title: BciD Is a Radical S-Adenosyl-l-methionine (SAM) Enzyme That Completes Bacteriochlorophyllide e Biosynthesis by Oxidizing a Methyl Group into a Formyl Group at C-7.
abstract: Green bacteria are chlorophotorophs that synthesize bacteriochlorophyll (BChl) c, d, or e, which assemble into supramolecular, nanotubular structures in large light-harvesting structures called chlorosomes. The biosynthetic pathways of these chlorophylls are known except for one reaction. Null mutants of bciD, which encodes a putative radical S-adenosyl-l-methionine (SAM) protein, are unable to synthesize BChl e but accumulate BChl c; however, it is unknown whether BciD is sufficient to convert BChl c (or its precursor, bacteriochlorophyllide (BChlide) c) into BChl e (or BChlide e). To determine the function of BciD, we expressed the bciD gene of Chlorobaculum limnaeum strain DSMZ 1677Tin Escherichia coli and purified the enzyme under anoxic conditions. Electron paramagnetic resonance spectroscopy of BciD indicated that it contains a single [4Fe-4S] cluster. In assays containing SAM, BChlide c or d, and sodium dithionite, BciD catalyzed the conversion of SAM into 5'-deoxyadenosine and BChlide c or d into BChlide e or f, respectively. Our analyses also identified intermediates that are proposed to be 71-OH-BChlide c and d Thus, BciD is a radical SAM enzyme that converts the methyl group of BChlide c or d into the formyl group of BChlide e or f This probably occurs by a mechanism involving consecutive hydroxylation reactions of the C-7 methyl group to form a geminal diol intermediate, which spontaneously dehydrates to produce the final products, BChlide e or BChlide f The demonstration that BciD is sufficient to catalyze the conversion of BChlide c into BChlide e completes the biosynthetic pathways for all "Chlorobium chlorophylls."
authors.affiliation: From the Departments of Biochemistry and Molecular Biology and.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Thweatt
authors.affiliation: From the Departments of Biochemistry and Molecular Biology and.
authors.forename: Bryan H
authors.initials: BH
authors.lastname: Ferlez
authors.affiliation: From the Departments of Biochemistry and Molecular Biology and.|Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 and.
authors.forename: John H
authors.initials: JH
authors.lastname: Golbeck
authors.affiliation: From the Departments of Biochemistry and Molecular Biology and dab14@psu.edu.|the Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717.
authors.forename: Donald A
authors.identifier: 0000-0003-2782-5988
authors.initials: DA
authors.lastname: Bryant
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001429:Bacteriochlorophylls / Q000096:biosynthesis* / Q000235:genetics; D019414:Chlorobi / Q000201:enzymology* / Q000235:genetics; D007506:Iron-Sulfur Proteins / Q000235:genetics / Q000378:metabolism*; D008716:Methionine Adenosyltransferase / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D007506:Iron-Sulfur Proteins; D008716:Methionine Adenosyltransferase
keywords: S-adenosylmethionine (SAM); bacteriochlorophyll e; chlorophyll; green sulfur bacteria; iron-sulfur protein; light-harvesting complex (antenna complex); photosynthesis; radical SAM protein