Red Blood cell membrane

Red cell membrane The red cell membrane consists of: Proteins~50% Lipids~43% Carbohydrates~7%

Certain blood group Ags are associated with specific membrane structures Many Ags are proteins Rhesus, Duffy, Kidd Others are carbohydrates ABO, Lewis, P Others are a combination of glyolipids & proteins M,N

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.

Majority of monosaccharides on red cell membrane are six carbon sugar In solution the six carbon atoms form a ring The carbons are numbered around the ring from 1 - 6

Seven different monosaccharides are present on the RBC membrane external surface N-acetylneuraminic acid Fucose Glucose Galactose Mannose N-acetylglucosamine N-acetylgalactosamine (-ve charge)

Monosaccharides linkage Monosaccharides are linked together by specific enzymes – glycosyltransferases Each transferase catalyses the transfer of one sugar onto acceptor sugar The enzyme is named according to the sugar that it transfers

Composition of the RBC memb. (cont.) Lipids Lipid components of the red cell membrane are: 60% Phospholipids 30% cholesterol. 10% sphingolipids.

Phospholipids Fatty acid tails Phosphate group head hydrophobic Phosphate group head hydrophilic Arranged as a bilayer Fatty acid

Phospholipid bilayer

Composition of the red cell membrane Proteins These are either: Peripheral (can be removed without disturbing the membrane) Integral, penetrating the lipid bilayer (removed if lipid bilayer is disturbed)-

Integral 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 Ankyrin Protein 4.1 and Actin

Spectrin is the most abundant, consists of two chains, a and b, wound around each other Spectrin gives the cell membrane its flexibility & strength If denatured, red cell takes spherical shape and loses flexibility

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).

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.

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.