SECONDARY STRUCTURE AND BIOPHYSICAL ACTIVITY OF SYNTHETIC ANALOGS OF THE PULMONARY SURFACTANT POLYPEPTIDE SP-C

Native pulmonary-surfactant-associated lipopolypeptide SP-C, its chemi cally depalmitoylated form and several synthetic analogues lacking the palmitoylcysteine residues were analysed for secondary structure in p hospholipid micelles and for biophysical activity in ero-3-phosphochol ine/phosphatidylglycerol/palmitic acid (68:22:9, by wt.). Compared wit h the native molecule, with the entire poly-valyl part in a known alph a-helical conformation, depalmitoylated SP-C was found to be still mai nly alpha-helical, but with an approx. 20% decrease in the helical con tent. A synthetic hybrid polypeptide where the entire poly-valyl alpha -helical part of native SP-C had been replaced with the amino acid seq uence of a transmembrane helix of bacteriorhodopsin is also predominan tly alpha-helical, In contrast, synthetic SP-C analogues lacking only the palmitoyl groups, by replacement of the palmitoylcysteine residues with cysteine, phenylalanine or serine, or lacking the positively cha rged amino acids by replacement with alanine, are considerably less al pha-helical than both native and depalmitoylated SP-C. The data indica te that the SP-C palmitoyl groups are important for maintenance of the alpha-helical conformation in parts of the polypeptide, and that the poly-valyl alpha-helical conformation is not fully formed in synthetic SP-C polypeptides. Furthermore, the helical structure of both native and depalmitoylated SP-C in dodecylphosphocholine micelles is very res istant to thermal denaturation, exhibiting ordered structure at 90 deg rees C. The alpha-helical content grossly parallels the peptide-induce d acceleration of the spreading of phospholipids at an air/water inter face and the increase of surface pressure. The data suggest that the a lpha-helical conformation itself, rather than just the covalent struct ure, is of prime importance for the biological function of synthetic p ulmonary-surfactant peptides.