title: Cation Effects on the Electron-Acceptor Side of Photosystem II.
abstract: The normal pathway of electron transfer on the electron-acceptor side of photosystem II (PSII) involves electron transfer from quinone A, QA, to quinone B, QB. It is possible to redirect electrons from QA(-) to water-soluble Co(III) complexes, which opens a new avenue for harvesting electrons from water oxidation by immobilization of PSII on electrode surfaces. Herein, the kinetics of electron transfer from QA(-) to [Co(III)(terpy)2](3+) (terpy = 2,2';6',2″-terpyridine) are investigated with a spectrophotometric assay revealing that the reaction follows Michaelis-Menten saturation kinetics, is inhibited by cations, and is not affected by variation of the QA reduction potential. A negatively charged site on the stromal surface of the PSII protein complex, composed of glutamic acid residues near QA, is hypothesized to bind cations, especially divalent cations. The cations are proposed to tune the redox properties of QA through electrostatic interactions. These observations may thus explain the molecular basis of the effect of divalent cations like Ca(2+), Sr(2+), Mg(2+), and Zn(2+) on the redox properties of the quinones in PSII, which has previously been attributed to long-range conformational changes propagated from divalent cations binding to the Ca(II)-binding site in the oxygen-evolving complex on the lumenal side of the PSII complex.
authors.affiliation: †Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States.
authors.forename: Sahr
authors.initials: S
authors.lastname: Khan
authors.affiliation: ‡Department of Molecular, Cellular, and Developmental Biology, Yale University, 219 Prospect Street, New Haven, Connecticut 06520-8114, United States.
authors.forename: Jennifer S
authors.initials: JS
authors.lastname: Sun
authors.affiliation: †Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States.
authors.forename: Gary W
authors.initials: GW
authors.lastname: Brudvig
mesh_terms: D002412:Cations / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D003035:Cobalt / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects; D004583:Electrons; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry / Q000378:metabolism; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism
chemical_list: D002412:Cations; D009942:Organometallic Compounds; D045332:Photosystem II Protein Complex; D003035:Cobalt