IDENTIFICATION OF PROTON-TRANSFER PATHWAYS IN THE X-RAY CRYSTAL-STRUCTURE OF THE BACTERIAL REACTION-CENTER FROM RHODOBACTER-SPHAEROIDES

Structural features that have important implications for the fundament al process of transmembrane proton transfer are examined in the recent ly published high resolution atomic structures of the reaction center (RC) from Rhodobacter sphaeroides in the dark adapted state (DQ(A)Q(B) ) and the charged separated state (D(+)Q(A)Q(B)(-)); the latter is the active state for proton transfer to the semiquinone. The structures h ave been determined at 2.2 Angstrom and 2.6 Angstrom resolution, respe ctively, as reported by Stowell et al. (1997) [Science 276: 812-816]. Three possible proton transfer pathways (P1, P2, P3) consisting of wat er molecules and/or protonatable residues were identified which connec t the Q(B) binding region with the cytoplasmic exposed surface at Asp H224 & Asp M240 (P1), Tyr M3 (P2) and Asp M17 (P3). All three represen t possible pathways for proton transfer into the RC. P1 contains an un interrupted chain of water molecules. This path could, in addition, fa cilitate the exchange of quinone for quinol during the photocycle by a llowing water to move into and out of the binding pocket. Located near these pathways is a cluster of electrostatically interacting acid res idues (Asp-L213, Glu-H173, Asp-M17, Asp H124, Asp-L210 and Asp H170) e ach being within 4.5 Angstrom of a neighboring carboxylic acid or a br idging water molecule. This cluster could serve as an internal 'proton reservoir' facilitating fast protonation of Q(B)(-) that could occur at a rate greater than that attainable by proton uptake from solution.