Cholesterol-Induced Conformational Change in the Sphingomyelin Headgroup

Area of Science:

• Biochemistry
• Biophysics
• Membrane Biology

Background:

• Sphingomyelin (SM) and cholesterol (Cho) are crucial lipids for forming functional membrane domains like lipid rafts.
• The precise interaction between cholesterol and the SM headgroup's conformation remains incompletely understood.

Purpose of the Study:

• To elucidate the effects of cholesterol on the headgroup conformation of sphingomyelin.
• To investigate the structural basis for cholesterol-SM interactions in model lipid bilayers.

Main Methods:

• Solid-state deuterium (²H) NMR spectroscopy on specifically labeled stearoyl-SM (SSM).
• ²H NMR measurements of quadrupolar splitting in SSM-only and SSM/Cho (1:1) bilayers.
• ²H NMR and ¹³C-³¹P REDOR data analysis to determine choline moiety conformation and orientation.
• Comparison with molecular dynamics simulations.

Main Results:

• Cholesterol incorporation altered the conformational ensemble of the SM headgroup, reducing gauche conformations at the Cα-Cβ torsion.
• The presence of cholesterol led to a slight increase in quadrupolar coupling at the α methylene group and significant decreases at Cβ and Cγ.
• These conformational changes suggest an enhanced 'umbrella effect' stabilizing cholesterol within the SM-rich environment.
• Chiral centers in the sphingosine chain were found to induce a slight diastereomeric excess in the SM headgroup conformation.

Conclusions:

• Cholesterol significantly influences sphingomyelin headgroup conformation, favoring a more extended orientation.
• The observed conformational changes likely contribute to the stabilization of cholesterol within lipid rafts.
• Chirality within sphingomyelin plays a subtle role in modulating headgroup conformation.