REVERSIBLE BINDING OF PROTONS BY SPINACH PHOTOSYSTEM-2 SUBCHLOROPLASTS AS A FUNCTION OF PH OF THE REACTION MEDIUM

Interaction of protons of the reaction medium with oxygen evolving subchloroplast particles of photosystem (PS) 2 was studied using precision acid-base titration at pH 7.0-3.0. Proton-acceptor groups of PS 2 proteins with pK within this pH range were shown to bind transiently about 600 H+ per reaction centre. Comparison of the experimental titration curves with the theoretically predicted ones and with the number of residues in the sample determined by amino acid analysis indicates that virtually all amino acids residues of PS 2 proteins are accessible to titrant. Diffusion of protons to the binding site took no less than 2 min at pH > 5.5 and was accelerated by a factor of 4 when pH was decreased to 5.0, which can be attributed to low accessibility of the groups with pK 6.2 located in the hydrophobic region of the protein complex. These groups are suggested to be the histidine residues of hydrophobic proteins essential for PS 2 photochemical activity.