Harpers

RED & WHITE BLOOD CELLS / 615Table 52–5. Summary of biochemicalinformation about the membrane of the humanred blood cell.• The membrane is a bilayer composed of about 50% lipidand 50% protein.• The major lipid classes are phospholipids and cholesterol;the major phospholipids are phosphatidylcholine (PC),phosphatidylethanolamine (PE), and phosphatidylserine(PS) along with sphingomyelin (Sph).• The choline-containing phospholipids, PC and Sph, predominatein the outer leaflet and the amino-containingphospholipids (PE and PS) in the inner leaflet.• Glycosphingolipids (GSLs) (neutral GSLs, gangliosides, andcomplex species, including the ABO blood group substances)constitute about 5–10% of the total lipid.• Analysis by SDS-PAGE shows that the membrane containsabout 10 major proteins and more than 100 minor species.• The major proteins (which include spectrin, ankyrin, theanion exchange protein, actin, and band 4.1) have beenstudied intensively, and the principal features of their disposition(eg, integral or peripheral), structure, and functionhave been established.• Many of the proteins are glycoproteins (eg, the glycophorins)containing O- or N-linked (or both) oligosaccharidechains located on the external surface of the membrane.(Figure 52–3), several of which have been shown to beglycoproteins. Their migration on SDS-PAGE wasused to name these proteins, with the slowest migrating(and hence highest molecular mass) being designatedband 1 or spectrin. All these major proteins have beenisolated, most of them have been identified, and considerableinsight has been obtained about their functions(Table 52–6). Many of their amino acid sequencesalso have been established. In addition, it hasbeen determined which are integral or peripheral membraneproteins, which are situated on the external surface,which are on the cytosolic surface, and which spanthe membrane (Figure 52–4). Many minor componentscan also be detected in the red cell membrane byuse of sensitive staining methods or two-dimensionalgel electrophoresis. One of these is the glucose transporterdescribed above.The Major Integral Proteins of the RedBlood Cell Membrane Are the AnionExchange Protein & the GlycophorinsThe anion exchange protein (band 3) is a transmembraneglycoprotein, with its carboxyl terminal end onthe external surface of the membrane and its amino terminalend on the cytoplasmic surface. It is an exampleof a multipass membrane protein, extending across theSpectrinAnkyrinandisoforms122.12.22.32.6Anion exchange protein 34.14.2ActinG3PD567GlobinMembraneCoomassieblue stainSkeletonMembranePAS stainGlycophorinsFigure 52–3. Diagrammatic representation of themajor proteins of the membrane of the human redblood cell separated by SDS-PAGE. The bands detectedby staining with Coomassie blue are shown in the twoleft-hand channels, and the glycoproteins detected bystaining with periodic acid-Schiff (PAS) reagent areshown in the right-hand channel. (Reproduced, withpermission, from Beck WS, Tepper RI: Hemolytic anemiasIII: membrane disorders. In: Hematology, 5th ed. Beck WS[editor]. The MIT Press, 1991.)bilayer at least ten times. It probably exists as a dimer inthe membrane, in which it forms a tunnel, permittingthe exchange of chloride for bicarbonate. Carbon dioxide,formed in the tissues, enters the red cell as bicarbonate,which is exchanged for chloride in the lungs,where carbon dioxide is exhaled. The amino terminalend binds many proteins, including hemoglobin, proteins4.1 and 4.2, ankyrin, and several glycolytic enzymes.Purified band 3 has been added to lipid vesiclesin vitro and has been shown to perform its transportfunctions in this reconstituted system.Glycophorins A, B, and C are also transmembraneglycoproteins but of the single-pass type, extendingacross the membrane only once. A is the major glycophorin,is made up of 131 amino acids, and is heavilyglycosylated (about 60% of its mass). Its amino terminalend, which contains 16 oligosaccharide chains (15 ofwhich are O-glycans), extrudes out from the surface of