Fluorescence and Fourier-transform infrared spectroscopic studies on the role of disulfide bond in the calcium binding in the 33 kDa protein of Photosystem II.

The 33 kDa protein of Photosystem II has one intrachain disulfide bond. Fluorescence spectroscopy shows that the major groups in the protein that bind to Ca(2+) should be the carboxylic side groups of glutamic acid and/or aspartic acid. Fluorescence and Fourier-transform infrared (FTIR) spectroscopic studies indicate that the conformation of the 33 kDa protein is altered upon reduction, while the reduced protein still retains the secondary structure. FTIR spectroscopy also shows that the metal ions induce a relative decrease of unordered structure and β-sheet, and a substantial increase of α-helix in both the intact and the reduced 33 kDa protein. This indicates that the addition of cations results in a much more compact structure and that both the intact and the reduced 33 kDa proteins have the ability to bind calcium. The above results may suggest that the disulfide bridge is not essential for calcium binding.