Figure 12.

Synthesis of bacteriochlorophylls c, d, e, and f from chlorophyllide a. Purified BciD is active with both BChlide d and BChlide c as substrates. The conversion of the C7 methyl group to a formyl group proceeds via a geminal diol intermediate that spontaneously dehydrates to produce the formyl group. Esterifying alcohols are added by BchK in all cases. BChls c and d are characteristically found in green-colored green bacteria, whereas BChls e and f are produced by brown-colored green sulfur bacteria. Note that BChl f has not been observed in nature; however, it has been generated by mutation of bchU in C. limnaeum and can still produce functional chlorosomes (see section “Chlorobium Chls: BChls c, d, e, and f” for more details). In green sulfur bacteria, the esterifying alcohols of BChls c, d, e, and f are usually farnesol (F) (see inset, showing a farnesol group attached to ring D of a partial macrocycle). However, in members of the Chloroflexi and C. thermophilum, the esterifying alcohols are often highly variable and are frequently straight-chain alcohols or geranylgeraniol and its reduction products. All alcohols are added by BchK enzymes that are specific to individual organisms. In green sulfur bacteria and some other green bacteria, the ethyl side chain at C8 can be methylated by BchQ to produce propyl, isobutyl, and neopentyl side chains (see inset). Similarly, the methyl group at C12 can be methylated by BchR to produce an ethyl side chain. The asterisk near the hydroxyl group at C3¹ indicates that this is a chiral center that is mostly R but is S when the side chains at C8 and C12 are more extensively methylated. The shaded boxes surrounding the names of some compounds coordinate with other pathway figures and the summary in Fig. 14.