Kinetic and binding studies with purified recombinant proteins ferredoxin reductase, ferredoxin and cytochrome P450 comprising the morpholine mono‐oxygenase from Mycobacterium sp. strain HE5

The P450mor system from Mycobacterium sp. strain HE5, supposed to catalyse the hydroxylation of different N‐heterocycles, is composed of three components: ferredoxin reductase (FdRmor), Fe3S4 ferredoxin (Fdmor) and cytochrome P450 (P450mor). In this study, we purified Fdmor and P450mor as recombinant proteins as well as FdRmor, which has been isolated previously. Kinetic investigations of the redox couple FdRmor/Fdmor revealed a 30‐fold preference for the NADH‐dependent reduction of nitroblue tetrazolium (NBT) and an absolute requirement for Fdmor in this reaction, compared with the NADH‐dependent reduction of cytochrome c. The quite low Km (5.3 ± 0.3 nm) of FdRmor for Fdmor, measured with NBT as the electron acceptor, indicated high specificity. The addition of sequences providing His‐tags to the N‐ or C‐terminus of Fdmor did not significantly alter kinetic parameters, but led to competitive background activities of these fusion proteins. Production of P450mor as an N‐terminal His‐tag fusion protein enabled the purification of this protein in its spectral active form, which has previously not been possible for wild‐type P450mor. The proposed substrates morpholine, piperidine or pyrrolidine failed to produce substrate‐binding spectra of P450mor under any conditions. Pyridine, metyrapone and different azole compounds generated type II binding spectra and the Kd values determined for these substances suggested that P450mor might have a preference for more bulky and/or hydrophobic molecules. The purified recombinant proteins FdRmor, Fdmor and P450mor were used to reconstitute the homologous P450‐containing mono‐oxygenase, which was shown to convert morpholine.