Crystal structures and low-affinity complex formation of halogenase CtcP and FAD reductase CtcQ from the chlortetracycline biosynthetic pathway

Enzymatic halogenation in natural products has been intensely investigated due to its potential utility as a tool to improve pharmacological and pharmaceutical properties of drug leads. Chlortetracycline (CTC), the first tetracycline (TC) antibiotic discovered nearly eight decades ago, contains a chlorine group. This chlorine is installed enzymatically by the flavin adenine dinucleotide (FAD)-dependent halogenase CtcP. CtcP and the FAD reductase CtcQ, which is also encoded in the CTC biosynthetic gene cluster, function as a two-component system. Structural information on CtcP and CtcQ has been lacking. In this study, we determined crystal structures of CtcP from Kitasatospora aureofaciens in a complex with polyethylene glycol and sulfate ions and in a complex with FAD, and a crystal structure of CtcQ in a complex with FAD and NAD. The structures of CtcP revealed a close similarity of this enzyme to the phenolic halogenase PltM, despite a large difference in the sizes of their respective substrates, presumably TC and phloroglucinol. The CtcP structure showed a conserved dimeric organization also found in PltM crystals. We showed that dimerization of CtcP is allosterically influenced by a distant C-terminal helical hairpin. A closed substrate-binding cavity of CtcP suggested that conformational changes were required to allow a substrate, likely not TC, to bind CtcP. We demonstrated that CtcP and CtcQ weakly bound each other. The dimeric structures of CtcP and CtcQ prompted us to propose approximate models of a 2:2/CtcP:CtcQ complex, where FAD(H₂) would shuttle between the two enzymes for chlorination and reduction.