PARENT SESSION

Symposium S1A Proton-coupled electon transport and ATPase
Monday August 30th, 2004 10:20 AM-12:20 PM Room 511D
Chair: Colin Wraight
Co-Chair: Wolfgang Junge

Energetics of water oxidation in Photosystem II, coupling between proton and electron transfer. Fabrice Rappaport^*,1, Bruce Diner², Jérôme Lavergne³, Alain Boussac⁴, ¹ Institut de Biologie Physico-Chimique, Paris, France² Experimental Station, E. I. du Pont de Nemours & Co., Wilmington, DE, USA³ UMR 6191 CNRS-CEA-Aix Marseille II, Saint Paul lez Durance, France⁴ Service de Bioénergétique, DBJC, Gif-sur-Yvette, France

ABSTRACT- Oxidizing water into dioxygen requires the accumulation in a single catalytic unit of at least 810 mV at pH 7. This emphasizes the importance of the catalytic mechanisms in tuning the driving force of the different reactions which lead to water splitting. First, recent data on the pathways of charge recombination in photosystem II will be presented and the redox potential of the primary electron donor (P⁺) which determines the available driving force for water splitting will be discussed. Further, the general problem of coupling electron and proton transfer will be discussed from this viewpoint and the interest of the "weak coupling" case with respect to the "strong coupling" one will be illustrated. Weak coupling can be achieved by facilitating deprotonation either before (proton-first path) or after (electron-first path) the oxidation step. The proton-first path is probably relevant to the oxidation of tyrosine YZ by P-680. Histidine D1-190 is believed to play a key role as a proton acceptor facilitating YZ deprotonation. The pKa of an efficient proton acceptor is submitted to conflicting requirements, since a high pKa favors proton transfer from the donor, but also from the medium. H-bonding between YZ and His, together with the Coulombic interaction between negative tyrosinate and positive imidazolium, are suggested to play a decisive role in alleviating these constraints. These theoretical considerations will be further discussed in the light of a recent study of the pH dependence of the rate of P⁺ reduction in PSII reaction centers in which the Tyr residues were uniformly substituted by a Fluorinated Tyr.

KEY WORDS: proton transfer, electron transfer, midpoint potential