Photosynthetic water oxidation in plants and
cyanobacteria is catalyzed by a Mn4CaO5 cluster within the
photosystem II (PSII) protein complex. Two Cl− ions bound
near the Mn4CaO5 cluster act as indispensable cofactors, but
their functional roles remain to be clarified. We have
investigated the role of the Cl− ion interacting with D2-
K317 (designated Cl-1) by Fourier transform infrared
spectroscopy (FTIR) analysis of the D2-K317R mutant of
Synechocystis sp. PCC 6803 in combination with Cl−/NO3
−
replacement. The D2-K317R mutation perturbed the bands in
the regions of the COO− stretching and backbone amide
vibrations in the FTIR difference spectrum upon the S1 → S2 transition. In addition, this mutation altered the 15N isotope-edited
NO3
− bands in the spectrum of NO3
−-treated PSII. These results provide the first experimental evidence that the Cl-1 site is
coupled with the Mn4CaO5 cluster and its interaction is affected by the S1 → S2 transition. It was also shown that a negative band
at 1748 cm−1 arising from COOH group(s) was altered to a positive intensity by the D2-K317R mutation as well as by NO3
−
treatment, suggesting that the Cl-1 site affects the pKa of COOH/COO− group(s) near the Mn4CaO5 cluster in a common
hydrogen bond network. Together with the observation that the efficiency of the S3 → S0 transition significantly decreased in the
core complexes of D2-K317R upon moderate dehydration, it is suggested that D2-K317 and Cl-1 are involved in a proton
transfer pathway from the Mn4CaO5 cluster to the lumen, which functions in the S3 → S0 transition.