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

Edited by Joseph JezGreen 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."

A physiological perspective on the origin and evolution of photosynthesis

“…Another radical SAM enzyme, BciD, probably arose later, but it also has a related activity (Thweatt et al. 2017 ). BciD oxidizes the C-7 methyl group of BChl c to form the C-7 formyl group of BChl e , a 4-electron oxidation, by hydroxylating the methyl group twice to form a geminal -diol intermediate, which then apparently dehydrates spontaneously to form the formyl group.…”

Advances in the members and biosynthesis of chlorophyll family

“…BChlide d can be converted to BChlide c by transferring a methyl to the C-20 methine bridge position of BChlide d. The enzyme that catalyzes this step is BchU methyltransferase (Maresca et al 2004, Harada et al 2005. The methyl group at the C-7 position of BChlide c is oxidized by a radical SAM enzyme, BciD, to form the formyl group by which to form BChlide e. BChlide d can be converted to BChlide f through the same reaction by the BciD enzyme (Thweatt et al 2017). The last specific synthase in the biosynthesis of BChl c, d, e, and f is BchK, which adds the farnesyl 'tail' to BChlide c, d, e, and f (Frigaard et al 2002).…”

“…The main pathway of chlorophyll synthesis was discovered in 1948 (Granick 1948). Some chlorophyll synthesis pathways have only been discovered in recent years (Harada et al 2014, Thweatt et al 2017, and some are still unclear. This paper summarizes the research on chlorophyll from the aspects of structure, property, function, distribution, and biosynthesis in order to better understand this complex bio-pigment and photosynthesis.…”

BciC‐Catalyzed C132‐Demethoxycarbonylation of Metal Pheophorbide a Alkyl Esters

“…The methyl group is introduced by a methyltransferase, BchU . BChl e is the C7‐formylated form of BChl c and oxygenation at the C7 1 ‐position is responsible for the BciD enzyme . BChl c , d , and e molecules are differentiated by their C7 or C20 substituents, but have common structural motifs, namely, the presence of a hydroxy group at the C3 1 ‐position and the absence of a methoxycarbonyl group at the C13 2 ‐position, compared with those of other peptide‐binding chlorophyll (Chl) and BChl molecules .…”