Effects of Including an N-Terminal Insertion Region and Arginine-Mimetic Side Chains in Helical Peptoid Analogues of Lung Surfactant Protein B

Abstract

Surfactant protein B (SP-B) is one of two helical, amphipathic proteins critical for the biophysical functioning of lung surfactant (LS) and hence is an important therapeutic protein. This small, complex 79mer has three internal disulfide bonds and homodimerizes via another disulfide bridge. A helical, amphipathic 25mer from the amino terminus (SP-B(1-25)) exhibits surface-active properties similar to those of full-length, synthetic SP-B. In previous work, we created helical, non-natural mimics of SP-B(1-25) based on sequence-specific peptoid 17mers and demonstrated their biomimetic surface activity. Like SP-B(1-25), the peptoids were designed to adopt helical structures with cationic and nonpolar faces. Here, we compare the surface activities of six different helical peptoid analogues of SP-B(1-25) to investigate the importance of mimicking its N-terminal insertion domain as well as its two arginine residues, both thought to be important for the peptide's proper function. Although the peptoid analogues of SP-B(1-25) studied here share many similar features and all functionally mimic SP-B(1-25) to some degree, it is notable that small differences in their sequences and side chain chemistries lead to substantial differences in their observed interactions with a lipid film. A peptoid comprising a hydrophobic, helical insertion region with aromatic side chains shows more biomimetic surface activity than simpler peptoids, and even better activity, by comparison to natural LS, than SP-B(1-25). However, the substitution of lysine-like side chains for arginine-like side chains in the peptoid has little effect on biomimetic surface activity, indicating that interactions of the guanidino groups with lipids may not be critical for the function of these SP-B mimics.

Functional importance of the NH2-terminal insertion sequence of lung surfactant protein B

“…As to the first point, the formation of stacks as superstructures has been observed before in bulk solutions of lipid discoid bicelles (58). Furthermore NMR experiments with bicelle solutions have shown a strong propensity for spontaneous alignment of bicelles in a magnetic field (52,65,68).…”

“…The remainder of the sequence, residues 10 -25 (LCRALIKRIQAMPIAMIPKG), is an amphipathic helix with the hydrophilic side lined with four cationic residues and the hydrophobic side dominated by alanine, leucine, and isoleucine residues (38). The functional effect of structural motifs found in SP-B 1-25 has been tested by the synthesis of peptoids based on this general design with a hydrophobic, helical insertion region with aromatic side chains, which show comparable biomimetic surface activity to SP-B 1-25 (58).…”

Enantiorecognition ability of peptoids with α-chiral, aromatic side chains

“…[20][21][22][23][24] Peptoids differ from polypeptides in that their side chains are appended to the amide nitrogen rather than to the a-carbons. Peptoids with a-chiral, aromatic side chains can adopt stable helices in organic or aqueous solutions.…”

The Dichotomy Between Understanding and Treating Emphysema

“…The protein with such astounding surface active properties is a 79 residue peptide now called surfactant protein B. What is even more amazing is that subsequent biophysical studies demonstrated that the first 25 amino acids possesses essentially identical surface active properties [58,59] to the whole protein (Figure 3). Further confirmation of the importance of the first domain of surfactant protein B comes from Discovery Labs with their Phase III clinical studies that one dose of (Lys-Leu-Leu-Leu) 4 [60] a mimetic of SP-B 1-25 added to cow lavage dramatically reduces mortality in severely preterm infants [61].…”