Experimental and computational studies of the effect of lung surfactant protein-B fragment, SP-B₁₋₉, on m lipid bilayers

Jyothini, Abinu Ajithan (2021) Experimental and computational studies of the effect of lung surfactant protein-B fragment, SP-B₁₋₉, on m lipid bilayers. Masters thesis, Memorial University of Newfoundland.

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Abstract

The effect of lung surfactant protein fragment, SP-B₁₋₉, on a model lipid bilayer was investigated. Lung surfactants are lipid-protein complex mixtures that help in reducing the effort needed for breathing by lowering the surface tension that builds up at the alveolar air-liquid interface. Recent research using deuterium NMR to examine the impact of SP-B fragments on bilayer model membranes found that the SP-B fragment, SP-B(1-25, 63-78), had a greater effect on the lipid chain acyl chain orientational order than the fragment, SP-B (8-25, 63- 78). Both of these SP-B fragments include the first and last helices of SP-B, however they vary in the presence or absence of the insertion motif SP-B₁₋₇. This indicates that the insertion motif may contribute to the ability of SP-B to promote the bilayer reorganisation required for lung surfactant function. To gain a better understanding of the insertion motif’s interaction with surfactant lipids, we used deuterium NMR and GROMACS molecular dynamic simulations to examine the effect of SP-B₁₋₉, on the acyl chain order of a lipid bilayer comprised of the lipids DPPC and POPG in a 7:3 ratio. 2H NMR studies with DPPC-d62/POPG (7:3) and SP-B₁₋₉ at peptide-to-lipid ratios of 0.066 and 0.098 revealed no detectable effects in the first moment and order parameter profiles. Even after freezing and thawing the samples, no significant impact of the peptide was detected. On the other hand, MDsimulations of lipid bilayers containing DPPC/POPG(7:3) and SP-B₁₋₉ at a peptide-to-lipid ratio of 0.031 showed a reduction in the acyl chain orientational order of the lipid chain. This difference between the simulation results and the experiments could be due to the aggregation of the peptides in the experiments. MD simulations also reveal the peptide’s average orientation and conformation in the lipid bilayer. It was found that the residues at peptide positions 5, 6, and 7 (Leucine, Proline, and Tyrosine, respectively) had the potential to be part of a helical segment with an average helicity of approximately 45%. The peptide seems to slope into the bilayer, with the last few residues at the N-terminal end remaining horizontal to the bilayer plain, not going closer to the bilayer centre.

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