Mg-protoporphyrin IX monomethyl ester cyclase from Rhodobacter capsulatus: radical SAM-dependent synthesis of the isocyclic ring of bacteriochlorophylls

David Bollivar, Milan Wiesselmann, Stefanie Hebecker, José M Borrero-de Acuña, Manfred Nimtz, Lothar Jänsch, Jürgen Moser, Dieter Jahn

Research output: Journal ArticleArticlepeer-review


During bacteriochlorophyll a biosynthesis, the oxygen-independent conversion of Mg-protoporphyrin IX monomethyl ester (Mg-PME) to protochlorophyllide (Pchlide) is catalyzed by the anaerobic Mg-PME cyclase termed BchE. Bioinformatics analyses in combination with pigment studies of cobalamin-requiring Rhodobacter capsulatus mutants indicated an unusual radical S-adenosylmethionine (SAM) and cobalamin-dependent BchE catalysis. However, in vitro biosynthesis of the isocyclic ring moiety of bacteriochlorophyll using purified recombinant BchE has never been demonstrated. We established a spectroscopic in vitro activity assay which was subsequently validated by HPLC analyses and H218O isotope label transfer onto the carbonyl-group (C-131-oxo) of the isocyclic ring of Pchlide. The reaction product was further converted to chlorophyllide in the presence of light-dependent Pchlide reductase. 
Original languageAmerican English
JournalBiochemical Journal
StatePublished - Dec 2020


  • BchE
  • chlorophyll
  • Mg-protoporphyrin IX monomethyl ester cyclase
  • radical SAM enzyme
  • Rhodobacter capsulatus


  • Life Sciences
  • Biology
  • Microbiology

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