Abstract
Bicarbonate is known to be a ligand to the non-heme iron located between QA and QB in PSII. It has also been proposed that bicarbonate is a ligand to the Mn cluster and controls O2-evolving reactions. However, details of its structural relationship remain unclarified. In this study, structural coupling of bicarbonate with the Mn cluster was studied using Fourier transform infrared (FTIR) spectroscopy. Flash-induced FTIR difference spectra of S-state cycle were recorded using PSII in the presence of H12CO3- or H13CO3-. In 12C -minus-13C double difference spectra, bicarbonate bands were observed at the 1st flash but no bands were detected at the 2nd-4th flashes. These bicarbonate bands were identical to those in the non-heme iron spectrum measured with Mn-depleted PSII. Thus, no bicarbonate band arising from the S-state cycle was detected in FTIR spectra. This result strongly suggests that bicarbonate is not a ligand to the Mn cluster.
