Influence of the H-subunit and Fe2+ on electron transport from I- to QA in Fe(2+)-free and/or H-free reaction centers from Rhodobacter sphaeroides R-26.

From reaction centres (RC) of Rhodobacter sphaeroides R-26 two LM preparations with 0.90 Fe2+/RC (LM) and 0.10 Fe2+/RC (LM/dFe) were prepared. Reconstitution of LM/dFe with the H-subunit and subsequently with Zn2+ yielded LMH/dFe and LMH/dFe and LMH/dFe + Zn preparations, respectively. In these four samples the decay of the primary radical pair P+I- was studied by means of transient absorption spectroscopy and compared with that in native RC. In LMH/dFe the reduction of QA by Bpheo a occurred in 5 ns, with concomitant increase in the yield of PT, the triplet state of the primary donor. In the LM/dFe, LM and LMH/dFe + Zn preparations the decay of I- had the same rate (200 ps)-1 as in native RC. Thus, neither the H-subunit in the RC nor a divalent metal as Fe2+ or Zn2+ are necessary per se for fast reduction of QA. Only demetallation in the presence of the H-subunit slows down the reduction of QA.