Photosynthetic water oxidation performed at the MnCaO cluster in photosystem II plays a crucial role in energy production as electron and proton sources necessary for CO fixation. Molecular oxygen, a byproduct, is a source of the oxygenic atmosphere that sustains life on earth. However, the molecular mechanism of water oxidation is not yet well-understood. In the reaction cycle of intermediates called S states, the S → S transition is particularly important; it consists of multiple processes of electron transfer, proton release, and water insertion, and generates an intermediate leading to O-O bond formation. In this study, we monitored the reaction process during the S → S transition using time-resolved infrared spectroscopy to clarify its molecular mechanism. A change in the hydrogen-bond interaction of the oxidized Y radical, an immediate electron acceptor of the MnCaO cluster, was clearly observed as a ∼100 μs phase before the electron-transfer phase with a time constant of ∼350 μs. This observation provides strong experimental evidence that rearrangement of the hydrogen-bond network around Y, possibly due to the movement of a water molecule located near Y to the Mn site, takes place before the electron transfer. The electron transfer was coupled with proton release, as revealed by a relatively high deuterium kinetic isotope effect of 1.9. This proton release, which decreases the redox potential of the MnCaO cluster to facilitate electron transfer to Y, was proposed to determine, as a rate-limiting step, the relatively slow electron-transfer rate of the S → S transition.

Photosynthetic water oxidation takes place at the Mn4CaO5 cluster in photosystem II through a light-driven cycle of intermediates called S states (S0–S4). To unravel the mechanism of water oxidation, it is essential to understand the coupling of electron- and proton-transfer reactions during the S-state transitions. Here, we monitored the reaction process in the S0 → S1 transition using time-resolved infrared (TRIR) spectroscopy. The TRIR signals of the pure contribution of the S0 → S1 transition was obtained by measurement upon a flash after dark adaptation following three flashes. The S0 → S1 traces at the vibrational frequencies of carboxylate groups and hydrogen bond networks around the Mn4CaO5 cluster showed a single phase with a time constant of ∼45 μs. A relatively small H/D kinetic isotope effect of ∼1.2 together with the absence of a slower phase suggests that proton release is coupled with electron transfer, which is a rate-limiting step. The high rate of proton-coupled electron transfer, which is even higher than pure electron transfer in the S1 → S2 transition, is consistent with the previous theoretical prediction that a hydroxo bridge of the Mn4CaO5 cluster gives rise to barrierless deprotonation upon S1 formation through a strongly hydrogen-bonded water molecule.

Tissue accumulation of polymorphonuclear neutrophils (PMN) in Inflammatory Bowel disease (IBD) might be, in part, due to a delay in apoptotic processes associated with the effects of their specific growth factors and inflammatory cytokines. We addressed this hypothesis by examining the activity of granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage CSF (GM-CSF) in the organ culture supernatants of colonic mucosal specimens and their regulatory effects on PMN apoptosis in patients with IBD. The contents of G-CSF and GM-CSF in the supernatants were measured by the enzyme-linked immunosorbent assays and PMN apoptosis was evaluated by acridine orange/ethidium bromide staining, respectively. Mucosal specimens obtained from patients with active IBD exhibited higher levels of G-CSF and GM-CSF activity than controls. Notably, the levels of G-CSF activity were approximately 1000-fold higher than those of GM-CSF activity. Freshly isolated PMN showed a time-related increase in the proportion of cells with characteristic features of apoptosis when they were incubated with the culture medium alone and exposure of PMN to recombinant G-CSF and GM-CSF caused a concentration-dependent inhibition of apoptosis. Incubation of PMN with the supernatants from patients with active IBD induced an inhibitory effect on PMN apoptosis; this effect was abrogated to a significant degree by pre-incubation of the supernatants with anti-G-CSF serum. This study suggests that PMN apoptosis may be delayed under the influence of soluble mediators, especially G-CSF, in the microenvironment of IBD-affected mucosa, thus providing possible mechanisms for tissue accumulation of PMN in IBD.

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