Pulmonary surfactant-associated polypeptide C in a mixed organic solvent transforms from a monomeric alpha-helical state into insoluble beta-sheet aggregates.
In the 35-residue pulmonary surfactant-associated lipopolypeptide C (SP-C), the stability of the valyl-rich alpha-helix comprising residues 9-34 has been monitored by circular dichroism, nuclear magnetic resonance, and Fourier transform infrared spectroscopy in both a mixed organic solvent and in phospholipid micelles. The alpha-helical form of SP-C observed in freshly prepared solutions in a mixed solvent of CHCl3/CH3OH/0.1 M HCl 32:64:5 (v/v/v) at 10 degrees C undergoes within a few days an irreversible transformation to an insoluble aggregate that contains beta-sheet secondary structure. Hydrogen exchange experiments revealed that this conformational transition proceeds through a transition state with an Eyring free activation enthalpy of about 100 kJ mol(-1), in which the polypeptide segment 9-27 largely retains a helical conformation. In dodecylphosphocholine micelles, the helical form of SP-C was maintained after seven weeks at 50 degrees C. The alpha-helical form of SP-C thus seems to be the thermodynamically most stable state in this micellar environment, whereas its presence in freshly prepared samples in the aforementioned mixed solvent is due to a high kinetic barrier for unfolding. These observations support a previously proposed pathway for in vivo synthesis of SP-C through proteolytic processing from a 21-kDa precursor protein.
Szyperski
T
T
Institut für Molekularbiologie und Biophysik, Eidgenössische Technische Hochschule Hönggerberg, Zürich, Switzerland.
Vandenbussche
G
G
Curstedt
T
T
Ruysschaert
J M
JM
Wüthrich
K
K
Johansson
J
J
D000595:Amino Acid Sequence; D002942:Circular Dichroism; D009682:Magnetic Resonance Spectroscopy; D008823:Micelles; D008969:Molecular Sequence Data; D009930:Organic Chemicals / Q000737:chemistry; D010767:Phosphorylcholine / Q000031:analogs & derivatives / Q000737:chemistry; D011487:Protein Conformation; D017510:Protein Folding; D011510:Proteolipids / Q000737:chemistry*; D011663:Pulmonary Surfactants / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared
D008823:Micelles; D009930:Organic Chemicals; D011510:Proteolipids; D011663:Pulmonary Surfactants; D012997:Solvents; D010767:Phosphorylcholine; C028810:dodecylphosphocholine