Influence of Mutagenic Replacement of Tryptophan M 250 on Electron Transfer Rates Involving Primary Quinone in Reaction Centers of Rhodobacter capsulatus

The photosynthetic reaction center (RC) of the purple bacteria is a transmembrane pigment-protein complex consisting of three subunits (L, M and H) which catalyze a light-driven charge separation across the bacterial membrane [1–3]. The photochemical reactants include a dimer or “special-pair” of bacteriochlorophylls (comprising the primary electron donor, P), two monomeric bacteriochlorophylls (B), two bacteriopheophytins (H), two quinones (Q) and a single non-heme iron atom (Fe). High resolution X-ray crystal structures of Rhodopseudomonas viridis and Rhodobacter sphaeroides RCs show that the chromophores, which are anchored to the L and M subunits, are arranged in two approximately symmetrical branches [4–6]. Spectroscopic evidence indicates, however, that only one of the pathways (the so-called active, or A-branch) participates in vectorial electron transfer [7–10]. Excitation of P by absorption of a photon leads to the kinetic intermediate P+HA −. The subsequent reaction $$ \rm P^ + {H_A}^-Q_A \to P^ + H_A {Q_A}^ - $$ (1) involves electron transfer over a distance of 14 A and reduction of the primary quinone acceptor, QA [11–15]. The component QA is bound to the RC more tightly than is the secondary acceptor QB [16,33], which accepts an electron from QA −.