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

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. BchE activity was stimulated by increasing concentrations of NADPH or SAM, and inhibited by S-adenosylhomocysteine. Subcellular fractionation experiments revealed that membrane-localized BchE requires an additional, heat-sensitive cytosolic component for activity. BchE catalysis was not sustained in chimeric experiments when a cytosolic extract from E. coli was used as a substitute. Size-fractionation of the soluble R. capsulatus fraction indicated that enzymatic activity relies on a specific component with an estimated molecular mass between 3 and 10 kDa. A structure guided site-directed mutagenesis approach was performed on the basis of a three-dimensional homology model of BchE. A newly established in vivo complementation assay was used to investigate 24 BchE mutant proteins. Potential ligands of the [4Fe-4S] cluster (Cys²⁰⁴, Cys²⁰⁸, Cys²¹¹), of SAM (Phe²¹⁰, Glu³⁰⁸ and Lys³²⁰) and of the proposed cobalamin cofactor (Asp²⁴⁸, Glu²⁴⁹, Leu²⁹, Thr⁷¹, Val⁹⁷) were identified.