تسجيل الدخول

تسجيل

Carbohydrates Occur in Cell Membranes & in Lipoproteins

المؤلف:  Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil

المصدر:  Harpers Illustrated Biochemistry

الجزء والصفحة:  32nd edition.p155

2025-06-10

665

 Approximately 5% of the weight of cell membranes is the carbohydrate part of glycoproteins and glycolipids. Their presence on the outer surface of the plasma membrane (the glycocalyx) has been shown with the use of plant lectins, proteins that bind specific glycosyl residues. For example, concanavalin A binds α-glucosyl and α-mannosyl residues. Glycophorin is a major integral membrane glycoprotein of human erythrocytes. It has 130 amino acid residues and spans the lipid membrane, with polypeptide regions outside both the external and internal (cytoplasmic) surfaces. Carbohydrate chains are attached to the amino terminal portion outside the external surface. Carbohydrates are also present in apolipoprotein B of plasma lipoproteins. ABO blood groups are defined by different immunogenic molecules on erythrocytes. ABO blood group antigens are oligosaccharide chains that are attached to proteins. Polymorphisms in the enzyme ABO glycosyltransferase determine which single saccharide is placed in the oligosaccharide chain. The blood group A expresses A-transferase that places N-acetylgalactos amine. The blood group B expresses the B-transferase which places galactose. Individuals with blood type O have a mutation that produces an inactive ABO glycosyltransferase; no oligosaccharide is added to the protein. Adding oligosaccharides to cell surface proteins make them an antigen. Individuals with blood type O do not have this modification are thus universal blood donors.