The NMR Structure of the Pulmonary Surfactant-Associated Polypeptide SP-C in an Apolar Solvent Contains a Valyl-Rich .alpha.-Helix

Abstract

The nuclear magnetic resonance (NMR) structure of the pulmonary surfactant-associated lipoplypeptide C (SP-C) was determined in a mixed solvent of C2H3Cl/C2H3OH/ 1 M HCl 32:64:5 (v/v). Sequence-specific 1H NMR assignments and the collection of conformational constraints were achieved with two-dimensional 1H NMR, and the structure was calculated with the distance geometry program DIANA. The root mean square deviations for the well-defined polypeptide segment of residues 9-34 calculated for the 20 best energy-minimized DIANA conformers relative to their mean are 0.5 and 1.3 A for the polypeptide backbone atoms N, C alpha, and C', and for all heavy atoms, respectively. The 35-residue polypeptide chain of SP-C forms an alpha-helix between positions 9 and 34, which includes two segments of seven and four consecutive valyls that are separated by a single leucyl residue. The N-terminal hexapeptide segment, which includes two palmitoylcysteinyls, is flexibly disordered. The length of the alpha-helix is about 37 A, and the helical segment of residues 13-28, which contains exclusively aliphatic residues with branched side chains, is 23-A long and about 10 A in diameter. The alpha-helix is outstandingly regular, with virtually identical chi 1 angles for all valyl residues. The observation of a helical structure of SP-C was rather unexpected, considering that Val is generally underrepresented in alpha-helices, and it provides intriguing novel insights into the structural basis of SP-C functions as well as into general structural aspects of protein-lipid interactions in biological membranes.

The 21‐residue surfactant peptide (LysLeu4)4Lys(KL4) is a transmembrane α‐helix with a mixed nonpolar/polar surface

“…FTIR spectroscopy revealed that SP-C adopts mainly an ahelical conformation with its a-helix oriented parallel to the lipid acyl chains [11,12]. These restults are supported by those obtained by NMR spectroscopy showing that SP-C in mixed organic solvents forms a very regular a-helix between positions 9 and 34 [13] which is perfectly suited to span a DPPC bilayer [14]. The small sizes of SP-B and SP-C have raised the possibility of producing bioactive analogues, for formulating synthetic peptide-containing surfactant preparations.…”

“…KL4 does accelerate the spreading of lipids at an air/water interface but it is less effective than both native SP-C and the SP-C/BR hybrid in this respect. Perhaps the most prominent differences between KL4 on the one hand and native SP-C and SP-C/BR on the other are that (i) the KL4 helix is shorter (about 30 A, compared to 37,4, in native SP-C [13,14] and presumably also in SP-C/BR with the it is calculated that the helix axis and the all-trans lipid acyl chains make a maximum tilt from a normal to the ATR plate of 27 ° and 20 °, respectively. This means that the KL4 peptide is oriented practically parallel to the lipid acyl chains in a DPPC/PG bilayer.…”

Pulmonary surfactant‐associated polypeptide C in a mixed organic solvent transforms from a monomeric α‐helical state into insoluble β‐sheet aggregates

“…1) and its conversion into an aggregate containing p-sheet secondary structure. The study was stimulated by the observation, made during the determination of the NMR solution structure of native SP-C (Johansson et al, 1994) at 10 "C in the same mixed organic solvent (Fig. l), that within approximately two weeks SP-C undergoes a transformation from the soluble state to a gel-like state for which no NMR signals could be detected.…”

“…For six N-terminal and three C-terminal residues, the H/D exchange was too fast to be registered with 2D N M R spectroscopy, indicating a lower limit of 0.01 min" for the corresponding rate constants (Johansson et al, 1994). To establish a reference value for amide proton exchange rates of random coil structures in the presently used mixed organic solvent, we studied three oligopeptides.…”

“…The intensity of the cross peak volume, I, is plotted versus time. A: Cross-peak between the wproton and one of the 8-methyl groups of Leu35 (Johansson et al, 1994). The decay rate constant, kdec, is indicated.…”