INFLUENCE OF PREFORMED ALPHA-HELIX AND ALPHA-HELIX INDUCTION ON THE ACTIVITY OF CATIONIC ANTIMICROBIAL PEPTIDES

One prominent class of cationic antibacterial peptides comprises the a lpha-helical class, which is unstructured in free solution but folds i nto an amphipathic alpha-helix upon insertion into the membranes of ta rget cells. To investigate the importance of alpha-helicity and its in duction on interaction with membranes, a series of peptides was constr ucted based on a hybrid of moth cecropin (amino acids 1-8) and bee mel ittin (amino acids 1-18) peptides. The new peptides were predicted to have a high tendency to form alpha-helices or to have preformed alpha- helices by virtue of construction of a lactam bridge between glutamate and lysine side-chains at positions i and i + 4 at various locations along the primary sequence. In two examples where the use of lactam br idge constraints induced and stabilized alpha-helical structure in ben ign (aqueous buffer) and/or hydrophobic medium, there was a decrease i n antibacterial activity relative to the linear counterparts. Thus the preformation of alpha-helix in solution was not necessarily beneficia l to antimicrobial activity. In the one case where the lactam bridge d id result in increased antibacterial activity (lower minimal inhibitor y concentration values) it did not increase alpha-helical content in b enign or hydrophobic medium. Broadly speaking, good activity of the pe ptides against Pseudomonas aeruginosa correlated best (r(2) = 0.88) wi th a helican parameter which was calculated as the induction of alpha- helix in a membrane-mimicking environment divided by the alpha-helix f ormation under benign conditions. Interestingly, the activity of the l actam bridge peptide constructs correlated in part with alterations in bacterial outer or cytoplasmic membrane permeability..