Glycosaminoglycan is a kind of heteropolysaccharide, which mainly exists in connective tissues of higher animals, and is also found in plants.
Figure 1. Chondroitin sulfate.
General structure of glycosaminoglycan
Glycosaminoglycans are heteropolysaccharides, which are unbranched, long-chain polymers. The general formula of glycosaminoglycan is: [hexuronic acid-hexosamine] n, n varies according to the type, generally between 30 and 250. Disaccharide units have at least one monosaccharide residue with a negatively charged carboxyl or sulfate group, so glycosaminoglycans are anionic polysaccharide chains that are acidic. Glycosaminoglycans exist as proteoglycans in the body, with the exception of hyaluronic acid.
According to the type of monosaccharide residues, the type of linkage between residues, and the number and position of sulfate groups, glycosaminoglycans can be divided into 5 main categories:
Hyaluronic acid is also called hyaluronic acid, which means glass is bright and transparent, while uronic acid refers to uronic acid. Hyaluronic acid has no relationship with uric acid. Hyaluronic acid is also called hyaluronic acid, which means glass is bright and transparent, while uronic acid refers to uronic acid. Hyaluronic acid has no relationship with uric acid. Biochemical drugs with high clinical value are widely used in various types of ophthalmic surgery, such as lens implantation, corneal transplantation and anti-glaucoma surgery. It can also be used to treat arthritis and accelerate wound healing. Used in cosmetics, it can play a unique role in protecting the skin, can keep the skin moist and smooth, delicate and soft, full of elasticity, and has the functions of anti-wrinkle, anti-wrinkle, beauty health care and restore the physiological functions of the skin.
Figure 2. Chemical structure of Hyaluronic acid (HA).
Chondroitin sulfate (CS) is a class of glycosaminoglycans that are covalently linked to proteins to form proteoglycans. Chondroitin sulfate is widely distributed on the extracellular matrix and cell surface of animal tissues. The sugar chain is composed of alternating glucuronic acid and N-acetylgalactosamine (also known as N-acetylgalactosamine) disaccharide units. The linking region is connected to the serine residue of the core protein. Chondroitin sulfate exists in all organisms from nematodes to humans except plants, and plays many important physiological functions. Although the structure of the main chain of the polysaccharide is not complicated, the degree of sulfated, sulfate group and two kinds of differences in the distribution of the isouronic acid rechain are highly heterogeneous. The fine structure of chondroitin sulfate determines the specificity of the function and its interaction with various protein molecules.
Figure 3. Chemical structure of Chondroitin sulfate (CS).
The B enzyme degrades chondroitin sulfate B (dermatin sulfate). This enzyme acts on β1-4 glycosides between aminohexose and L-iduronic acid residues via β elimination mechanism and cuts them off. The products of the degradation reaction are oligosaccharides, mainly unsaturated disaccharides. Chondroitin sulfate B can be quantified by measuring unsaturated disaccharide products.
Figure 4. Structure of Dermatan sulfate (DS).
Keratin sulfate, a protein-forming complex, is found in mammalian cornea, intervertebral plates, cartilage and arteries, which is a mucopolysaccharide in the form of proteoglgcan. In most cases, it coexists with chondroitin sulfate, and sometimes there is only one protein part. Proteoglycans in fetal cartilage hardly contain keratan sulfate, but their content increases with age, which can be used as an example of changes in connective tissue of animals with age. As shown in the figure, keratan sulfate uses disaccharides formed by D-galactose and N-acetylglucosamine-6-sulfuric acid as the main repeating unit, and a part of galactose is sulfated or branched into another at the 6 position. Sugar chain. Some keratan sulfates also contain a small amount of fucose, aluminosilicic acid, this fine heterogeneous structure, which is tissue specific. In terms of protein-bound forms, keratan sulfate in the cornea is N-acetylglucosamine and aspartic acid is bound by N-glycoside bonds, while keratan sulfate in cartilage and other skeletal systems is N-acetylglucosamine. Amine, serine and methionine are different from each other by O-glycosidic bonds. In either case, several molecules of mannose are still present near the binding region.
Figure 5. Chemical structure of Keratin sulfate (KS).
Heparin was first discovered from the liver and got its name. It is a mucopolysaccharide sulfate composed of glucosamine, L-iduronic glycoside, N-acetylglucosamine, and D-glucuronic acid. The average molecular weight is 15KD, which is strongly acidic. It is also found in lungs, blood vessel walls, intestinal mucosa and other tissues, and is a natural anticoagulant substance in animals. Naturally found in mast cells, it is now mainly extracted from bovine lung or porcine small intestine mucosa. As an anticoagulant, it is a polymer composed of two kinds of polysaccharides alternately connected, and has anticoagulant effect in vivo and in vitro. It is mainly used clinically for thromboembolic diseases, myocardial infarction, cardiovascular surgery, cardiac catheterization, extracorporeal circulation, and hemodialysis. With the development of pharmacology and clinical medicine, the application of heparin continues to expand.
Figure 6. Chemical structure of heparin (HP)
The molecular weight, disaccharide structure, and sulfate of glycosaminoglycans vary widely. This is because GAG synthesis is not driven by templates like proteins or nucleic acids, but is constantly changed by processing enzymes. GAG is divided into four categories based on the core disaccharide structure. Heparin/heparan sulfate (HSGAG) and chondroitin sulfate / dermatan sulfate (CSGAG) are synthesized in the Golgi apparatus, where the protein core formed in the rough endoplasmic reticulum is O-glycosylated the base transferase is translated into a glycoprotein, forming a proteoglycan. Keratin sulfate can modify core proteins by N-linked glycosylation or O-linked glycosylation of proteoglycans. The fourth type of GAG, hyaluronic acid is synthesized by intact membrane synthase, which immediately secretes a dynamically elongated disaccharide chain.