Basic Introduction Chinese Name: glycoprotein English Name: Glycoprotein Water Solubility: Soluble Glycoprotein Soluble in Water Sugar Content: Less than protein? Type: Combined with protein's chemical introduction, basic structure, connection mode, sugar chain structure, biological function and evaluation, the chemical introduction glycoprotein is a complex carbohydrate formed by covalent connection of branched oligosaccharide chains and polypeptide chains, with a short main chain and a sugar content less than that of protein in most cases. In glycoprotein, the composition of sugar is often complicated, including mannose, galactose, fucose, glucosamine, galactosamine, sialic acid and so on. Oligosaccharides and protein can be combined in two ways: (1) hemiacetal hydroxyl of sugar and amino acids containing hydroxyl (serine, threonine, hydroxylysine, etc. ) through O- glycosidic bond; (2) Semi-acetyl hydroxyl group of sugar and amide group of asparagine are bound by N- glycosidic bond. It is widely distributed in nature. Among the sixty or seventy kinds of plasma proteins that have been studied, most of them are glycoproteins. Some enzymes and hormones are glycoproteins. Glycoprotein is also an important component of plasma membrane, intercellular substance and plasma mucus. More and more glycoproteins are isolated from plants, especially lectins. At present, many lectins have been isolated from vertebrates, invertebrates and even many microorganisms, and most of them belong to glycoproteins. Glycoproteins are species-specific. One kind of protein exists as glycoprotein in some animals, but it is different in another animal. Even glycoproteins may have different sugar contents. For example, pancreatic ribonucleases of cattle, sheep and pigs are glycoproteins, but the sugar content is 9.4%, 9.8% and 38% respectively. This enzyme in mice does not contain sugar. At the same time, glycoprotein is a binding protein. Glycoprotein is a molecule composed of short oligosaccharide chains covalently linked to protein, and its overall properties are closer to that of protein. Between sugar and protein, protein is dominant, and some short oligosaccharide chains are connected by covalent bonds. These oligosaccharide chains are usually branched heteropolysaccharide chains, and there are no repeated disaccharide series. They are usually composed of 2- 10 monomers (less than 15)=, and the terminal members are usually sialic acid or L- fucose. Usually, the sugar content per molecule is very small (about 4%). Some glycoproteins contain only one or several glycosyl groups, while others contain multiple linear or branched oligosaccharide side chains. Glycoproteins are usually secreted into body fluids or membrane proteins, which are located outside cells and have corresponding functions. Glycoproteins include enzymes, hormones, vectors, lectins, antibodies and so on. Composition β-D- glucose (Glc) α-D- mannose (Man) α-D- galactose (Gal)α-D- xylose (Xyl) α-D- arabinose (Ara) α-L- fucose (Fuc) glucuronic acid (GlcuA) Aidoo uronic acid (IduA) N- α-D- galactose (Gal), α-D- xylose (Xyl), α-D- arabinose (Ara) and α-DN- acetylneuraminic acid (NeuNAC), namely sialic acid (Sia) and glycopeptide bond of protein glycoprotein, referred to as glycopeptide bond for short. The types of glycopeptide chains can be summarized as follows: ① N- glycosidic bond type: oligosaccharide chain (β-hydroxyl of GlcNAC) is connected with amide group of Asn, N-terminal a- amino group, Lys or W- amino group of Arg; ② O- glycosidic bond type: oligosaccharide chain (α-hydroxyl group of GalNAC) is connected with hydroxyl groups of Ser, Thr, hydroxylysine and hydroxyproline. ③ S- glycosidic bond type: glycopeptide bond with cysteine as the connection point. ④ Ester glycosidic bond type: the free carboxyl group of aspartic acid and glutamic acid is the connection point. Structure of sugar chains The sugar chains in glycoproteins vary greatly and contain abundant structural information. Oligosaccharide chains are usually the recognition sites of receptors and enzymes. 1, N- glycosidic bond type N- glycosidic bond type mainly has three types of oligosaccharide chains: ① high mannose type, consisting of GlcNAc and mannose; ② Compounds: Besides GlcNAc and mannose, there are fructose, galactose and sialic acid; ③ Heterozygous subtype, including the characteristics of ① and ②. Pentose core 2, O- glycosidic bond type (O- bond) has no pentose core. Such as: human plasminogen; Human immunoglobulin IgA: N- glycopeptide bonds, such as β- GlcNAc-Asn and O- glycopeptide chains, such as α-Galnac-Thr/Ser, β-Gal-Hyl, β-L-Araf-Hyp and N- linked oligosaccharide chains all contain a common structural fancy, which is called core pentose or mannose. The structure of O- sugar chain is simpler than that of N- sugar chain, but there are more connecting forms than that of N- sugar chain. The sialic acid residue at the end of glycoprotein oligosaccharide chain, whose biological function carries the information of protein metabolism, determines whether some protein exists in the blood stream or is eliminated by the liver. Ceruloplasmin in vertebrate blood. Hepatocytes can degrade ceruloplasmin which has lost sialic acid, and the elimination of sialic acid may be one of the ways to mark the "old" protein in the body. B. The sialic acid content of newborn erythrocyte membrane is much higher than that of mature erythrocyte membrane. The newborn red blood cells were reinjected into the body after being treated with sialidase, and all disappeared after a few hours. However, red blood cells without enzyme treatment can still survive normally in the body after a few days of reinjection. The key function of oligosaccharide chain lymphocytes normally returns to the spleen, but after sialic acid is removed, the result actually returns to the liver. Eukaryotic genes expressed in prokaryotic cells cannot be glycosylated. Glycoproteins can be cytolytic or membrane-bound, and can exist in intracellular or intercellular