Unraveling the implications of multiple histidine residues in the potent antimicrobial peptide Gaduscidin-1.

The development of antimicrobial peptides (AMPs) as potential therapeutics requires resolving the foundational principles behind their structure-activity relationships. The role of histidine residues within AMPs remains a mystery despite the fact that several potent peptides containing this amino acid are being considered for further clinical development. Gaduscidin-1 (Gad-1) is a potent AMP from Atlantic cod fish that has a total of five His residues. Herein, the role of His residues and metal-potentiated activity of Gad-1 was studied. The five His residues contribute to the broad-spectrum activity of Gad-1. We demonstrated that Gad-1 can coordinate two Cu ions, one at the N-terminus and one at the C-terminus, where the C-terminal binding site is a novel Cu binding motif. High affinity Cu binding at both sites was observed using mass spectrometry and isothermal titration calorimetry. Electron paramagnetic resonance was used to determine the coordination environment of the Cu ions. Cu binding was shown to be responsible for an increase in antimicrobial activity and a new mode of action. Along with the traditional AMP mode of action of pore formation, Gad-1 in the presence of Cu (per)oxidizes lipids. Importantly, His3, His11, His17, and His21 were found to be important to lipid (per)oxidation. This insight will help further understand the inclusion and role of His residues in AMPs, the role of the novel C-terminal binding site, and can contribute to the field of designing potent AMPs that bind metal ions to potentiate activity.