Whole‐cell studies of substrate and inhibitor specificity of isoprene monooxygenase and related enzymes

Co‐oxidation of a range of alkenes, dienes, and aromatic compounds by whole cells of the isoprene‐degrading bacterium Rhodococcus sp. AD45 expressing isoprene monooxygenase was investigated, revealing a relatively broad substrate specificity for this soluble diiron centre monooxygenase. A range of 1‐alkynes (C(2)–C(8)) were tested as potential inhibitors. Acetylene, a potent inhibitor of the related enzyme soluble methane monooxygenase, had little inhibitory effect, whereas 1‐octyne was a potent inhibitor of isoprene monooxygenase, indicating that 1‐octyne could potentially be used as a specific inhibitor to differentiate between isoprene consumption by bona fide isoprene degraders and co‐oxidation of isoprene by other oxygenase‐containing bacteria, such as methanotrophs, in environmental samples. The isoprene oxidation kinetics of a variety of monooxygenase‐expressing bacteria were also investigated, revealing that alkene monooxygenase from Xanthobacter and soluble methane monooxygenases from Methylococcus and Methylocella, but not particulate methane monooxygenases from Methylococcus or Methylomicrobium, could co‐oxidise isoprene at appreciable rates. Interestingly the ammonia monooxygenase from the nitrifier Nitrosomonas europaea could also co‐oxidise isoprene at relatively high rates, suggesting that co‐oxidation of isoprene by additional groups of bacteria, under the right conditions, might occur in the environment.