Structural Coupling of Extrinsic Proteins with the Oxygen-Evolving Center in Red Algal Photosystem II As Revealed by Light-Induced FTIR Difference Spectroscopy

Effects of binding of extrinsic proteins (PsbO, PsbQ′, PsbV, and PsbU) on the structure of the oxygen-evolving center (OEC) in photosystem II core complexes from a red alga, Cyanidium caldarium, were studied using Fourier transform infrared (FTIR) spectroscopy. S₂-minus-S₁ FTIR difference spectra showed that the protein conformations of the OEC, revealed by the changes in amide I and II bands, were significantly altered upon depletion of all the extrinsic proteins, but mostly recovered when PsbV was rebound with the support of other extrinsic proteins. The recovery of protein conformations correlated well with O₂ evolution activity. This PsbV function of retaining a proper OEC conformation in red algae resembles that of PsbP in higher plants reported previously.