The manganese and calcium ions of photosynthetic oxygen evolution.

The profanation of the singly reduced quinones Q::\ and QJj and the doubly reduced quinone Q~is discussed. Q::\ and QJj are not protonated directly in contrast to Q~which forms Q8 H2 • Experimental evidence is presented showing that Q8 H2 leaves the reaction center (RC). It is shown that this process is compatible with the three-dimensional structure of the RC. A model for the profanation involving a chain of protonatable residues connecting Q8 with the aqueous interface was tested by replacing Glu-2I2 of the L subunit with the non-protonatable residue Gin. The rate of profanation, deduced from a cytochrome turnover assay, was greatly reduced in the mutant, implicating Glu-2I2 as a proton donor. Repl<Icing Glu-2 I 2 with the protonatable residue Asp did not significantly affect the profanation rate. From the pH dependence of the electron transfer from Q::\ to Q8 the pK of Glu-2 I 2 was determined to be 9.5.

Current Research in Photosynthesis

Dynamics of photosynthetic membrane composition and function

Chlorophyll a fluorescence induction1

This review article describes the progress that has been made in understanding the photosystem II/oxygen-evolving complex (PSII/OEC). Photosystem II forms the photochemical core of the system: upon light absorption PSII generates oxidizing equivalents or holes at sufficiently high potential to oxidize water. The OEC can exist in five redox states depending on the number of stored oxidizing equivalents. These redox states are designated S{sub 0}-S{sub 4}, with S{sub 4} being the most oxidizing and capable of oxidizing water. The rapid progress in researching this important membrane protein has been due to several factors. First, oxygen evolution has proven to be quite stable to biochemical manipulation. Second, the crystallization of the bacterial reaction center, the realization of the analogies between it and PSII, and the demonstration of the relevance of constructing experiments on the basis of the analogy have provided a shortcut to dissecting the oxygen-evolving system. Finally, because the water-splitting process, like all photosynthetic reactions, can be initiated by light, kinetic studies over the picosecond to second time regime have become possible. The majority of the current work is aimed at determining the structure of the complex.

Water oxidation in photosystem II: from radical chemistry to multielectron chemistry.

Optical characterization of intermediates in the water-splitting enzyme system of photosynthesis — possible states and configurations of manganese and water

An agreement on the quaternary oscillation of ultraviolet absorption changes accompanying the water splitting in isolated Photosystem II complexes from the cyanobacterium Synechococcus sp.

Ö. Saygin

Papers

Optical characterization of intermediates in the water-splitting enzyme system of photosynthesis — possible states and configurations of manganese and water

An agreement on the quaternary oscillation of ultraviolet absorption changes accompanying the water splitting in isolated Photosystem II complexes from the cyanobacterium Synechococcus sp.

Functional Mechanism in Reaction Center II Based on Analysis of 7 Time-Resolved Optical Difference Spectra and Hydroxylamine “Titration”