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

Abstract

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.