substances. Glycoproteins are typical in animals and plants, especially vertebrates, such as metal transporter (transferrin), ceruloplasmin, coagulation factor, complement system, some hormones, follicle-stimulating hormone (FSH), pituitary gland secretion, promoting the development of eggs and * * *), ribonuclease, membrane-bound protein (such as Na+-K+-ATPase of animal cell membrane) and major histocompatibility antigen (MHC). Most oligosaccharides of glycoprotein are functional centers of glycoprotein. Some glycoproteins have protective or lubricating effects on glycoproteins themselves. For example, bovine RNaseB (glycoprotein) is more heat-resistant than RNaseA, and a large amount of sialic acid can enhance the viscosity of saliva mucin, thus enhancing the lubricity of saliva. The sugar component of Antarctic fish antifreeze protein can form hydrogen bonds with water to prevent the formation of ice crystals, thus improving the frost resistance. Glycoproteins play a more complicated role in signal transmission between cells. The target cell binding protein GP 120 of Hiv is a glycoprotein, which can bind to CD4 receptor on the surface of human target cells and attach to the surface of target cells. If the sugar part of GP 120 is removed, it will not bind to CD4 receptor and lose its infectivity. Glycoproteins on the cell surface form glycocalyx of cells and participate in cell adhesion, which plays a key role in the growth, development and differentiation of embryos and tissues. Evaluation glycoprotein is a kind of sugary protein, which is formed by covalent connection of some amino acid residues in oligosaccharide chain and peptide chain through glycosidic bonds. Its main biological function is the biological recognition of cells or molecules. For example, when an egg is fertilized, it is necessary to recognize the corresponding glycoprotein on the cell membrane of the egg. Receptor proteins and tumor cell surface antigens are also glycoproteins. Glycoproteins are ubiquitous in animals, plants and microorganisms and have a wide range of types and functions. According to the existing mode, it can be divided into three types: glycoprotein 1 is formed in cells, and soluble glycoprotein exists in intracellular fluid, various body fluids and mucus secreted by cavity glands. Plasma proteins are glycoproteins other than albumin. Soluble glycoproteins include enzymes (such as nucleases, proteases and glycosidases), peptide hormones (such as chorionic gonadotropin, luteinizing hormone, thyrotropin and erythropoietin), antibodies, complements and some growth factors, interferons, somatostatin, lectins and toxins. 2. Membrane-bound glycoprotein, whose peptide chain consists of hydrophobic peptide and hydrophilic peptide. Hydrophobic peptide segments can be one to several, which are embedded in the membrane lipid bilayer through hydrophobic interaction. The hydrophilic peptide segment is exposed outside the membrane. The sugar chain is attached to the hydrophilic peptide segment and has strict directionality. The sugar chains on the surface of plasma membrane are outward; The inner membrane of the battery usually faces the cavity surface. Membrane-bound glycoproteins include enzymes, receptors, lectins and transporters. This glycoprotein usually participates in cell recognition and can be used as a surface marker or surface antigen for specific cells or cells at a specific stage. 3. Structural glycoproteins are insoluble macromolecular glycoproteins in extracellular matrix, such as collagen and various non-collagen glycoproteins (fibronectin, laminin, etc.). Their function is not only to support, connect and buffer as structural components of extracellular matrix, but also to participate in cell recognition, adhesion and migration, and to regulate cell proliferation and differentiation. Microscopically, glycoprotein oligosaccharide chain usually refers to a polymer composed of 2 ~ 10 monosaccharide groups connected by glycosidic bonds. Glycoprotein oligosaccharide chains are mostly branched. Because the terminal carbon (hetero-carbon) atom of monosaccharide has two configurations, α and β, and many hydroxyl groups in monosaccharide molecule can form glycosidic bonds, the diversity of sugar chain structure exceeds that of polynucleotide and peptide chain. The sugar chain structure can store enough identification information, which plays a decisive role in molecular recognition and cell recognition. The physiological functions of glycoprotein include coagulation, immunity, secretion, endocytosis, substance transport, information transmission, nerve conduction, regulation of growth and differentiation, cell migration, cell homing, wound repair and regeneration. The sugar chain of glycoprotein is also involved in maintaining its peptide chain in the natural conformation with biological activity and stabilizing the peptide chain structure, and endowing the whole glycoprotein with specific physical and chemical properties (such as lubricity, viscoelasticity, heat inactivation resistance, protease hydrolysis resistance and freezing resistance). Glycoprotein is related to the occurrence and development of many diseases, such as infection, tumor, cardiovascular disease, liver disease, nephropathy, diabetes and some hereditary diseases. Furthermore, glycoproteins and glycolipids on the cell surface can "fall off" to the surrounding environment or enter the blood circulation, which can provide information for clinical diagnosis as abnormal signs; When suffering from certain diseases, glycoproteins in body fluids often have specific changes, which may be helpful for diagnosis or prognosis judgment. Glycoproteins are also increasingly involved in treatment. For example, antibodies against specific sugar structures on specific cell surfaces can be used as targeted carriers for targeted therapeutic drugs. The use of carbohydrates (monosaccharides, oligosaccharides or glycopeptides) to fight infection and tumor metastasis has also appeared. glycoprotein