Lipids 2 - Membranes and Membrane Transport

Lipids Module 2 - Membranes and Membrane Transport

Glycerophospholipids

  • Most common phospholipid.
  • 1,2-diacylglycerol with a phosphate group esterified at carbon atom 3 of the glycerol backbone.
  • Also known as phosphoglyceride or glycerol phosphatide.
  • Essential components of cell membranes.
  • Phosphatidic acid is an important intermediate in the biosynthesis of glycerophospholipids.

Headgroups of ‘phosphatides’

  • Phosphate and esterified entities form a "head" group.
  • Phosphatides with choline or ethanolamine are phosphatidylcholine (lecithin) or phosphatidylethanolamine, respectively.
  • Common constituents of biological membranes.
  • Other common head groups include glycerol, serine, and inositol.
  • Diphosphatidylglycerol (Cardiolipin) is another glycerol phosphatide.
  • Cardiolipin has a phosphatidylglycerol esterified through the C-1 hydroxyl group.

Membrane Asymmetry

  • Membranes exhibit asymmetric and heterogeneous structures.
  • Lipid distribution varies between outer and inner faces.

Lipid Structures

  • Amphipathic lipids in water form micelles with hydrophobic tails in the center and polar head groups facing outward.
  • Micelles form above the critical micelle concentration (CMC).
  • Micelles are preferred for detergents and soaps.

Fluid Mosaic Model

  • Describes membrane dynamics: Fluid (lipids move) and Mosaic (full of non-lipid components).

Peripheral Membrane Proteins

  • Associate loosely with the membrane via ionic and H-bond interactions.

Transmembrane Proteins

  • Single transmembrane helix spanning proteins, e.g., Glycophorin A.
  • Multi-transmembrane proteins have multiple a-helical segments, e.g., Bacteriorhodopsin.
  • Embedded segments may not always be oriented perpendicular to the membrane plane.

Membrane Anchors

  • Proteins can be anchored by covalently coupled lipids, including thioester-linked fatty acyl anchors and thioether-linked prenyl anchors.

Glycosyl Phosphotidylinositol (GPI) anchors

  • Modify the C-terminus of a target protein via an ethanolamine residue linked to an oligosaccharide.