In photosystem II, light-induced water oxidation occurs at the Mn4CaO5 cluster. Here we demonstrate proton releases, dioxygen formation, and substrate water incorporation in response to Mn4CaO5 oxidation in the protein environment, using a quantum mechanical/molecular mechanical approach and molecular dynamics simulations. In S2, H2O at the W1 site forms a low-barrier H-bond with D1-Asp61. In the S2-to-S3 transition, oxidation of OW1H– to OW1•–, concerted proton transfer from OW1H– to D1-Asp61, and binding of a water molecule Wn-W1 at OW1•– are observed. In S4, Wn-W1 facilitates oxo-oxyl radical coupling between OW1•– and corner μ-oxo O4. Deprotonation via D1-Asp61 leads to formation of OW1=O4. As OW1=O4 moves away from Mn, H2O at W539 is incorporated into the vacant O4 site of the O2-evolved Mn4CaO4 cluster, forming a μ-oxo bridge (Mn3–OW539–Mn4) in an exergonic process. Simultaneously, Wn-W1 is incorporated as W1, recovering the Mn4CaO5 cluster.

We report Em values of (bacterio-)chlorophylls for one-electron reduction in both electron-transfer branches of PbRC, PSI, and PSII.

The cDNA encoding rat cGMP-binding, cGMP-specific phosphodiesterase (cGB-PDE) was isolated from a rat lung cDNA library. Although the deduced amino acid sequence showed 93.4% similarity with that of bovine cGB-PDE, the N-terminal portion of rat cGB-PDE was extremely different from that of bovine. Northern blot analysis indicated that cGB-PDE transcripts in rats were expressed not only in aorta and lung, but also in several other tissues including cerebellum. In situ hybridization analysis demonstrated that cerebellar expression of cGB-PDE was confined to Purkinje cell layers in adult rats. To clarify the role of cGB-PDE in the cerebellum, we investigated expression of cGB-PDE mRNA in rats of various ages. cGB-PDE mRNA was not observed in the cerebellum of newborn rats, but levels of a cGB-PDE mRNA were markedly increased between 4 days and 28 days of age and reached a maximum in eight-week-old rats. In this study, we suggest that cGB-PDE plays important roles not only in regulating the relaxation of vascular vessels, but also in establishing neuronal networks in the cerebellum at an early postnatal stage. In addition the NO/cGMP/cGB-PDE pathway appears to be essential for the induction of long-term depression.

In photosystem II (PSII), water oxidation occurs in the Mn 4 CaO 5 cluster with the release of electrons via the redox-active tyrosine (TyrZ) to the reaction-center chlorophylls (P D1 /P D2 ). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials (E m ) of these cofactors in the PSII protein environment. The E m values suggest that the Mn 4 CaO 5 cluster, TyrZ, and P D1 /P D2 form a downhill electron transfer pathway. E m for the first oxidation step, E m (S 0 /S 1 ), is uniquely low (730 mV) and is ∼100 mV lower than that for the second oxidation step, E m (S 1 /S 2 ) (830 mV) only when the O4 site of the Mn 4 CaO 5 cluster is protonated in S 0 . The O4-water chain, which directly forms a low-barrier H-bond with the Mn 4 CaO 5 cluster and mediates protoncoupled electron transfer in the S 0 to S 1 transition, explains why the second lowest oxidation state, S 1 , is the most stable and S 0 is converted to S 1 even in the dark.

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