Open AccessPhotosynthetic oxygen evolution: H/D isotope effects and the coupling between electron and proton transfer during transitions S 2 ⟹ 3 and S 3 ⟹S 4 →S 0
Author(s) -
Bögershausen Oliver,
Haumann Michael,
Junge Wolfgang
Publication year - 1996
Publication title -
berichte der bunsengesellschaft für physikalische chemie
Language(s) - English
Resource type - Journals
ISSN - 0005-9021
DOI - 10.1002/bbpc.19961001210
Subject(s) - electron transfer , chemistry , deprotonation , photosystem ii , electron transport chain , proton coupled electron transfer , redox , proton , thylakoid , protonation , oxygen , kinetic isotope effect , oxygen evolution , acceptor , histidine , crystallography , photochemistry , photosynthesis , inorganic chemistry , electrochemistry , deuterium , chloroplast , atomic physics , ion , physics , condensed matter physics , amino acid , biochemistry , quantum mechanics , organic chemistry , electrode , gene
The oxygen evolving complex (OEC) of photosystem II (PS II) incorporates a tetra Mn‐cluster, tyrosine (Y z ) and probably one histidine residue (X) as redox cofactors. Four quanta of light drive the OEC through the increasingly oxidized states S 0 ⟹S 1 ⟹S 2 ⟹S 3 ⟹S 4 to yield O 2 during S 4 →S 0 . It has been speculated that some oxidized cofactor abstracts hydrogen from bound water. This implies that its oxidoreduction is electroneutral and linked to its deprotonation. To identify such steps we investigated the rates of electron transfer and proton release as function of the D 2 O/H 2 O ratio, the pH, and the temperature in thylakoids and PS II core particles. Upon oxidation of X on S 2 ⟹S 3 , a rise of the pH from 5 to 8 increased the rate of the electron transfer to Y z by a factor of 2.5 and substitution of D 2 O for H 2 O gave an isotopic ratio of 2.1. Contrastingly, during all other transitions, including the O 2 ‐evolving step S 4 →S 0 , the electron transfer rate was much less sensitive to these parameters (factors of ≤ 1.4). These results suggest a kinetical steering role of proton transfer only during S 2 ⟹S 3 . We propose that X * (His * ?) serves as a hydrogen acceptor for bound water during S 4 ⟹S 0 .