Kinamycin B

AlpK is an essential monooxygenase involved in the biosynthesis of kinamycin. It catalyzes the C5-hyfroxylattion of the crucial benzo[b]-fluorence intermediate in kinamycin synthesis. However, the structure and mechanism of AlpK is unclear. Here, we report the first structure of AlpK in complex with FAD. Our structure sheds light on the catalytic mechanism of AlpK.

Inactivation of the flavoenzyme-encoding gene flsO1 in fluostatin biosynthesis led to the isolation of four new angucyclinone derivatives (11, 12, 14, and 15), among which fluostarenes A (14) and B (15) featured the unprecedented 6/6/5/6/6 pentacyclic skeleton with fusion of a benzo[b]fluorene and a six-membered lactone ring. Both 14 and 15 were putatively generated via quinone methide-mediated nonenzymatic reactions. Fluostarene B (15) exhibited cytotoxicity against several cancer cell lines with IC50 values ranging from 7 to 10 μM. Fluostarenes A (14), B (15), and PK1 (16) showed α-glucosidase inhibition activity with IC50 of 0.89, 1.58, and 0.13 μM, respectively. Successful complementation of the ΔflsO1 mutant with alpK from kinamycin biosynthesis suggests that FlsO1 should function equivalently to AlpK as a putative C-5 hydroxylase.

The biosynthesis of diazobenzofluorene kinamycins requires a hitherto uncharacterized B-ring contraction. Via detailed genetic and enzymatic analyses we unambiguously characterized the conserved pairs of oxidases, AlpJ and AlpK homologs, as nature's machinery for benzofluorenone formation, which paves the way for the investigation of the following diazo assembly.