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 proteinsRhesus, Duffy, KiddOthers are carbohydratesABO, Lewis, POthers are a combination of glyolipids & proteinsM,N

Composition of the red cell membraneCarbohydrates

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 sugarIn solution the six carbon atoms form a ringThe carbons are numbered around the ring from 1 - 6

Seven different monosaccharides are present on the RBC membrane external surfaceN-acetylneuraminic acidFucoseGlucoseGalactoseMannoseN-acetylglucosamineN-acetylgalactosamine (-ve charge)

Monosaccharides linkageMonosaccharides are linked together by specific enzymes glycosyltransferasesEach transferase catalyses the transfer of one sugar onto acceptor sugarThe 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% Phospholipids30% cholesterol.10% sphingolipids.

PhospholipidsFatty acid tailshydrophobicPhosphate group head hydrophilicArranged as a bilayerFatty acidPhosphate

Phospholipid bilayer

Composition of the red cell membraneProteins

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 ProteinsThe 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 abundantAnkyrin Protein 4.1 and Actin

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

Integral ProteinsThese 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 antigensBlood 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 membraneDefects 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.