A four-enzyme pathway for 3,5-dihydroxy-4-methylanthranilic acid formation and incorporation into the antitumor antibiotic sibiromycin.

The antitumor antibiotic sibiromycin belongs to the class of pyrrolo[1,4]benzodiazepines (PBDs) that are produced by a variety of actinomycetes. PBDs are sequence-specific DNA-alkylating agents and possess significant antitumor properties. Among them, sibiromycin, one of two identified glycosylated PBDs, displays the highest DNA binding affinity and the most potent antitumor activity. In this study, we report the elucidation of the precise reaction sequence leading to the formation and activation of the 3,5-dihydroxy-4-methylanthranilic acid building block found in sibiromycin, starting from the known metabolite 3-hydroxykynurenine (3HK). The investigated pathway consists of four enzymes, which were biochemically characterized in vitro. Starting from 3HK, the SAM-dependent methyltransferase SibL converts the substrate to its 4-methyl derivative, followed by hydrolysis through the action of the PLP-dependent kynureninase SibQ, leading to 3-hydroxy-4-methylanthranilic acid (3H4MAA) formation. Subsequently the NRPS didomain SibE activates 3H4MAA and tethers it to its thiolation domain, where it is hydroxylated at the C5 position by the FAD/NADH-dependent hydroxylase SibG yielding the fully substituted anthranilate moiety found in sibiromycin. These insights about sibiromycin biosynthesis and the substrate specificities of the biosynthetic enzymes involved may guide future attempts to engineer the PBD biosynthetic machinery and help in the production of PBD derivatives.