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
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 1677 T in 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 7 1 -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.”
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