The electrogenicity of electron and proton transfer at the oxidizing s
ide of PSII was monitored by transmembrane electrochromism of caroteno
ids in thylakoids and, independently, by electrometry in oxygen-evolvi
ng photosystem II core particles. It yielded dielectrically weighted d
istances between cofactors, They were related to the one between Y-Z(o
x) and Q(A)(-) (=100%). The electron transfer from Y-Z to P-680(+) ran
ged 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 cofac
tor X by Y-Z(ox) during S-2 --> S-3 ranged over 10%. We tentatively at
tributed 7% to proton transfer into the lumen and 3% to electron trans
fer, in line with our notion that one proton is liberated from X-ox it
self. This placed X at the same depth in the membrane as Mn. Proton re
lease upon the final oxidation of water during the oxygen-evolving ste
p S-4 --> S-0 revealed relative electrogenic components of 5.5% in car
e 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 fo
regoing transitions, A tentative scheme for the arrangement of cofacto
rs al the oxidizing side of photosystem II is presented.