Conformational changes in pediocin AcH upon vesicle binding and approximation of the membrane-bound structure in detergent micelles

Pediocin AcH is a 44-residue antimicrobial peptide with bactericidal potenc y against Gram-positive bacteria such as Listeria. It belongs to a family o f bacteriocins that, when membrane-associated, is predicted to contain P-sh eet and a-helical regions. All bacteriocins in this family have a conserved N-terminal disulfide bond. An additional C-terminal disulfide bond in pedi ocin AcH is thought to confer enhanced potency and broader specificity rang e against sensitive bacteria. The C-terminal disulfide bond may also affect the conformation of the C-terminus. The secondary structures of pediocin A cH in aqueous solution and vesicles from susceptible cells, as well as the ability of trifluoroethanol (TFE) and detergent systems to induce secondary structures like those induced in vesicles, were studied by circular dichro ism (CD) spectroscopy. Like related peptides, pediocin AcH was highly unord ered in aqueous solution, 56%. However, it also contained 20% beta -strand and 15% beta -turn structures. Upon complete binding to vesicles, 32% alpha -helical structure formed, the unordered structure decreased to 32%, and t he beta -strand and beta -turn structures remained largely unchanged. Thus, a, beta alpha domain structure formed in vesicles. The helical structure l ikely forces the C-terminal tail to loop back on the helix so that the C24- C44 disulfide bond can form. Detergent micelles were superior to TFE in the ir ability to induce secondary structural fractions in pediocin AcH compara ble to those observed in vesicles. This demonstrates the importance of a hy drocarbon-water interface to pediocin AcH structure induction and suggests that it is preferable to use detergent micelles as solvents in NMR studies of pediocin AcH structure.