Cofactor X of photosynthetic water oxidation, electron transfer, proton release, and electrogenic behaviour in chloride-depleted Photosystem II

Four quanta of light, absorbed by Photosystem II (PS II), drive the catalytic center of oxygen evolution (OEC) through five transitions which are named S-0 double right arrow S-1 to S-3 double right arrow S-4 --> S-0 [1]. Manganese (Mn-4), tyrosine (Y-Z) and a chemically ill-defined compound, X, serve as redox cofactors. Transient optical absorption spectra of PS II core particles have led us to propose that the same cofactor X is oxidized on S-2 double right arrow S-3 in controls and on S-1* double right arrow S-2* in Cl--depleted centers [2]. In this work this particular transition was scrutinized by monitoring UV-transients, proton release and transmembrane electrochromism, both in Cl--depleted and in control thylakoids. The oxidation of X by Y-Z(OX) caused biphasic proton release: the fast component (t(1/2) approximate to 35 mu S) was attributable to electrostatically induced pK-shifts of peripheral amino acid residues, It was transient and disappeared concomittantly with the rise of the slow component (t(1/2) approximate to 220 mu s) that was attributed to proton liberation from X itself. The stoichiometric extent of 'chemical' proton release per X was 1:1. The transfer of a proton from X into the lumen of thylakoids was electrogenic with a relative extent of 10% of the one attributable to the formation of the charge pair Y-Z(OX)/Q(A)(-). The oxidation of X by Y-Z(OX), proton release and the 10% rise of the transmembrane voltage were all characterized by the same half-rise time of 220 mu s. We propose that the membrane embedded X, after its oxidation and deprotonation during S-2 double right arrow S-3, serves as the postulated hydrogen acceptor during the final oxygen evolving step S-3 double right arrow S-4 --> S-0. (C) 1997 Elsevier Science B.V.