Structure and functions of a dimeric form of surfactant protein SP-C : a Fourier transform infrared and surfactometry study

Abstract Surfactant proteins SP-B ( M r = 8700, reduced) and SP-C ( M r = 3000–6000, major form, non-reduced) interact with surfactant phospholipids to enhance their surface active properties. In the present study, we describe the structural and functional characteristics of a novel dimeric form of bovine SP-C ( M r = 9000, non-reduced), which is identified as [SP-C] 2 . Dimeric SP-C exhibits surface tension-lowering properties differing from those of monomeric SP-C and enhances the surface properties of bovine SP-B/phospholipid mixtures. Chemical analysis indicated that [SP-C] 2 was not acylated at the cysteinyl residues. Fourier transform-infrared spectroscopy (FT-IR) was utilized to determine the secondary structures of [SP-C] 2 in DPPC films. Relative percentages of α-helical, β-sheet, β-turn and random coil structures were calculated by peak fit analysis of the amide I band of the FT-IR spectra indicating that, in contrast to the helical structure of monomeric SP-C, [SP-C] 2 exhibits almost exclusively β-sheet structure. In addition, only 10% of the amide (backbone) hydrogens exchanged with deuterium of D 2 O, indicating that the remaining 90% of amide hydrogens were not accessible to D 2 O due to strong hydrogen bonding or their location in a hydrophobic environment. Dimerization of SP-C effects a major change in secondary structure, a factor which may play a role in the interaction of SP-C with phospholipids in pulmonary surfactant.