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 B_12 (B_12)-requiring mutants of the bluE and bluB genes of Rhodobacter capsulatus, grown without B_12, 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 B_12- dependent mutants required B_12 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 13~1-radical of MPE. Withdrawal of an electron gives the 13~1-cation of MPE. Hydroxyl ion attack on the cation gives 13~1-hydroxy-MPE. Withdrawal of three hydrogen atoms leads successively to 13~1-keto-MPE, its 13~2-radical, and cyclization to PChlide.