title: S-state dependence of the calcium requirement and binding characteristics in the oxygen-evolving complex of photosystem II.
abstract: The functional role of the Ca (2+) ion in the oxygen-evolving complex of photosystem II is not yet clear. Current models explain why the redox cycle of the complex would be interrupted after the S 3 state without Ca (2+), but the literature shows that it is interrupted after the S 2 state. Reinterpretation of the literature on methods of Ca (2+) depletion [Miqyass, M., van Gorkom, H. J., and Yocum, C. F. (2007) Photosynth. Res. 92, 275-287] led us to propose that all S-state transitions require Ca (2+). Here we confirm that interpretation by measurements of flash-induced S-state transitions in UV absorbance. The results are explained by a cation exchange at the Ca (2+) binding site that, in the absence of the extrinsic PsbP and PsbQ polypeptides, can occur in minutes in low S-states and in seconds in high S-states, depending on the concentration of the substituting cation. In the S 2(K (+)) or S 2(Na (+)) state a slow conformational change occurs that prevents recovery of the slow-exchange situation on return to a lower S-state but does not inhibit the S-state cycle in the presence of Ca (2+). The ratio of binding affinities for monovalent vs divalent cations increases dramatically in the higher S-states. With the possible exception of S 0 to S 1, all S-state transitions specifically require Ca (2+), suggesting that Ca (2+)-bound H 2O plays an essential role in a H (+) transfer network required for H (+)-coupled electron transfer from the Mn cluster to tyrosine Z.
authors.affiliation: Department of Biophysics, Huygens Laboratory, Leiden University, P.O. Box 9504, NL-2300 RA Leiden, The Netherlands.
authors.forename: Mohamed
authors.initials: M
authors.lastname: Miqyass
authors.forename: Marcell A
authors.initials: MA
authors.lastname: Marosvölgyi
authors.forename: Zachary
authors.initials: Z
authors.lastname: Nagel
authors.forename: Charles F
authors.initials: CF
authors.lastname: Yocum
authors.forename: Hans J
authors.initials: HJ
authors.lastname: van Gorkom
mesh_terms: D001665:Binding Sites; D002104:Cadmium / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002122:Calcium Chloride / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010100:Oxygen / Q000737:chemistry / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000378:metabolism*; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D012965:Sodium Chloride / Q000494:pharmacology; D018724:Spinacia oleracea / Q000378:metabolism
chemical_list: D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002104:Cadmium; D012965:Sodium Chloride; D004533:Egtazic Acid; D011189:Potassium Chloride; D002122:Calcium Chloride; D010100:Oxygen; D002118:Calcium