved in oxygen transport and activation as well as desaturation of fatty acids. Analysis of the products of the reaction indicates that AMO is capable of stereospecific epoxidation of alkenes producing the R-enantiomer in high yield, a reaction catalysed by very few oxygenase enzymes. Whole cells gave lower enantiomeric excess values for the epoxide and a stereospecific epoxidase enzyme has been proposed to account for this difference. Although alkene monooxygenase was not inhibited by ethyne, a potent inhibitor of soluble methane monooxygenase with which alkene monooxygenase shares many common features, it was weakly inhibited by propyne with an apparent Km value of 340 microM. The mechanistic implications of these physicochemical features of the enzyme are discussed.