The translocation mechanism of calcitriol through Helicobacter pylori lipid membrane and influence on water permeability

Helicobacter pylori exhibits a unique membrane lipid composition, including dimyristoyl phosphatidylethanolamine (DMPE) and cholesterol, unlike other Gram-negative bacteria. Calcitriol has antimicrobial activity against H. pylori, but cholesterol enhances antibiotics resistance in H. pylori. This study explored the changes in membrane structure and the molecular mechanisms of cholesterol/calcitriol translocation using well-tempered metadynamics (WT-MetaD) simulations and microsecond conventional molecular dynamics simulations. Our results showed that the average area per lipid and sterol tilt angles were slightly lower, while DP-P, DCG-CG, DAC-AC, and SCD were higher in cholesterol membrane systems than in calcitriol membrane systems. Cholesterol membrane systems were more ordered than calcitriol-containing membranes. Calcitriol facilitated water transport across the membrane, while cholesterol had the opposite effect. The differing effects might result from the tail 25-hydroxyl group and a wider range of orientations of calcitriol in the DMPE/ dimyristoyl phosphatidylglycerol (DMPG) (3:1) membrane. Calcitriol moves across the bilayer center without changing its orientation along the membrane Z-axis, becomes parallel to the membrane surface at the membrane-water interface, and then rotates approximately 90{o} in this interface. The translocation mechanism of calcitriol is quite different from the flip-flop of cholesterol. Moreover, calcitriol crossed from one layer to another more easily than cholesterol, causing successive perturbations to the hydrophobic core and increasing water permeation. These results improve our understanding of the relationship between cholesterol/ calcitriol concentrations and the lipid bilayer structure and the role of lipid composition in water permeation.