The effect of cholesterol on the solution structure of proteins of photosystem II. Protein secondary structure and photosynthetic oxygen evolution

Cholesterol induces large perturbations in the physical properties of membr anes, especially in the structural organization of the phospholipid bilayer s and the aggregation and solubility of proteins at physiological temperatu res. This study was designed to examine the interaction of cholesterol with lipid and proteins of chloroplasts photosystem II (PSII) submembrane fract ions in air dried film at pH 6-7 with cholesterol concentrations of 0.01 to 20 mM. Fourier transform infrared difference spectroscopy with its self-de convolution and second derivative methods as well as curve-fitting procedur es are used, in order to determine the cholesterol binding mode, the protei n conformational changes, and the structural properties of cholesterol-prot ein complexes. Correlations between the effect of cholesterol on the protei n secondary structure and the rate of oxygen evolution in PSII are also est ablished. Spectroscopic evidence showed that at low cholesterol concentrati on (0.01 and 0.1 mM), minor chol-protein and chol-lipid interactions (throu gh hydrogen bonding) occur with no major perturbations of the protein secon dary structure. As cholesterol concentration increases (5 and 10 and 20 mM) , major alterations of the protein secondary structure are observed from th at of the alpha-helix 47% (uncomplexed protein) to 43-39% (complexes) and t he beta-sheet structure 18% (uncomplexed protein) to 22-26% (complexes). Th ose changes coincide with a partial decrease in the rate of the oxygen evol ution (8-33%) is observed in the presence of cholesterol at high concentrat ion. (C) 1999 Academic Press.