The effect of C-terminal deamidation on bacterial susceptibility and resistance to modelin-5

The C-terminal amide carried by antimicrobial peptides (AMPs) can play a variable role in their antibacterial action and here, this role is investigated here for the synthetic peptide modelin-5 (M5-NH₂). The peptide showed potent activity against Pseudomonas aeruginosa (MLC = 5.9 µM), with strong binding to the cytoplasmic membrane (CM) (K_d = 21.5 μM) and the adoption of high levels of amphiphilic α-helical structure (80.1%) which promoted strong CM penetration (9.6 mN m^-1) and CM lysis (89.0%). In contrast, Staphylococcus aureus was resistant to M5-NH₂ (MLC = 139.6 µM), probably due electrostatic repulsion effects mediated by Lys-PG in the organism's CM. These effects promoted weak CM binding (K_d = 120.6 μM) and the formation of low levels of amphiphilic α-helical structure (30.1%), with low levels of CM penetration (4.8 mN m^-1) and lysis (36.4%). C-terminal deamidation had a variable influence on the antibacterial activity of M5-NH₂, and in the case of S. aureus, loss of this structural moiety had no apparent effect on activity. The resistance of S. aureus to M5-NH₂ isoforms appeared to be facilitated by the high level of charge carried by these peptides, as well as the density and distribution of this charge. In the case of P. aeruginosa, the activity of M5-NH₂ was greatly reduced by C-terminal deamidation (MLC = 138.6 µM), primarily through decreased CM binding (K_d = 118.4 μM) and amphiphilic α-helix formation (39.6%) that led to lower levels of CM penetration (5.1 mN m^-1) and lysis (39.0%).