In some cases, there are also qualitative differences due to the expression of genes that are silent in the differentiated normal cells. Oncogenic transformation by viral infection or chemical mutagens usually leads to alterations in the cell surface pattern of glycosphingolipids such that certain types increase greatly in quantity. In addition, certain glycosphingolipids of the ganglioside class have been found recently to inhibit the mitogenic response of cell growth factors by allosteric modulation of their cell surface receptors (Bremer et al., 1986). Many of the cell surface functions of glycoproteins have also been proposed for the neutral and acidic glycosphingolipids. ![]() Recognition sites on cell surfaces for binding and uptake of hormones and for interactions with other cells, viruses and bacteria are also glycoproteins. The recognition and binding of lysosomal enzymes to specific receptors in the Golgi apparatus and on the cell surface involves one or more phosphorylated mannose residues on N-linked oligosaccharide chains. Most of the animal and plant lectins are glycoproteins, as are the lysosomal enzymes. Many human plasma proteins such as fetuin, transferrin, and ceruloplasmin are glycoproteins, as are several of the hormones such as chorionic gonadotropin and thyrotropin. Important immunological molecules of the glycoprotein class include the immunoglobulins, histocompatibility antigens, blood group antigens of the ABO and Lewis types, complement in the blood clotting mechanism, and interferon. ![]() Mucins are significant as lubricants and protective agents in mucous secretions. ![]() Collagen is an important structural element in the extracellular space and in cartilage, bone and basement membranes. Glycoproteins have many functions in higher organisms.
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