title: Thermodynamic limitations of photosynthetic water oxidation at high proton concentrations.
abstract: In oxygenic photosynthesis, solar energy drives the oxidation of water catalyzed by a Mn(4)Ca complex bound to the proteins of Photosystem II. Four protons are released during one turnover of the water oxidation cycle (S-state cycle), implying thermodynamic limitations at low pH. For proton concentrations ranging from 1 nm (pH 9) to 1 mm (pH 3), we have characterized the low-pH limitations using a new experimental approach: a specific pH-jump protocol combined with time-resolved measurement of the delayed chlorophyll fluorescence after nanosecond flash excitation. Effective pK values were determined for low-pH inhibition of the light-induced S-state transitions: pK(1)=3.3 ± 0.3, pK(2)=3.5 ± 0.2, and pK(3)≈pK(4)=4.6 ± 0.2. Alkaline inhibition was not observed. An extension of the classical Kok model facilitated assignment of these four pK values to specific deprotonation steps in the reaction cycle. Our results provide important support to the extended S-state cycle model and criteria needed for assessment of quantum chemical calculations of the mechanism of water oxidation. They also imply that, in intact organisms, the pH in the lumen compartment can hardly drop below 5, thereby limiting the ΔpH contribution to the driving force of ATP synthesis.
authors.affiliation: Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany.
authors.forename: Ivelina
authors.initials: I
authors.lastname: Zaharieva
authors.forename: Jörg M
authors.initials: JM
authors.lastname: Wichmann
authors.forename: Holger
authors.initials: H
authors.lastname: Dau
mesh_terms: D000255:Adenosine Triphosphate / Q000096:biosynthesis / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000502:physiology*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D011522:Protons; D018724:Spinacia oleracea / Q000201:enzymology*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry / Q000378:metabolism*
chemical_list: D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011522:Protons; D014867:Water; D000255:Adenosine Triphosphate