BPh and other prosthetic groups

Reaction centers isolated from the Rhodospirillaceae contain four molecules of BChl, two molecules of BPh, one or two quinones (depending on the isolation procedure), and one atom of nonheme Fe [21, 116]. As mentioned above, the quinones can be either ubiquinone or menaquinone, depending on the species. The Fe can be replaced by Mn, Zn or other metals with only minor effects on photochemical activity [42,117,118]. In reaction centers from Rp. viridis the BChl and 

BPh are BChl b and BPh b in most of the other species that have been characterized, they are BChl a and BPh a. (BChl b differs from BChl a in having a vinyl group on ring II in place of an ethyl group. Thiocapsa pfennigii, another bacterial species that contains BChl b, resembles Rp.viridis in its photochemical activities [119,171].) Reaction centers isolated from Cf. aurantiacus are unusual in having three molecules of BChl a and three of BPh a, instead of four BChls and two BPhs [46,93]. 

The nonheme Fe atom appears to have five ligands two histidine residues of the L subunit, two histidiftes of the M subunit, and a glutamyl residue of M (Fig. 2). The coordination to four histidine nitrogens and the finding that the Fe is not attached directly to either of the quinones are in accord with measurements of the EXAFS spectrum of the Fe [121,122]. 

which is menaquinone-9 in Rp. viridis, is located near the BPh that is attached to the L subunit (BPhL, Fig. 4), but the quinone itself is surrounded mainly by amino acid residues of subunit M. Near the headgroup of the quinone are His M217, which is one of the ligands of the Fe, Trp M250, and a peptide nitro- 

The center-to-center distance from either of the BChls of P to the nearest heme in the cytochrome subunit is about 21 A. A tyrosine residue of the protein sits squarely in the path from the heme to P [102]. Because the complete amino acid sequence of the cytochrome subunit has not yet been fitted to the crystallographic map of the reaction center, it is not clear which two of the four hemes are the low-potential hemes, and which two the high-potential, but information on this point should be available shortly. 

---