Connective tissue, glycoproteins

Collagen is a major component of connective tissue that becomes exposed at the subendothelium of injured blood vessels. It contributes to platelet adhesion and also plays a role in platelet activation by binding to several receptors on platelets such as integrin a 2(3 1 or glycoprotein VI (GP VI). 

The carboxy-terminal region in apolipoprotein (a) closely resembles the protease domain in plasminogen [eight amino acid substitutions, nine amino acid deletions, and one insertion in apo(a) relative to plasminogen, with 94% overall nucleotide sequence identity] (G28). The most important difference is the substitution of arginine by serine in the site responsible for proteolytic activity (position 4308) (G28). As a result, Lp(a) has no protease activity towards substrates for plasmin (J3). Salonen (SI) reported a serine-protease activity of Lp(a) towards fibronectin, a glycoprotein present in connective tissue matrices. 

Unlike the acellular basement membranes, the interstitial connective tissue consists of cells distributed in meshwork of collagen fibers, glycoproteins (e.g., fibronectin), proteoglycans, and hyaluronic acid. The main forms of collagen found in interstitial connective tissue are known as type I, II, and III or interstitial collagen. 

N-acetylneuraminic acid (sialic acid, 5), is a characteristic component of glycoproteins. Other acidic monosaccharides such as D-glu-curonic add, D-galacturonic add, and liduronic add, are typical constituents of the glycosa-minoglycans found in connective tissue. 

Proteoglycans are widely distributed throughout the body, being most abundant in connective tissue, where they may contribute up to 30% of that tissue s dry weight. They consist of a polypeptide backbone to which heteropolysaccharide chains are attached. However, unlike glycoproteins, proteolycans consist of up to, or in excess of, 95% carbohydrate and their properties resemble those of polysaccharides more than those of proteins. 

In this Section, the nature of the linkages between the protein chain and the carbohydrate moieties in glycoproteins will be considered, and some indication of their occurrence will be given. It will become clear that, in a number of examples, a given polypeptide chain is glycosidically substituted at more than one point. Sometimes, the amino acid residue at two such points is of a different type, so that more than one type of carbohydrate-peptide bond occurs in the same protein, It is likely that more examples of this kind of structure will be found as more features of the constitution of glycoproteins involved in membranes and connective tissue are determined. 

Function and location of elastin Cause of Marfan syn drome Elastin is a connective tissue protein with rubber-like properties. Elastic fibers composed of elastin and glycoprotein microfibrils, such as fibrillin, are found in the lungs, the walls of large arteries, and elastic ligaments. [Note Mutations in the fibrillin gene are responsible for Marfan syndrome]... 

Fibronectin is a large glycoprotein normally present in blood and connective tissue that is made up of a number of different domains serving as binding sites to various cells via specific integrin receptors... 

It has been proven that it is mainly TNFa that stimulates fibroblasts to release metalloproteases—compounds leading to the degradation of proteoglycans, collagen, and glycoproteins, and consequently damage the substance of basic connective tissue. As a result, the mucous barrier becomes less hermetic and a subsequent atrophy of intestinal villi and reduction of absorption surfaces occurs (Schuppan, 2000). 

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