PROTON-TRANSFER TO UBIQUINONE Q(B) IN THE PHOTOSYNTHETIC REACTION-CENTER OF RPS VIRIDIS - THE ROLE OF ELECTROSTATIC INTERACTIONS

Electrostatic calculations of the pK alpha of ionizable groups in the reaction center of Rhodopseudomonas (Rps.) viridis were carried out to investigate three possible mechanisms for proton transfer to the sing ly reduced acceptor ubiquinone Q(B) The program DelPhi, which solves t he Poisson-Boltzmann equation given the distribution of charges and di electric boundaries, was used to determine the electrostatic potential . The shift in pK alpha of the titratable residues in the Q(B) binding packet in response to the one-electron reduction and following proton ation of Q(B) was obtained from calculated interactions with the react ion held, background protein dipoles, charged cofactors, and other ion izable residues. A limited number of bound waters was also included in the computations as titrating sites. Their titration behavior was sho wn to be strongly coupled to neighboring ionizable sites. The results show that strong electrostatic interaction between the radical anion Q (B)(-.) and a neighboring serine residue (SER L 223) as well as the pr otein environment stabilize a system in which the incoming proton is l ocalized on serine and only shared in a hydrogen-bonding relationship with Q(B)(-.) These results hint to the possibility that actual proton transfer to Q(B)(-.) only occurs after a second negative charge has b een added to the system through transfer of a second electron either t o the menaquinone Q(A), with formation of the Q(B)(-.) Q(A)(-.) system , or to Q(B)(-.), leading to the doubly reduced Q(B)(2-.) This preposi tion is consistent with spectroscopical and electron nuclear double re sonance (ENDOR) experimental results for bacterial reaction centers (R CS) failing to find evidence for the existence of the protonated semiq uinone Q(B)H(.). (C) 1993 John Wiley & Sons, Inc.