The Red Cell Membrane PDF

The red cell membrane By Dr C. Efobi  The red cell is essentially a bag of haemoglobin. It is unique to other eukaryotic cells in the sense that it has no nucleus or cytoplasmic organelles.  An 8 micron cell needs to be able to deform to pass through 3 micron capillari...

The red cell membrane By Dr C. Efobi  The red cell is essentially a bag of haemoglobin. It is unique to other eukaryotic cells in the sense that it has no nucleus or cytoplasmic organelles.  An 8 micron cell needs to be able to deform to pass through 3 micron capillaries/ reticuloendothelial system without fragmentation.  Structure of the red cell membrane include:  Lipid bilayer(40%): 20% phospholipids and 20% cholesterol  Membrane proteins(52%): peripheral or integral  Carbohydrates (8%): occur only on the external surface  Plains of design  Vertical interaction which stabilizes the lipid bilayer membrane  Horizontal interaction which support the integrity Membrane lipids  The membrane lipids consists of phospholipids, cholesterol and glycolipids  The phospholipids are arranged in the form of a bilayer in an asymmetric manner.  Outer layer of the lipid bilayer is made up of phosphatidyl choline and sphingomyelin which are uncharged  The inner layer is made up of phosphatidyl ethanolamine, phosphatidyl inositol and phosphatidyl serine  The asymmetric phospholipid distribution among the lipid bilayer is the result of the function of phospholipid transport proteins:  Flippases:move phospholipids from the outer to the inner monolayer  Floppases: move phospholipids from the inner to the outer monolayer  Maintenance of asymmetric phospholipid distribution in the lipid bilayer is critical for cell integrity and function due to the following reasons:  Macrophages recognize and phagocytose red cells that expose Phosphatidyserine at their outer surface  Premature destruction of thallassemic and sickle red cells has been due to disruption of lipid asymmetry leading to exposure of PS to the outer monolayer  Exposure of PS can initiate adhesion of red cells to endothelial cells, preventing normal transit through the microvasculature  The unesterified cholesterol lies between the two layers of the lipid bilayer.  Note: mature red cells cannot synthesize lipids invivo and hence requires exchange between plasma and membrane lipids.  Exposure of phosphatidyl serine to the outer layer leads to recognition by macrophages and subsequent phagocytosis of the red cell. Membrane proteins  Integral proteins are embedded within the lipid bilayer. They include:  Band 3, glycophorin, Aquaporin.  Peripheralproteins are located on the cytoplasmic surface of the lipid bilayer and constitute the membrane skeleton. They are anchored by the integral proteins and are responsible for membrane elasticity and stability. They include:  Spectrin  Actin  Protein 4.1  Pallidin(band 4.2)  Ankyrin  Adducin  Tropomyosin  Tropomodulin  Proteins of the cell membrane skeleton are responsible for deformability, flexibility and durability of the red cells, allowing it to squeeze through capillaries less than half the diameter of the red blood cell and recovering its discoid shape as soon as it stop receiving compressive forces  There are over 50 membrane proteins and about 25 carry various blood groups such as A, B and Rh antigens.  Proteins involved with vertical connections include:  Betachain of spectrin, ankyrin, band 3, protein 4.2  Proteins involved in horizontal connections include  Alphaand beta spectrin chains, protein 4.1,actin, tropomyosin  These proteins can be separated according to molecular size by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS- PAGE)  Thenetwork of cytoskeletal proteins provides mechanical support and resilience for the red cell membrane, while allowing them be flexible enough to pass through pores smaller than their diameter.eg capillaries, splenic sinusoids.  Thedifferent bands can be visualized by staining with protein stains such as Coomassie blue.  Onelectron microscopy the skeletal proteins appear to be organised in the form of a hexagonal lattice. Functions of the red cell membrane  The membrane provides mechanical strength and flexibility to the red cell to withstand the shearing forces in the circulation.  Servesto maintain red cell volume by cation pump which is maintained by Na/K ATPase. It serves to regulate intracellular concentration of sodium and potassium.  Membrane ATPase also keeps intracellular calcium at a very low level.  Band3 protein which is an anion transporter. It exchanges HCO3 (formed from tissue CO2) in the  Adhesion and interaction with other cells such as endothelial cells.  As signalling receptors  Band 3 is an anion transporter. It defines the Diego blood group  Aquaporin 1: water transporter, defines the colton blood group  Glut 1- glucose and L- dehydroascorbic acid transporter  Kidd antigen protein- Urea transporter  Rh Antigen: gas transporter, probably CO2. defines the Rh blood group and the unusual blood group called Rh null. Biochemistry of the red cell.  The red cell require energy to preserve the integrity of the cell membrane, for active transport of cations, for nucleotide salvage and for synthesis of glutathione.