title: Distribution of the cationic state over the chlorophyll pair of the photosystem II reaction center.
abstract: The reaction center chlorophylls a (Chla) of photosystem II (PSII) are composed of six Chla molecules including the special pair Chla P(D1)/P(D2) harbored by the D1/D2 heterodimer. They serve as the ultimate electron abstractors for water oxidation in the oxygen-evolving Mn(4)CaO(5) cluster. Using the PSII crystal structure analyzed at 1.9 Å resolution, the redox potentials of P(D1)/P(D2) for one-electron oxidation (E(m)) were calculated by considering all PSII subunits and the protonation pattern of all titratable residues. The E(m)(Chla) values were calculated to be 1015-1132 mV for P(D1) and 1141-1201 mV for P(D2), depending on the protonation state of the Mn(4)CaO(5) cluster. The results showed that E(m)(P(D1)) was lower than E(m)(P(D2)), favoring localization of the charge of the cationic state more on P(D1). The P(D1)(•+)/P(D2)(•+) charge ratio determined by the large-scale QM/MM calculations with the explicit PSII protein environment yielded a P(D1)(•+)/P(D2)(•+) ratio of ~80/~20, which was found to be due to the asymmetry in electrostatic characters of several conserved D1/D2 residue pairs that cause the E(m)(P(D1))/E(m)(P(D2)) difference, e.g., D1-Asn181/D2-Arg180, D1-Asn298/D2-Arg294, D1-Asp61/D2-His61, D1-Glu189/D2-Phe188, and D1-Asp170/D2-Phe169. The larger P(D1)(•+) population than P(D2)(•+) appears to be an inevitable fate of the intact PSII that possesses water oxidation activity.
authors.affiliation: Career-Path Promotion Unit for Young Life Scientists, Graduate School of Medicine, Kyoto University, Yoshida-Konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Saito
authors.forename: Toyokazu
authors.initials: T
authors.lastname: Ishida
authors.forename: Miwa
authors.initials: M
authors.lastname: Sugiura
authors.forename: Keisuke
authors.initials: K
authors.lastname: Kawakami
authors.forename: Yasufumi
authors.initials: Y
authors.lastname: Umena
authors.forename: Nobuo
authors.initials: N
authors.lastname: Kamiya
authors.forename: Jian-Ren
authors.initials: JR
authors.lastname: Shen
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ishikita
mesh_terms: D000595:Amino Acid Sequence; D002118:Calcium / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D008345:Manganese / Q000737:chemistry*; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000737:chemistry*
chemical_list: D002412:Cations; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D008345:Manganese; D002118:Calcium; D000077194:Chlorophyll A