Question

Describe six different molecules that compose the extracellular matrix.

Answer 1

Extracellular matrix is a complex network of non-cellular components present within tissues and organs. Their organization plays a role in cell division, motility, adhesion. The extracellular matrix supports cell, tissue development and is involved in homeostasis.Molecules that compose the extracellular matrix are as follows.

Glycosaminoglycans: They are highly negatively charged, long unbranched polysaccharide usually found covalently linked to protein by way of proteoglycans. They constitute 95% sugar residue of the total weight, whose repeating unit is either N-acetylglucosamine or N-acetylgalactosamine.Among the types of GAGs are hyaluronan, chondroitin sulfate, heparan sulfate and keratan sulfate.

Hyaluronan- Simplest GAG which consists of glucuronic acid and N-acetylglucosamine.These are not linked to proteins.

Chondroitin sulfate- Most abundant GAG, usually associated with protein to form proteoglycans.

Heparan sulfate- Found associated with protein forming heparan sulfate proteoglycans.

Proteoglycans: Many GAGs are linked to core protein(synthesized on rough endoplasmic reticulum) forming proteoglycans. Proteoglycans have various types of GAG chains and their protein core are rich in Ser and Thr residue, which permits multiple sites of polymeric GAG attachment.

Collagen: These are major structural protein in the extracellular matrix. They are long, inelastic, triple-stranded helical structure whose fundamental unit is tropocollagen. The repeating tripeptide unit is Gly-X-Y, where X is often proline and Y is often 3 or 4-hydroxyproline or 5-hydroxylysine. Proline and hydroxyproline confer rigidity in the collagen molecule. Prolyl hydroxylase and lysyl hydroxylase are the enzymes that are involved in hydroxylation reaction, hydroxylation further result in interchain H-bonds. Ascorbate act as a cofactor for these enzymes and hence their deficiency can result in the impaired synthesis of collagen, a situation of scurvy.

Elastin: These are highly hydrophobic connective tissue protein for extensibility and elasticity. The fundamental unit is tropoelastin, rich in non-polar amino acids. The tropoelastin is composed of alternating hydrophobic segments and alanine and lysine-rich segments.

Fibronectin: These are the specialized protein whose role is to attach cells to a variety of extracellular matrices. Fibronectin exists as dimer consisting of two nearly identical monomers linked by a pair of disulfide bonds. Fibronectins contains three types of domain FN-I, FN-II, and FN-III all of which consist of antiparallel beta strands.

Laminins: These are heterotrimeric matrix glycoprotein which consists of alpha, beta and gamma chains. Laminins constitute the structural scaffolding of all basement membranes. They are found to bind with integrins and numerous other receptors.