Sulfhydryl modifying reagents inhibit Q A (-) oxidation in reaction centers from Rhodobacter sphaeroides and Capsulatus, but not Rhodopseudomonas viridis.

The effects of various sulfhydryl-modifying reagents on reaction centers (RCs) from purple photosynthetic bacteria have been examined, with particular emphasis on the activity of the acceptor quinones, QA and QB, comprising the two electron gate. Mercurial reagents, especially p-chloromercuribenzenesulfonate (pCMBS), were effective in inhibiting QB function in RCs from Rhodobacter sphaeroides and Rb. capsulatus, but not in Rhodopseudomonas viridis. The inhibition was fully reversible by dialysis against dithiothreitol (DTT). The effect on QB function was not an apparent one mediated by an alteration in the redox potential of QA. N-ethylmaleimide (NEM) had no effect on any of the quinone functions, even at very high concentrations. Comparison of the X-ray structures of the RCs from Rb. sphaeroides and Rp. viridis and the known amino acid sequences for all three bacterial RCs suggest that a cysteine residue at position 108 in the L subunit of the Rhodobacter species is the most likely candidate for the site of action of the mercurial reagents. This was strongly supported by the absence of any effect of pCMBS on a site specific mutation of Rb. sphaeroides (L108CS) with residue L108 changed from cysteine to serine. These results imply a long distance (>20 Å) effect on the functioning of QB, perhaps involving a relatively gross structural alteration.