An aromatic hydroxylation reaction catalyzed by a two-component FMN-dependent Monooxygenase. The ActVA-ActVB system from Streptomyces coelicolor.

The ActVA-ActVB system from Streptomyces coelicolor isatwo-component flavin-dependent monooxygenase that belongs to an emerging class of enzymes involved in various oxidation reactions in microorganisms. The ActVB component is a NADH:flavin oxidoreductase that provides a reduced FMN to the second component, ActVA the proper monooxygenase. In this work, we demonstrate that the ActVA-ActVB system catalyzes the aromatic monohydroxylation of dihydrokalafungin by molecular oxygen. In the presence of reduced FMN and molecular oxygen, the ActVA active site accommodates and stabilizes an electrophilic flavin FMN-OOH hydroperoxide intermediate species as the oxidant. Surprisingly, we demonstrate that the quinone form of dihydrokalafungin is not oxidized by the ActVA-ActVB system, whereas the corresponding hydroquinone is an excellent substrate. The enantiomer of dihydrokalafungin, nanaomycin A, as well as the enantiomer of kalafungin, nanaomycin D, are also substrates in their hydroquinone forms. The previously postulated product of the ActVA-ActVB system, the antibiotic actinorhodin, was not found to be formed during the oxidation reaction.