Journal article
Anaerobic chlorophyll isocyclic ring formation in Rhodobacter capsulatus requires a cobalamin cofactor
Departments of Physiology and Chemistry, Carlsberg Laboratory, Gamle Carlsberg Vej 10, 2500 Valby, Denmark. gough@biobase.dk1
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.
Language: | English |
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Publisher: | National Academy of Sciences |
Year: | 2000 |
Pages: | 6908-13 |
ISSN: | 10916490 and 00278424 |
Types: | Journal article |
DOI: | 10.1073/pnas.97.12.6908 |