Photosystem II of green plants. Oxidation and deprotonation of the same component (histidine?) on S-1(*)double right arrow S-2* 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 oxidizing equivalents to produce molecular oxygen from water. At least two equivalents are stored on the catalytic tetra-manganese cluster. How many and which steps oxidize an amino acid residue instead is under debate. We studied the progression towards higher oxidation states in dark-adapted, chloride-depleted Photosystem II core particles and thylakoids from pea, and monitored from nano- to milliseconds absorption transients from the near-UV into the near-IR spectral regions that directly or by electrochromism reflect the oxidation/reduction of the primary (P-680), secondary (Y-Z) and tertiary electron donors (Mn-4, histidine?). When starting from the first oxidation state and with the cofactor Cl- removed, we found that the OEC stored only two oxidizing equivalents 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 second equivalent was stored on YZ itself. Based on a spectral analysis we propose that the same component X (His?) stores the oxidizing equivalent during the second transition, S-2 double right arrow S-3, in unperturbed material. Our measurements at high time resolution of the concomitant proteolytic reactions led us to propose that during the transition 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 the midpoint potential of Mn-4 relative to X (His?) and Y-Z.