THE STRUCTURE OF THE MAMMALIAN ANTIBACTERIAL PEPTIDE CECROPIN-P1 IN SOLUTION, DETERMINED BY PROTON-NMR

Cecropins are peptides with antibacterial activity originally found in insects. Recently a cecropin-type peptide was isolated from pig intestine. This peptide, porcine cecropin Pl, which has 31 amino acid residues and is not amidated in the C-terminus, has been synthesized, purified, and investigated by CD and two-dimensional H-1-NMR at pH 5.0 in aqueous solution with 30% (by vol.) 1,1,1,3,3,3-hexafluoro-2-propanol. All proton resonances have been assigned except for the N-terminal serine. Using constraints derived from NOE connectivities and 3J(NHalpha)-coupling constants, three-dimensional structures have been calculated by means of a distance-geometry program. Some of these structures have been refined by energy minimization and restrained molecular dynamics. The structures reveal an alpha-helix of approximately seven turns along nearly the full length of the peptide. The central part of the helix is very well defined by the NMR constraints. Also the chemical shifts of the alpha protons and the results of CD measurements are in accord with this structure, which is different from the helix-hinge-helix structure earlier found in cecropin A and related peptides. In the alpha-helix of cecropin PI there is a long amphipathic section, of 4 - 5 turns, and a short hydrophobic section of one to two tums, with an intervening Glu-Gly sequence, which is a potential bend-forming section. The helix can easily span a lipid membrane.