The effects of spermine and spermidine on the structure of photosystem II proteins in relation to inhibition of electron transport.

Polyamines (PAs) are ubiquitous in cells of higher plants and play an important role in many biological functions. Because PAs affect photosynthetic oxygen evolution, this study is designed to investigate the interaction of spermine (Spm) and spermidine (Spd) cations with proteins of photosystem II (PSII) using PSII-enriched submembranes fraction with polyamine concentrations of 0.01-10 mM. Fourier transform infrared (FTIR) difference spectroscopy with its self-deconvolution and second derivative resolution enhancement as well as curve-fitting procedures was applied, in order to determine the cation binding mode, the protein conformational changes and the structural properties of cation-protein complexes. It is shown that at low polyamine concentration, cation-protein interaction (H-bonding) is through the polypeptide C=O groups with no major perturbation of the protein secondary structure. As cation concentration increases, the polyamine complexation causes significant alterations of the protein secondary structure with a decrease of the alpha-helical domains from 47% (uncomplexed PSII) up to 37% (cation complexes) and an increase in the beta-sheet structure from 18% (uncomplexed PSII) up to 29% (cation complexes). Correlations between the effects of polyamines on protein secondary structure and on the rate of oxygen evolution in PSII are also established.