Proton and hydrogen currents in photosynthetic water oxidation

Abstract

The photosynthetic processes that lead to water oxidation involve an evolution in time from photon dynamics to photochemically-driven electron transfer to coupled electron/proton chemistry. The redox-active tyrosine, Y(Z), is the component at which the proton currents necessary for water oxidation are switched on. The thermodynamic and kinetic implications of this function for Y(Z) are discussed. These considerations also provide insight into the related roles of Y(Z) in preserving the high photochemical quantum efficiency in Photosystem II (PSII) and of conserving the highly oxidizing conditions generated by the photochemistry in the PSII reaction center. The oxidation of Y(Z) by P(680)(+) can be described well by a treatment that invokes proton coupling within the context of non-adiabatic electron transfer. The reduction of Y(.)(Z), however, appears to proceed by an adiabatic process that may have hydrogen-atom transfer character.

No Evidence from FTIR Difference Spectroscopy That Glutamate-189 of the D1 Polypeptide Ligates a Mn Ion That Undergoes Oxidation during the S0 to S1, S1 to S2, or S2 to S3 Transitions in Photosystem II

“…• state by perturbing the network of hydrogen bonds that is believed (17)(18)(19) to facilitate electron transfer from the Mn 4 cluster to Y Z…”

“…• during the higher S state transitions (17)(18)(19). This network is thought to be disrupted by inhibitory treatments that prevent advancement beyond the S 2 Y Z…”

The mechanism for proton–coupled electron transfer from tyrosine in a model complex and comparisons with Y Z oxidation in photosystem II

“…In PSII, the P ϩ/0 680 potential has been estimated to ca. ϩ1.12 V (versus SCE) (see Tommos & Babcock 2000;Diner (2001) and references therein), that is ca. 0.14 V below that for our Ru(bpy) 3ϩ 3 oxidant.…”

“…There is also the question whether Y Z is hydrogen bonded to one or more bases under all conditions (pH, redox state and presence or absence of Mn) and where the Y Z proton goes upon oxidation and from where it comes back, etc. (for recent examples, see Ahlbrink et al 1998;Diner et al 1998;Hays et al 1998Hays et al , 1999Tommos & Babcock 2000;Diner 2001;Rappaport & Lavergne 2001;Renger 2001 and references therein). This discussion was fuelled by a proposal that Y Z is not just an electron-transfer intermediate, but is intimately involved in the water-oxidation mechanism by abstracting hydrogen atoms from water bound to the Mn cluster (Tommos et al 1995;Hogansson & Babcock 1997).…”

“…This discussion was fuelled by a proposal that Y Z is not just an electron-transfer intermediate, but is intimately involved in the water-oxidation mechanism by abstracting hydrogen atoms from water bound to the Mn cluster (Tommos et al 1995;Hogansson & Babcock 1997). In this model, the maintenance of charge neutrality was emphasized (Tommos & Babcock 2000), so as to avoid the energy penalty for creating charges in a protein.…”