Lecture 1 + 2

• Biological membranes


  • Lipids and membrane proteins
• Eukaryotes: each organelle has a unique membrane
• Membranes are dynamic: HIV particle budding (electron micrograph)
• Lipids


  • Membrane building blocks
• Types of lipids


  • Insoluble in water
  • Amphipatic molecules - Hydrophobic tail, Hydrophilic head group
• Functions of lipids


  • Structural functions - Membrane components, Protein modification
  • Metabolic functions - Energy storage
  • Other functions - Cellular signalling, including hormones, Enzyme cofactors, Electron carriers, Pigments
• Unsaturated fats stop close packing
• Glycerophospholipids


  • Major membrane components, Glycerophospholipid head groups
• Sphingolipids


  • Derivatives of the amino alcohol sphingosine, N-acyl fatty-acyl derivatives of sphingosine are called ceramides
• Steroids


  • Mostly of eukaryotic origin, Most common is cholesterol (also known as sterol), Cholesterol is a major component of the plasma membrane
• Biological membranes


  • Define external boundaries of cells/intracellular compartments (eukaryotic cells), Regulate traffic across this boundary, Functions - Signal transduction, Cell communication, Complex reaction sequences, Energy transduction, Special properties - Flexible, Self-sealing/can fuse, Selectively permeable, Two-dimensional
• Membrane: fluid mosaic model - Lipid bilayer (~30-40 Å thick), Lipids are in constant motion, Free lateral diffusion, Almost no unassisted flipping, Membrane proteins also diffuse laterally
• Lipid aggregates - Amphipatic nature of phospholipids critical to the structure of biomembranes, Hydrophobic tails aggregate to exclude water
• Which structure forms is determined by - Size of the fatty acyl chains, Degree of saturation of the fatty acyl chains, Size of hydrophilic head group, Temperature
• Gorter and Grendel (1925) - Red blood cells – estimated the surface area, The membrane was made up of a bilayer
• The membrane was measured to be twice the area estimated for the blood cell, leading to the deduction that it was made up of a bilayer
• Discovery of lipid bilayers involved two compensating errors: no consideration for membrane proteins and not all of the solvent evaporated
• Under the right conditions, phospholipids form a bilayer with two leaflets: outer leaflet and inner leaflet, with a hydrophobic core that is 3-4 nm thick
• Stabilisation of bilayers


  • Aggregation of hydrophobic tails due to the hydrophobic effect
• Study of phospholipid bilayers

  Bilayers can be made in the laboratory from chemically pure lipids, and their properties can then be studied
• Lipid mobility in phospholipid bilayers

  Lipids have two main types of motion: spinning without changing location and lateral diffusion within the same leaflet
• Lipids can diffuse several mm/s at 37°C, giving them a viscosity similar to olive oil, which means that membranes act like fluids
• Heat can disorder the interactions between the fatty acid tails, changing the membrane from a gel to a fluid state
• Lipids determine membrane properties

  • A cell has many types of membranes, each with different properties due to their protein and lipid composition
• Composition determines thickness


  • Sphingomyelin (SM) associates into a thicker, more gel-like bilayer than phospholipids. Cholesterol increases thickness by ordering fatty acid tails and stabilising head group interactions
• Composition and curvature

  • Curvature is determined by the relative size of the head group to the size of the fatty acid tails. Different types of lipids result in different membrane curvatures
• Some aspects of membrane function require curvature


  • Viruses budding, formation of vesicles, stability of curved structures
• Leaflets differ in composition

  • Most membranes have an asymmetric distribution of lipids in their leaflets. For example, human red cells have different lipid compositions in their exoplasmic and cytosolic leaflets
• Two-faced nature of bilayers


  • Two faces can be defined: cytosol-facing and exoplasmic-facing
• Phospholipids distribution in leaflets

  • Cytosolic leaflet - rich in PE/PS/PI
  • Cholesterol is relatively evenly distributed in both leaflets
• Asymmetric distribution of phospholipids between inner and outer leaflets of red blood cell plasma membrane

  • Two-faced nature of bilayers
  • Two faces: cytosolic, exoplasmic
• How do we know phospholipid distribution?

  1. Use enzymes called phospholipases to determine which PLs are on the outside of membranes

  2. Enzymes only remove head-groups exposed on one face of the membrane
• How does asymmetry arise?

  1. Lipids do not spontaneously flip from one leaflet to the other

  2. Specific enzymes catalyse translocations

  3. Examples: Sphingomyelin synthesis in exoplasmic face of Golgi, Glycerophospholipids synthesis on cytosolic face of ER, PC transported to other leaflet by "flippase" enzymes requiring ATP hydrolysis
• Membrane microdomains


  • Control lateral diffusion
  • Example: Stable associations of sphingolipids and cholesterol - lipid rafts
• Membrane microdomains disruptors


  • methyl-3-b-cyclodextrin (removes cholesterol from membranes)
  • antibiotic filipin (sequesters cholesterol)
• Membrane proteins


  • Proteins located in or on the membrane bilayer
  • Different membranes have different composition in terms of lipid:protein ratio
  • Some membranes have more protein than lipid (e.g., bacterial, mitochondrial, chloroplast)
• Functions of membrane proteins

  • Transporters
  • Receptors
  • Adhesion molecules
  • Lipid synthesis
  • Energy transduction (mitochondria, chloroplasts)
  • ...
• Membrane protein:membrane interaction


  Hydrophobic interactions with lipids - bilayer shell (annulus)
• Membrane proteins


  • Aquaporin
  • V-type Na+-ATPase
• Membrane protein structure


  • Structures of transmembrane portions of integral membrane proteins show a limited repertoire
  • Transmembrane alpha-helices are most common
  • Transmembrane beta-sheet (beta-barrel)