Hiroshi Ishikita

Learn More
Photosystem II uses light to drive water oxidation and plastoquinone (PQ) reduction. PQ reduction involves two PQ cofactors, Q(A) and Q(B), working in series. Q(A) is a one-electron carrier, whereas Q(B) undergoes sequential reduction and protonation to form Q(B)H(2). Q(B)H(2) exchanges with PQ from the pool in the membrane. Based on the atomic coordinates(More)
In photosynthesis, light is captured by antenna proteins. These proteins transfer the excitation energy with almost 100% quantum efficiency to the reaction centers, where charge separation takes place. The time scale and pathways of this transfer are controlled by the protein scaffold, which holds the pigments at optimal geometry and tunes their excitation(More)
The crystal structure of photosystem II (PSII) at 3.0-A resolution suggests that titratable residues on the lumenal side of D1/D2 and PsbO form a polar channel, which might serve as a proton exit pathway associated with water oxidation on the Mn-cluster. With full account of protein environment, we calculated the pK(a) of these residues by solving the(More)
At the heart of photosynthetic reaction centers (RCs) are pairs of chlorophyll a (Chla), P700 in photosystem I (PSI) and P680 in photosystem II (PSII) of cyanobacteria, algae, or plants, and a pair of bacteriochlorophyll a (BChla), P870 in purple bacterial RCs (PbRCs). These pairs differ greatly in their redox potentials for one-electron oxidation, E(m).(More)
The absence of the PsaC subunit in the photosystem I (PSI) complex (native PSI complex) by mutagenesis or chemical manipulation yields a PSI core (P700-F(X) core) that also lacks subunits PsaD and PsaE and the two iron-sulfur clusters F(A) and F(B), which constitute an integral part of PsaC. In this P700-F(X) core, the redox potentials (E(m)) of the two(More)
In mammals, xanthine oxidoreductase can exist as xanthine dehydrogenase (XDH) and xanthine oxidase (XO). The two enzymes possess common redox active cofactors, which form an electron transfer (ET) pathway terminated by a flavin cofactor. In spite of identical protein primary structures, the redox potential difference between XDH and XO for the flavin(More)
Water oxidation generating atmospheric oxygen occurs in photosystem II (PSII), a large protein-pigment complex located in the thylakoid membrane. The recent crystal structures at 3.2 and 3.5 A resolutions provide novel details on amino acid side chains, especially in the D1/D2 subunits. We calculated the redox potentials for one-electron oxidation of the(More)
Using quantum mechanics/molecular mechanics calculations and the 1.9-Å crystal structure of Photosystem II [Umena Y, Kawakami K, Shen J-R, Kamiya N (2011) Nature 473(7345):55-60], we investigated the H-bonding environment of the redox-active tyrosine D (TyrD) and obtained insights that help explain its slow redox kinetics and the stability of TyrD(•). The(More)
In O(2)-evolving complex Photosystem II (PSII), an unimpeded transfer of electrons from the primary quinone (Q(A)) to the secondary quinone (Q(B)) is essential for the efficiency of photosynthesis. Recent PSII crystal structures revealed the protein environment of the Q(A/B) binding sites. We calculated the plastoquinone (Q(A/B)) redox potentials (E(m)) for(More)
The absolute values of the one-electron redox potentials of the two quinones (Q(A) and Q(B)) in bacterial photosynthetic reaction centers from Rhodobacter sphaeroides were calculated by evaluating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation at pH 7.0. The redox potential for Q(A) was calculated to be between(More)