Boar sperm proacrosin infrared investigation: secondary structure analysis after autoactivation and suramin binding

Fourier transform infrared spectroscopy was used to investigate the secondary structure of boar sperm proacrosin at p2H 5.5, to determine the structural changes following protein autoactivation to beta-acrosin at p2H 8.0 and to study the effect of suramin binding on the protein secondary structure. At p2H 5.5, proacrosin contents of alpha-helix, beta-sheet, turns, and unordered structures were estimated to be 9, 49-51, 16-18, and 24%, respectively. At p2H 8.0, the protein was partially insoluble; spectral analysis of the soluble fraction, which contained beta-acrosin, showed an overall secondary structure quite similar to that of proacrosin at p2H 5.5. However, p2H 8.0 spectra of the soluble protein (beta-acrosin), together with the thermal denaturation experiments, indicated that, compared to proacrosin, beta-acrosin showed an increased content of beta-sheets exposed to the solvent as well as a different tertiary structure. Proacrosin/suramin interaction at p2H 5.5 resulted in the formation of soluble and insoluble complexes and the relevant infrared spectra showed only minor differences with respect to the native proacrosin. However, the thermal denaturation curves revealed that suramin induced a destabilization of proacrosin structure. The data also indicated that suramin could modify the interaction characteristics of proacrosin aspartyl and glutamyl residues, thus suggesting competition of suramin with these two residues for ionic interactions.