Biochemistry and roles of glycophorin A

Abstract

SAAD TAYYAB* and MOHAMMAD ABUL QASIM Department of Biochemistry, Faculty of Medicine Aligarh Muslim University Aligarh-202001, India Introduction Membranes play important roles in many cellular phenomena. The transmembrane proteins contain carbohydrates, and are usually implicated in such important biological functions of the cell as virus binding, cell-surface antigenicity, cell-cell recognition, cell-cell communication, cellular transformation, transport, energy transduction, etc. These proteins are embedded in the lipid milieu of the membrane which provides the suitable environment for their action. The erythrocyte membrane is the most thoroughly studied. 1-3 About six proteins and four sialoglycoproteins may be observed by SDS- polyacrylamide gel electrophoresis, but the actual number of polypeptides is in fact much higher. 4 The glycoproteins of the membrane have been classified into asialo and sialoglyco- proteins depending upon the presence of sialic acid. All the sialic acid of the membrane appears to be bound to proteins. 5 These glycoproteins comprise about 10% of the total protein of human erythrocyte membrane and their sialic acid residues are responsible for most of the negative charge on the cell surface. Cell-surface carbohydrates may be involved in the stability of the cell and also contribute to the specificity of blood group substances. 6 In human erythrocytes, there are about 20 million sialic acid molecules per cell. 7,s The human erythrocyte membrane, after solubilization in 1% SDS, gives four PAS- stainable bands, PAS-1, PAS-2, PAS-3 and PAS-4, on SDS-polyacrylamide gel electro- phoresis. 9 These sialoglycoproteins were called