Nishant Mohan Bahuguna

CARDIOLIPIN IN THE BACTERIAL MEMBRANES DETERMINES THE BINDING OF CM15 ANTIMICROBIAL PEPTIDE

In the growing global landscape of antimicrobial resistance, antimicrobial peptides (AMPs) hold promise for the development of novel therapeutics. AMPs are a part of the innate immune system of a large number of animal species and are generally considered to be more resilient to antimicrobial resistance. AMPs target the bacterial inner membrane, which is primarily composed of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin lipids. Although the concentration of cardiolipin depends on the bacterial strain, the role of cardiolipin in modulating the interactions with AMPs is unknown. In this study, we examined the influence of cardiolipin concentration on the insertion-free energy of CM15. CM15 is a positively charged synthetic AMP. It is disordered in aqueous solution, however, upon insertion into the membrane, it acquires a helical structure, causing membrane disruption by introducing defects. We have performed string method free energy calculations for the insertion of a single CM15 in the bacterial inner membrane with varying cardiolipin concentrations. Membrane insertion takes place in three main steps – first unfolding occurs in the extracellular space, followed by membrane binding and insertion below the bilayer headgroups. Subsequent refolding is followed by reorientation of the folded peptide along the lipid tails. The dominant free energy barrier is associated with the insertion of the peptide
below the membrane headgroups. In the absence of cardiolipin, the free energy barrier for the initial membrane insertion is lower since the peptide is able to freely insert below the headgroups. This suggests that cardiolipin plays a key role in protecting bacteria against membrane disruption caused by CM15. Although the increase in cardiolipin content has been implicated in assisting the survival of bacteria in extreme environments, our study suggests that cardiolipin content has broader implications on antimicrobial activity.