Biophysics · MU Sofia

8. The erythrocyte membrane - characteristics

Covers glycoproteins, their composition and attachment to serine, threonine and asparagine, followed by protein classification by function and by position in the membrane. It then describes the fluid-mosaic model with its three features, lipid rafts, and the main methods for studying biomembranes, including TEM, freeze-etching and several other techniques. The erythrocyte membrane closes the note, covering its lipid bilayer, transmembrane proteins such as band 3 and glycophorins, and the spectrin-based cytoskeleton.

Inside: 1 section · 15 flashcards · 2 min

Topic 8 of 30 in the MU Sofia syllabus for Biophysics (Medicine, Year 2). The free AceMed note covers it in 1 section, about 350 words (2 min of reading), with 15 flashcards.

In this note

  1. Membrane proteins and models

Membrane proteins and models

Membrane proteins and glycoproteins - functions, localization and binding. Membrane models and methods of investigation of biomembranes. The erythrocyte membranes.

Glycoproteins are made of amino acids and carbohydrates. They are monosaccharides or short oligosaccharide chains, covalently bonded to serine, threonine & asparagine.

Protein classification by function:

  1. 1. Transport proteins
  2. 2. Structural proteins
  3. 3. Receptor proteins
  4. 4. Enzyme proteins

Protein classification by position and type of connection:

  1. 1. Peripheral
  2. 2. Integral
  3. 3. Lipid-anchored

Membrane model; Fluid-Mosaic model.

  1. 1. The membrane proteins are dispersed and individually inserted into the phospholipid bilayer.
  2. 2. Lipids in the bilayer are in liquid crystalline state.
  3. 3. The two sides of the membrane are asymmetrical.

Other model: lipid rafts. These are small, ordered membrane domains rich in cholesterol and sphingolipids. They gather certain proteins and take part in cell signaling and membrane trafficking.

Methods for investigation the biomembrane structure: Transmission Electron Microscopy (TEM); Freeze-fracture, Freeze-etching Other methods:

  1. 1. X-ray diffraction Analysis (XRD)
  2. 2. Atomic Force Microscopy (AFM)
  3. 3. Electron Spin Resonance (ESR)
  4. 4. Fluorescent Microscopy
  5. 5. Nuclear Magnetic Resonance (NMR)
  6. 6. Differential Scanning Calorimetry (DSC)

The erythrocyte membranes

The red blood cell membrane has three main parts: a lipid bilayer, transmembrane proteins and a cytoskeletal network.

Lipid Bilayer

The bilayer is an asymmetric, two-dimensional liquid crystal. Phosphatidylcholine and sphingomyelin dominate the outer leaflet, phosphatidylserine and phosphatidylethanolamine dominate the inner leaflet, and cholesterol is found in both. Enzymes called flippases help maintain this asymmetry.

Transmembrane Proteins

The main transmembrane proteins are band 3 and the glycophorins. Band 3 is an anion exchanger found as dimers and tetramers, and its cytoplasmic domain anchors the bilayer to the skeleton through ankyrin and spectrin. Glycophorins are single-pass proteins that carry most of the cell's surface sugars, and glycophorin C is a second anchoring point, through protein 4.1. Other proteins include aquaporin (a water channel), the Rh complex and GPI-linked proteins such as CD59.

The Cytoskeleton

The cytoskeleton is a flexible, roughly hexagonal network of spectrin, joined at junctional complexes by actin, protein 4.1 and other proteins. It gives the cell strength and lets it deform as it passes through narrow capillaries.

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