Anaerobic chlorophyll isocyclic ring formation in Rhodobacter capsulatus requires a cobalamin cofactor

摘要

The isocyclic ring of bacteriochlorophyll (BChl) is formed by the conversion of Mg-protoporphyrin monomethyl ester (MPE) to protochlorophyllide (PChlide). Similarities revealed by blast searches with the putative anaerobic MPE-cyclase BchE suggested to us that this protein also uses a cobalamin cofactor. We found that vitamin B12 (B12)-requiring mutants of the bluE and bluB genes of Rhodobacter capsulatus, grown without B12, accumulated Mg-porphyrins. Laser desorption/ionization time-of-flight (LDI-TOF) MS and NMR spectroscopy identified them as MPE and its 3-vinyl-8-ethyl (mvMPE) derivative. An in vivo assay was devised for the cyclase converting MPE to PChlide. Cyclase activity in the B12-dependent mutants required B12 but not protein synthesis. The following reaction mechanism is proposed for this MPE-cyclase reaction. Adenosylcobalamin forms the adenosyl radical, which leads to withdrawal of a hydrogen atom and formation of the benzylic-type 131-radical of MPE. Withdrawal of an electron gives the 131-cation of MPE. Hydroxyl ion attack on the cation gives 131-hydroxy-MPE. Withdrawal of three hydrogen atoms leads successively to 131-keto-MPE, its 132-radical, and cyclization to PChlide.