Dissecting the Structure-Activity Relationship of Hairpinin, a Plant Derived Antimicrobial Peptide

Abstract: Antibiotic resistance is a growing health issue that necessitates development of alternative drugs with antimicrobial properties. Antimicrobial peptides are a promising group of compounds in this respect and are used by all varieties of living organisms to defend against invading or competing organisms. Hairpinin is an antimicrobial peptide isolated from Echinochloa crus-galli that has previously been found to have antifungal activity. In this study, truncated variants of hairpinin were synthesized and their antifungal activity tested against Candida albicans, Aspergillus fumigatus, and Saccharomyces cerevisiae to identify the minimum structural element of hairpinin required for maintained activity. Hairpinin was active against all three fungi with a minimum inhibitory concentration ranging between 0.6 μM - 5 μM depending on strain and growth media. Two truncated versions were synthesized in this study by solid-phase peptide synthesis, also resulting in a dimer of one of the derivatives, and their antifungal activity was assessed together with four other truncated peptides previously synthesized. The findings indicated that hairpinins C-terminal end together with an inflexible central part stabilized by at least one disulfide bond was vital for activity. The mechanism of action in which hairpinin inhibits fungi was examined by liposome leakage assay of Escherichia coli and Saccharomyces cerevisiae model membranes. It was concluded that the mechanism of action did not involve membrane disruption, a common mechanism among similar antimicrobial peptides. Although hairpinin displayed potent antifungal activity, it was found to be proteolytically unstable in serum. To improve hairpinins value in pharmaceutical context stability has to be improved while preserving the important structural elements.