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 Ca2+ should be the carboxylic side groups of glutamic ac id and/or aspartic acid. Fluorescence and Fourier-transform infrared ( FTIR) spectroscopic studies indicate that the conformation of the 33 k Da protein is altered upon reduction, while the reduced protein still retains the secondary structure. FITR spectroscopy also shows that the metal ions induce a relative decrease of unordered structure and P-sh eet, and a substantial increase of a-helix in both the intact and the reduced 33 kDa protein. This indicates that the addition of cations re sults in a much more compact structure and that both the intact and th e reduced 33 kDa proteins have the ability to bind calcium. The above results may suggest that the disulfide bridge is not essential for cal cium binding.