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

Ž . Ž . Four quanta of light, absorbed by Photosystem II PS II , drive the catalytic center of oxygen evolution OEC through w x Ž . Ž . five transitions which are named S ́S to S ́S TMS 1 . Manganese Mn , tyrosine Y and a chemically 0 1 3 4 0 4 Z ill-defined compound, X, serve as redox cofactors. Transient optical absorption spectra of PS II core particles have led us to ) ) y w x propose that the same cofactor X is oxidized on S ́S in controls and on S ́S in Cl -depleted centers 2 . In this 2 3 1 2 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 caused biphasic proton release: the Z Ž . fast component t f35 ms was attributable to electrostatically induced pK-shifts of peripheral amino acid residues. It 1r2 Ž . was transient and disappeared concomittantly with the rise of the slow component t f220 ms that was attributed to 1r2 proton liberation from X itself. The stoichiometric extent of ‘chemical’ proton release per X Ø 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 YrQ. The oxidation of X by Y , proton release and the 10% rise of the transmembrane Z A Z voltage were all characterized by the same half-rise time of 220 ms. We propose that the membrane embedded X, after its oxidation and deprotonation during S ́S , serves as the postulated hydrogen acceptor during the final oxygen evolving 2 3 step S ́S TMS . q 1997 Elsevier Science B.V. 3 4 0