Bridging Thermodynamics, Antimicrobial Activity, and pH Sensitivity of Cationic Membranolytic Heptapeptides-A...

Understanding the thermodynamics of peptide:membrane binding and the factors that alter the stability is the key to designing potent and selective small antimicrobial peptides. Here, we report the thermodynamics, antimicrobial activity, and mechanism of a de novo designed seven residue long cationic antimicrobial peptide (P4: NH₃⁺-LKWLKKL-CONH₂, Charge +4) and its analogs (P5: Lysine's → Arginine's; P6: Lysine's → Uncharged-Histidine's; P7: Tryptophan → Leucine) by combining computation and experiments. Computer simulations predicted the order of decreasing peptide binding affinity to the membrane-mimetic systems (micelle/bilayer) as P5 > P4 > P7 ≫ P6. Antimicrobial assays of these peptides against P. aeruginosa and E. coli at physiological pH 7.4 confirmed P5 as the most potent peptide (followed by P4), whereas P6 showed inferior activity. P7 was found to be inactive against E. coli. Substitution of the uncharged-histidine (P6) by the charged-histidine (P6*) significantly favored micelle/bilayer binding. Thus, P6 was predicted to be an effective antimicrobial peptide only at low pH. Noticeable improvement in the antimicrobial activity of the histidine-peptide (P6) against E. coli (an acid-resistant bacteria) upon lowering the pH was demonstrated and validated the computational claim. The peptides displayed a membranolytic mode of action. The link between the structure and calculated energetics (ΔΔG) has been established, and the correlation between the calculated energetics and the antimicrobial activity has been highlighted. The histidine-peptide (P6) is reported to be active against acid-resistant bacteria, thus, a promising membranolytic pH-sensitive AMP.