Structure, design and activity of thanatin analogs: a host defense antimicrobial peptide

Abstract

Several frontline antibiotics are unable to kill Gram-negative bacteria due to the permeability barrier of the outer membrane (OM). Lipopolysaccharide (LPS) at the outer leaflet of the OM establishes an efficient mechanism of trapping antibiotics and antimicrobial agents. Thanatin, a natural AMP is investigated as an alternative therapeutic due to its broad spectrum antibacterial and antifungal activity. However, as shorter peptides are cost-effective and easier to synthesize, this study designed various N-terminal truncated analogs of thanatin that were 16, 15 and 14 residues long to evaluate their structure-activity correlations. All thanatin analogs, were potent displaying Minimal Inhibitory Concentration (MIC) values lesser than 8μM for most of the tested bacteria except for 14mer long peptide. All active peptides showcased the ability to permeate the OM and neutralise surface charge of OM substantially with tight binding to LptAm. Notably, inactive 14 mer peptide shared similar characteristics but demonstrated weak binding to LptAm. However, 16mer peptide lacking a disulfide bond had diminished antimicrobial activity and was ineffective in binding to both LPS and LptAm. Structural elucidation of active thanatin analogs in aqueous solution and in complex with LPS endowed a defined monomeric beta- hairpin structure with a connecting loop. However, significant distortion in the canonical beta- hairpin of the inactive peptides highlights the importance of the disulfide bond and the presence of an extra residue in the N-terminus for effective antimicrobial activity.