Reaction coordinate of P680+o reduction by YZ in PS II core complexes from spinach

Abstract

Photosynthetic water oxidation to molecular oxygen and four protons is energetically driven by the cation radical P680+ that is formed as a result of light induced primary charge separation. Electron transfer from the redox active tyrosine residue YZ of polypeptide D1 leads to rapid reduction of P680+ via surprisingly complex multiphasic kinetics which comprise two components (fast and slow) in the ns time domain. The present study reveals that in untreated isolated PSII core complexes from spinach the normalized extent of ns kinetics is significantly enhanced by addition of Ca2+. This effect is specific for Ca2+ and most pronounced at moderately acidic pH. Although Ca2+ also leads to an increase of the average oxygen yield per flash, a strict correlation does not exist between this parameter and the kinetic pattern of P680 reduction. The activation energy of electron transfer from YZ to P680+ in the ns time domain is of the order of 10 kJ/mol and virtually independent of Ca2+. It is concluded that the electron transfer via the fast ns kinetics is not limited by trigger reactions and thus follows the Marcus-relation for a nonadiabatic redox step. The slower reactions leading to further P680+ reduction are assumed to be triggered by short and long range relaxation processes that take place in the protein matrix with different underlying mechanisms and time constants.