Studies on the proton release pattern of the donor side of system II: Correlation between oxidation and deprotonization of donor D 1 in Tris-washed inside-out thylakoids

Abstract

The protonization pattern of the endogenous donor component D 1 which feeds electrons directly into chl- a + II has been analyzed in Tris-washed inside-out thylakoids with the aid of appropriate pH-indicators. It was found that under repetitive flash excitation the amount of protons released is proportional to the extent of D 1-oxidation, depending on the time between the flashes. The kinetics of D 1-oxidation (being practically the same as in normal Tris-washed chloroplasts) are faster than the proton release by two orders of magnitude. The results lead to the conclusion that D 1 is protonized in the reduced state with p K(D ox 1) < 5 and becomes deprotonized in the oxidized state with p K(D red 1) ≳ 8. The proton release is kinetically limited by a transport barrier. Implications on the interpretation of the proton release pattern in preparation with intact water oxidation are discussed.