Electrogenicity of electron and proton transfer at the oxidizing side of photosystem II
1997
Haumann, M. | Mulkidjanian, A. | Junge, W.
The electrogenicity of electron and proton transfer at the oxidizing side of PSII was monitored by transmembrane electrochromism of carotenoids in thylakoids and, independently, by electrometry in oxygen-evolving photosystem II core particles. It yielded dielectrically weighted distances between cofactors. They were related to the one between Yz (ox) and QA- (=100%). The electron transfer from Yz to P680+ ranged over a relative distance of 15%, while the one from Mn4 to YZ(ox) ranged over less than 3.5%. The latter result placed Mn4 and Yz at about the same weighted depth in the membrane. The oxidation of cofactor X by YZ(ox) during S2 leads to S3 ranged over 10%. We tentatively attributed 7% to proton transfer into the lumen and 3% to electron transfer, in line with our notion that one proton is liberated from X(ox) itself. This placed X at the same depth in the membrane as Mn. Proton release upon the final oxidation of water during the oxygen-evolving step S4 leads to S0 revealed relative electrogenic components of 5.5% in core particles and between 10.5% (pH 7.4) and 2% (pH 6.2) in thylakoids. The former likely reflected proton transfer from bound water into the lumen and the latter to intraprotein bases that were created in the foregoing transitions. A tentative scheme for the arrangement of cofactors at the oxidizing side of photosystem II is presented.
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