Electrogenicity of electron and proton transfer at the oxidizing side of photosystem II

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 YZox and QA- (=100%). The electron transfer from YZ to P680+ ranged over a relative distance of 15%, while the one from Mn4 to YZox 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 YZox during S2 --> 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 Xox 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 --> 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.