Red cell membrane Dr. Suhair Abbas Ahmed

objectives  After studying this lecture you should be able to: 1-List the main functions of the red cell membrane. 2-Outline the composition of the red cell membrane. 3-Differentiate between the integral and peripheral proteins in the red cell membrane. 4-Describe the structure and composition of the red cell cytoskeleton. 5-Outline the ABO and Rh systems.

Red cell membrane  The primary function of the red cell is the transport of respiratory gases to and from the tissues.  To achieve this task the red cell should be capable of traversing the microvascular system without mechanical damage, and that the cell should retain a shape which facilitates gaseous exchange.

Red cell membrane  In order to meet with the demands of function, the red cell membrane should be extremely tough yet highly flexible.  This strength and flexibility of the red cell membrane is due to the design of its protein cytoskeleton and the way the cytoskeleton interacts with the membrane lipid bilayer.

Functions of red cell membrane  To separate the contents of the cell from the plasma.  To maintain the characteristic shape of the red cell.  To regulate intracellular cation conc.  To act as the interface between the cell and its environment via membrane surface receptors.

Red cell membrane  The red cell membrane consists of: Proteins~50% Lipids~40% Carbohydrates~10%

Composition of the red cell membrane CARBOHYDRATES  They occur only on the external surface of the red cell.  They are composed of glycoproteins and glycolipids.

Composition of the red cell PROTEINS  These are either: -Peripheral or -Integral, penetrating the lipid bilayer.

Composition of the RBC memb. (cont.) LIPIDS Lipid components of the red cell membrane are: 30% free unesterified cholesterol. 10% glycerides and free fatty acids. 60% phospholipids

Red cell membrane lipids  All of the lipids associated with red cells is present in the cell membrane.  The mature red cell has no capacity to synthesize lipids, alteration in membrane lipid content can only occur by exchange with plasma lipids.

Lipids (cont)  The red cell membrane phospholipids, are: Phosphatidyl choline, Phosphatidyl ethanolamine, Sphingomyelin and Phosphatidyl serine.  Phospholipid molecules are characterized by a polar head group attached to a non-polar fatty acid tail.  The polar head group is hydrophilic ( water loving), while the fatty acid tail is hydrophobic (water fearing).  Thus the phospholipids in the cell membrane tend to arrange themselves in a bilayer with their hydrophilic heads pointing towards the inner and outer aqueous phases ( the cytoplasm and plasma), while the hydrophobic tails point towards each other.

Lipids (cont)  The membrane cholesterol is unesterified and lies between the two layers of the lipid bilayer.  The concentration of cholesterol in the membrane is an important determinant of membrane surface area and fluidity: an increase in membrane cholesterol leads to an increased surface area and decreased deformability.

Red cell membrane proteins PERIPHERAL PROTEINS  The red cell peripheral proteins interact to form a cytoskeleton.  The cytoskeleton acts as a tough supporting framework for the lipid bilayer.  Four peripheral proteins play a key role in the structure of the red cell cytoskeleton, these are : - Spectrin, which is the most abundant and consists of two chains  and  spectrin. - Ankyrin - Protein 4.1 and - Actin

Red cell membrane proteins SPECTRIN  is the most abundant, consists of two chains,  and , wound around each other to form heterodimers which then self- associate head to head to form tetramers.  These tetramersare linked at the tail end to actin and are attached to protein band 4.1.  At the head end, the -spectrin chains attach to ankyrin which connects to band 3(anion channel). Protein 4.2 enhances this interaction.

Red cell membrane proteins INTEGRAL PROTEINS  These penetrate the lipid bilayer and are firmly anchored within it via interactions with the core. -Band 3 (acts as anion transport channel). -Glycophorins A, B, and C. -Na+/K+ ATPase. -glucose transport protein. -surface receptors. (the most important is the transferrin receptors).

Red cell membrane proteins BAND 3  One of the integral proteins  Has two major functions within the red cell membrane: 1- Its primary function is to facilitate anion transport via the red cell membrane. 2- It is an important binding site for cytoskeletal and other red cell proteins.

Red cell membrane proteins Na + /K + ATPase  This enzyme catalyses the hydrolysis of ATP to ADP, liberating energy in this process.  Each ATP molecule hydrolysed via this system results in the ejection of three Na + ions from the cell and the transport of two K + ions into the cell.

Blood group antigens Blood group antigens: are antigens found on the red cell membrane, they are responsible for the determination of the blood group of the individual.  Blood group antigens are found on both the protein and the carbohydrate components of the membrane glycoproteins and the glycolipids.  The Rh and Fy a antigens are considered as minor membrane proteins.  The MN and Ss are found on larger membrane proteins.  Some blood group antigens are absorbed passively from the plasma, e.g. Lewis, Chido and Rogers.

Defects of the red cell membrane  Defects of the proteins may explain some of the abnormalities of the shape of the red cell membrane, e.g. hereditary spherocytosis and elliptocytosis.  While alterations in lipid composition because of congenital or acquired abnormalities in plasma cholesterol or phospholipids may be associated with other membrane abnormalities, e.g. target cells and acanthocytes.