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

AbstractFour quanta of light, absorbed by Photosystem II (PS II), drive the catalytic center of oxygen evolution (OEC) through five transitions which are named S0⇒S1 to S3⇒S4→S0[1]. Manganese (Mn4), tyrosine (YZ) 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 S2⇒S3 in controls and on S1*⇒S2* 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 YZox caused biphasic proton release: the fast component (t1/2≈35 μs) 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 (t1/2≈220 μs) 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 YZox/QA−. The oxidation of X by YZox, proton release and the 10% rise of the transmembrane voltage were all characterized by the same half-rise time of 220 μs. We propose that the membrane embedded X, after its oxidation and deprotonation during S2⇒S3, serves as the postulated hydrogen acceptor during the final oxygen evolving step S3⇒S4→S0