Activity and characterization of a pH-sensitive antimicrobial peptide

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• Antimicrobial peptides (AMPs) are vital for innate immunity, offering selective bacterial targeting and low resistance development.
• Histidine incorporation into the C18G peptide previously abolished its antimicrobial activity, necessitating investigation into the underlying mechanisms.

Purpose of the Study:

• To elucidate the role of pH in the activity and biophysical properties of the histidine-containing peptide C18G.
• To understand how histidine ionization affects peptide behavior and antimicrobial efficacy.

Main Methods:

• Minimal inhibitory concentration (MIC) assays to determine antimicrobial activity at varying pH.
• Trichloroethanol (TCE) quenching and red-edge excitation spectroscopy (REES) to assess peptide aggregation.
• Tryptophan (Trp) fluorescence and Dual Quencher Analysis (DQA) to study peptide-lipid interactions and membrane binding.
• Bacterial membrane permeabilization and vesicle leakage assays to evaluate membrane disruption.

Main Results:

• Antimicrobial activity increased as media pH decreased.
• Peptide aggregation in solution showed a clear pH dependence.
• Histidine ionization state influenced peptide binding and insertion into different lipid bilayers (anionic vs. zwitterionic).
• Membrane permeabilization varied with pH and lipid composition, with enhanced disruption at lower pH for bacterial membranes and neutral pH for specific lipid bilayers.

Conclusions:

• Histidine ionization significantly impacts AMP aggregation and conformational changes upon binding to lipid bilayers.
• Lipid composition and properties play subtle but important roles in AMP-induced membrane pore formation.
• Understanding pH-dependent mechanisms is key for designing effective AMP-based therapeutics.