Hyaluronic acid repeating units

Hyaluronan or hyaluronic acid is a linear mucopolysaccharide constituted by N-acetyl-D-glucosamine and D-glucuronic acid repeating units. The structure of hyaluronic acid is shown in Figure 8.2. 

Hyaluronic acid consists of an unbranched chain of repeating disaccharide units containing GlcUA and GlcNAc. Hyaluronic acid is present in bacteria and is widely distributed among various animals and tissues, including synovial fluid, the vitreous body of the eye, cartilage, and loose connective tissues. 

As shown in Figure 48-8, the keratan sulfates consist of repeating Gal-GlcNAc disaccharide units containing sulfate attached to the 6 position of GlcNAc or occasionally of Gal. Type 1 is abundant in cornea, and type II is found along with chondroitin sulfate attached to hyaluronic acid in loose connective tissue. Types I and II have different attachments to protein (Figure 48—8). 

We used this procedure in estimating the chain properties of hyaluronic acid 16). Hyaluronic acid is a regular polysaccharide with a disaccharide repeat unit. The two units are glucuronic acid (Figure 1) and N-acetyl glucosamine (Figure 2). 

Structure of the repeating unit of hyaluronic acid GlcUA/3(l, 3)GlcN Ac/3( 1,4). 

Whistler and Smart10 have proposed as the possible repeating unit in hyaluronic acid, a disaccharide (VI) of D-glucuronic acid and V-acetyl-D-glucosamine. 

Disaccharides of the hexuronic acid-hexosamine type constitute the fundamental repeating unit of many mucopolysaccharides, from which they are derived by hydrolytic degradation. Further hydrolytic treatment results generally in destruction of the uronic acid moiety of the disaccharide. Structural studies have been made on the disaccharides from hyaluronic acid (hyalobiouronic acid), chondroitinsulfate (chondrosine), and heparin (heparosine). 

Figure 9.13 Structures of repeating units of glycosaminoglycans (A) hyaluronic acid (B) chondroitin sulfate (C) dermatan sulfate (D) two types of keratan sulfate type I is N linked and type II is O linked (E) heparan sulfate. Sulfate residues have been omitted in all cases.

Hyaluronan (hyaluronic acid, HA) is a highly anionic unbranched linear polymer containing a -GlcAfSl,4GlcNAcP 1,4-repeating unit, which plays important roles in modulating cell adhesion, signaling, and motility. Several enzymes responsible for HA synthesis, namely Hyaluronic Acid Synthases, have been cloned from bacteria and mammals (107, 108). 

Hyaluronan, or hyaluronic acid, is synthesized at the plasma membrane (rather than in the ER or Golgi apparatus) by one of three distinct hyaluronan synthases, which allows it to be easily secreted directly to the ECM (47). Hyaluronan is the simplest GAG, consisting of the repeating unit -GlcA-pi-3-GlcNAc-pi-4- this GAG forgoes postsynthetic modification and remains unbound to surface proteins. 

Molecular weight hyaluronic acid molecules have a molecular weight of 300-2000 kDa as the number of repeating disaccharide units in each molecule is variable. In its natural form, hyaluronic acid exists as a high-molecular-weight polymer of 10 -10 Da. 

Different glycosaminoglicans are not very different in structure. For example, the repeating unit in chondroitin is - 4)-0-(p-D-glucopyranosyluronic acid)-(1 3)-0-(2-acetamido-2-deoxy-p-D-galactopyranosyl)-(1 ->. The number of units varies but can be in the range of hundreds. Hyaluronic acid has a rather similar structure, - 4)-0-(p-D-glucopyranosyluronic acid)-(1 ->3)-0-(2-acetamido-2-deoxy-p-D-glucopyranosyl)-(1 as shown below ... 

Another important glucosaminoglycan of the ECM in cormective tissues, especially cartilage and skin, is hyaluronic acid. It consists of repeating disaccharide units of d glucoronic acid and D-A-acetyl-glucosamine. For biomedical applications. 

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