pH Dependence of the Flash-Induced S-State Transitions in the Oxygen-Evolving Center of Photosystem II from Thermosynechoccocus elongatus as Revealed by Fourier Transform Infrared Spectroscopy

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Abstract
PH dependence of the efficiencies of the flash-induced S-state transitions in the oxygen-evolving center (OEC) was studied by means of Fourier transform infrared (FTIR) difference spectroscopy using photosystem II (PSII) core complexes from the thermophilic cyanabactrium Thermosynechoccocus elongatus. The PSII core complexes dark-adapted at different pHs in the presence of ferricyanide as an electron acceptor were excited by four consecutive saturating laser flashes, and FTIR difference spectra induced by each flash were recorded in the region of 1800−1200 cm-1. Each difference spectrum was fitted with a linear combination of standard spectra measured at pH 6.0, which represent the spectra upon individual S-state transitions, and the transition efficiencies were estimated from the fitting parameters. It was found that the S1 → S2 transition probability is independent of pH throughout the pH region of 3.5−9.5, while the S2 → S3, S3 → S0, and S0 → S1 transition probabilities decrease at acidic pH with pK values of 3.6 ± 0.2, 4.2 ± 0.3, and 4.7 ± 0.5, respectively. These findings, i.e., the pH-independent S1 → S2 transition probability and the pK values for the inhibition in the acidic range of the other three transitions, were in good agreement with recent results obtained by electron paramagnetic resonance measurements for PSII-enriched membranes of spinach [Bernát, G., Morvaridi, F., Feyziyev, Y., and Styring, S. (2002) Biochemistry41, 5830−5843]. On the basis of this correspondence for quite different types of PSII preparations exhibiting marked difference in the pH dependence of the apparent proton release pattern, it is concluded that the inhibition of the S2 → S3, S3 → S0, and S0 → S1 transitions in the acidic region is an inherent property of the OEC. This feature probably reflects proton release from substrate water in these three transitions. On the other hand, all of the S-state transitions remained generally efficient up to pH 9.5 in the alkaline region, except for a slight decrease of the S3 → S0 transition probability above pH 8 (pK ∼ 10). This observation partly differs from the tendency reported for spinach preparations, suggesting that a mechanism different from that in the acidic region is responsible for the transition efficiencies in the alkaline region.