Practice question

Intrinsic lipid shape contributes substantially to membrane bending energy as described by Helfrich elastic theory and packing parameter concepts. Phospholipids with small headgroup relative to bulky unsaturated tail volume possess negative intrinsic curvature favoring concave bending of monolayer. Phosphatidylethanolamine possesses ethanolamine headgroup substantially smaller than choline of phosphatidylcholine conferring cone shape with packing parameter greater than one, preferring inverted hexagonal HII phases under stress and reducing bilayer stability. Enrichment of PE in cytoplasmic leaflet promotes formation of fusion intermediates like stalks and hemifusion diaphragms required for SNARE-mediated exocytosis, supports high curvature of endocytic buds and cristae junctions stabilized by ATP synthase dimers and OPA1. Phosphatidylethanolamine is generated from phosphatidylserine decarboxylation by phosphatidylserine decarboxylase PSD in mitochondria inner membrane and via CDP-ethanolamine Kennedy pathway in ER. Cholesterol modulates bending rigidity but induces less curvature than PE, sphingomyelin cylindrical shape stabilizes flat domains. Therefore PE distribution is major lipid factor shaping vesicle budding and organelle morphology during trafficking.