Metal-binding ability of Calcitermin, an antimicrobial peptide of human airways

Calcitermin is a 15-mer peptide recently isolated in the human airways [1]. Its sequence (VAIALKAAHYHTHKE) exactly corresponds to the C-terminal domain of Calgranulin C, a calcium-binding protein of the S100 family which have been shown to have antimicrobial properties. While Calcitermin did not show any antimicrobial activity in phosphate buffer at pH 7.4, when pH was lowered to 5.4, it was active against E. coli, P. aeruginosa and C. albicans. In addition, it was demonstrated that zinc improves the antimicrobial activity of Calcitermin against both E. coli and L. monocytogenes [1]. Calcitermin possesses a putative metal-binding domain, containing three histidines separated by one different amino acid. The above results prompted us to deeply investigate complex-formation equilibria of Calcitermin with Zn(II) and Cu(II), two endogenic and competing metal ions. Three peptide analogues, where one His residue has been substituted by an alanine (VAIALKAAAYHTHKE, VAIALKAAHYATHKE, VAIALKAAHYHTAKE) have been also studied, for the sake of comparison, in order to shed light on the role played by each histidine in the sequence. The experiments have been performed in aqueous solution, at 25 °C and I = 0.1 M (KCl), by potentiometry, mass spectrometry and several spectroscopic techniques. The preliminary results show that all the investigated peptides are good ligands for the considered metal ions (see Fig. 1), although copper complexes are far more stable than zinc ones. The presence of three His residues makes wild-type Calcitermin the best ligand among the four peptides. [1] A.M. Cole, Y.-H. Kim, S. Tahk, T. Hong, P. Weis, A.J. Waring, T. Ganz, Calcitermin, a novel antimicrobial peptide isolated from human airway secretions. FEBS Lett. 504 (2001) 5-10.