PHOTOSYSTEM-II OF GREEN PLANTS - OXIDATION AND DEPROTONATION OF THE SAME COMPONENT (HISTIDINE-QUESTIONABLE) ON S-1(ASTERISK)DOUBLE-RIGHT-ARROW-S-2-ASTERISK IN CHLORIDE-DEPLETED CENTERS AS ON S-2-DOUBLE-RIGHT-ARROW-S-3 IN CONTROLS

The oxygen-evolving complex (OEC) of green plants accumulates four oxi dizing equivalents to produce molecular oxygen from water. At least tw o equivalents are stored on the catalytic tetra-manganese cluster. How many and which steps oxidize an amino acid residue instead is under d ebate. We studied the progression towards higher oxidation states in d ark-adapted, chloride-depleted Photosystem II core particles and thyla koids from pea, and monitored from nano- to milliseconds absorption tr ansients from the near-UV into the near-IR spectral regions that direc tly or by electrochromism reflect the oxidation/reduction of the prima ry (P-680), secondary (Y-Z) and tertiary electron donors (Mn-4, histid ine?). When starting from the first oxidation state and with the cofac tor Cl- removed, we found that the OEC stored only two oxidizing equiv alents if the cofactor Cl- had been removed. The first equivalent was passed via Y-Z to an as yet chemically ill-defined component, X, that resembled histidine according to its UV/VIS difference spectrum. The s econd equivalent was stored on YZ itself. Based on a spectral analysis we propose that the same component X (His?) stores the oxidizing equi valent during the second transition, S-2 double right arrow S-3, in un perturbed material. Our measurements at high time resolution of the co ncomitant proteolytic reactions led us to propose that during the tran sition X(red) --> X(ox) a proton is ejected into the aqueous phase. We interpret these data in terms of an electrostatic control by Cl- of t he midpoint potential of Mn-4 relative to X (His?) and Y-Z.