Do metal ions alter the protein secondary structure of a light-harvesting complex of thylakoid membranes?

Abstract

We report the effects of Hg(II), Fe(II), and Fe(III) on the solution structure of the light-harvesting complex (LHC-II) of chloroplast thylakoid membranes using metal cation concentrations of 0.01, 0.1, 1, 5, 10, and 20 mM. FTIR spectroscopy with its self-deconvolution and second derivative resolution enhancement methods and curve-fitting procedures were applied for quantitative analysis of protein conformation variations, metal ion binding sites, and protein aggregation in aqueous solution. A quantitative analysis of protein secondary structure of the free LHC-II complex showed a major α-helical structure (48%) with β-sheet (21%), turn (23%), and β-antiparallel (8%) as minor components of the protein conformation. Upon metal cation coordination, a major reduction of the α-helix was observed at high metal ion concentrations (10–20 mM), with Fe(III) ion inducing a larger perturbation of the protein secondary structure. At low metal ion concentrations (0.01 mM), metal-protein binding was negligible, whereas protein aggregation was predominant. At higher cation concentrations, Fe(II) binding was through polypeptide CO and CN groups with no participation of the tyrosine residue in protein complexation, while Fe(III) coordination was mainly to the peptide carbonyl group with the participation of tyrosine in metal-complex formation. The Hg(II) ion coordination was through 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) complexes.