Structure of phenoxazinone synthase from Streptomyces antibioticus reveals a new type 2 copper center

The multicopper oxidase phenoxazinone synthase (PHS) catalyzes the penultimate step in the biosynthesis of the antibiotic actinomycin D by Streptomyces antibioticus. PHS exists in two oligomeric forms: a dimeric form and a hexameric form, with older actinomycin-producing cultures containing predominately the hexameric form. The structure of hexameric PHS has been determined using X-ray diffraction to a resolution limit of 2.30 angstrom and is found to contain several unexpected and disctinctive features. The structure forms a hexameric ring that is centered on a pseudo 6-fold axis and has an outer diameter of 185 angstrom with a large central cavity that has a diameter of 50 angstrom. This hexameric structure is stabilized by a Iona loop connecting two domains; bound to this long loop is a fifth copper atom that is present as a type 2 copper. This copper atom is not present in any other multicopper oxidase, and its presence appears to stabilize the hexameric structure.