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

Abstract

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 Y-Z(ox) and Q(A)(-) (=100%). The electron transfer from Y-Z to P-680(+) ranged over a relative distance of 15%, while the one from Mn-4 to Y-Z(ox) ranged over less than 3.5%. The latter result placed Mn-4 and Y-Z at about the same weighted depth in the membrane. The oxidation of cofactor X by Y-Z(ox) during S-2 --> S-3 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 S-4 --> S-0 revealed relative electrogenic components of 5.5% in care 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 al the oxidizing side of photosystem II is presented.