DO METAL-IONS ALTER THE PROTEIN SECONDARY STRUCTURE OF A LIGHT-HARVESTING COMPLEX OF THYLAKOID MEMBRANES

We report the effects of Hg(II), Fe(II), and Fe(III) on the solution s tructure of the light-harvesting complex (LHC-II) of chloroplast thyla koid membranes using metal cation concentrations of 0.01, 0.1, 1, 5, 1 0, and 20 mM. FTIR spectroscopy with its self-deconvolution and second derivative resolution enhancement methods and curve-fitting procedure s were applied for quantitative analysis of protein conformational var iations, metal ion binding sites, and protein aggregation in aqueous s olution. A quantitative analysis of protein secondary structure of the free LHC-II complex showed a major alpha-helical structure (48%) with beta-sheet (21%), turn (23%), and beta-antiparallel (8%) as minor com ponents of the protein conformation. Upon metal cation coordination, a major reduction of the alpha-helix was observed at high metal ion con centrations (10-20 mM), with Fe(III) ion inducing a larger perturbatio n of the protein secondary structure. At low metal ion concentrations (0.01 mM), metal-protein binding was negligible, whereas protein aggre gation was predominant. At higher cation concentrations, Fe(II) bindin g was through polypeptide C=O and C-N groups with no participation of the tyrosine residue in protein complexation, while Fe(III) coordinati on was mainly to the peptide carbonyl group with the participation of tyrosine in metal-complex formation. The Hg(II) ion coordination was t hrough protein C=O and C-N groups with major Hg-sulfur binding, while metal-sulfur interaction was negligible for the Fe(II) and Fe(III) com plexes.