The cell. 2. Extracellular matrix.
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Carbohydrates are essential components of the extracellular matrix. Glycosaminoglycans are the most abundant carbohydrates.
Glycosaminoglycans are unbranched carbohydrates composed of repeated pairs of monosaccharides. They are highly hydrated and work as lubricant, counteract hydrostatic pressures and mechanical loads, etcetera.
Hyaluronan or hyaluronic acid is a glycosaminoglycan not covalently bound to polypeptides.
Proteglycans are glycosaminoglycans and polypeptides attached by covalent binding. Glycosominoglycans are chondroitin sulfate, dermatan sulfate, keratan sulfate and heparan sulfate.
Plant cell walls are mainly made up of carbohydrates: cellulose, hemicelluloses, and pectins.
In animal tissues, cells are surrounded by collagen proteins, elastic fibers, glycoproteins, and a large amount of non branched polymers of carbohydrates, and water. Carbohydrate polymers are glycosaminoglycans, also known as mucopolysaccharides. In plant tissues, the main extracellular carbohydrate is cellulose, whereas glycosaminoglycans are not present.
Glycosaminoglycans are unbranched carbohydrate polymers that can be very long. They are composed up of disaccharide repeating units containing an amine group on the first one (N-acetylgalactosamine or N-acetylglucosamine), whereas the second is usually galactose or glucuronic acid. Most of the hydration of extracellular matrix depends on glycosaminoglycans because water is strongly associated with the negative electric charges of carboxyl groups (COO-) and sulfate groups (SO3-), both being found in these saccharides. Glycosaminoglycans occupy large volumes because their molecular secondary structure is quite inflexible and, together with the high hydration, they get gel-like properties, which allow tissues to resist mechanical stresses. Furthermore, the diffusion of molecules through the extracellular matrix is highly facilitated. The most common glycosaminoglycans are hyaluronan and sulfated glycosaminoglycans, such as chondroitin sulfate, dermatan sulfate, keratan sulfate and heparan sulfate.
Hyaluronan, also known as hyaluronic acid, is a distinct type of glycosaminoglycan. First, it does not make covalent bindings with other extracellular matrix molecules. Second, it is synthesized in the cell membrane, not in the Golgi complex. Third, it does not contain sulfate groups. Hyaluronan is made up of disaccharide units, D-glucuronic acid and N-acetyl-glucosamine, repeated up to 20000 times. It is usually associated to collagen and proteoglycans and provides elasticity and resistance to tissues. It is also frequent in those places of the body where a high rate of cell proliferation occurs because hyaluronan makes easier the cell movement. As a large and rather inflexible molecule, hyaluronan occupies a large volume and provides many free spaces. It is also abundant in those body places suffering high friction forces such as joint cartilages.
All glycosaminoglycans, excepting hyaluronan, contain sulfate groups and are covalently bound to polypeptides. Chondroitin sulfate is composed of repetitions of N-acetylgalactosamine - glucuronic acid and is abundant in cartilage. Dermatan sulfate is made up of repetitions of glucuronic acid (or iduronic acid) plus N-acetyl-galactosamine. Keratan sulfate contains repetitions of N-acetyl-glucosamine and galactose, with different type of sulfation depending of the monosaccharide. Heparan sulfate is synthesised by most of the cells, whereas heparin, which is a molecular analogue, is only synthesized by mastocytes and used as anticoagulant. Heparin is formed by repetitions of N-acetyl-glucosamine plus glucuronic acid (or iduronic acid), with different chemical bindings than those of hyaluronan. All these glycosaminoglycans are chemically linked to polypeptides and form macromolecular structures known as proteoglycans.
Aggrecan proteoglycan (modified from Lamoureux, 2007).
Proteoglycans are molecules composed of a polypeptide and one or several glycosaminoglycans attached by chemical binding. They are found in every animal tissue. Proteoglycans are synthesized inside the cell. The polypeptide part is assembled in the endoplasmic reticulum, where some monosaccharides are also added. However, the elongation of the glycosaminoglycan chains, as well as the addition of sulfate groups, occurs in the trans site of the Golgi complex. Most of the proteoglycans are exocytosed to the extracellular space, but some of them will be part of the plasma membrane, where they are inserted among the fatty acid chains of lipids thanks to a sequence of hydrophobic amino acids of the polypeptide
Different proteoglycans show different amino acid sequence and polypeptide length (ranging from 100 to 4000 amino acids). Furthermore, they contain a different number and types of glycosaminoglycans. For example, decorin proteoglycan is made up of just one molecule of glycosaminoglycan attached to the polypeptide, whereas aggrecan proteoglycan contain more than 200 glycosaminoglycans.
Proteoglycans can be classified in a number of groups. Lecticans show a globular structure in the N-terminal part of the polypeptide, which interacts with hyaluronan. It mainly contains chondroitin sulfate, and occasionally keratan sulfate. Aggrecan, versican, neurocan and brevican are members of lectican group. The SLRP group of proteoglycans contain many leucine amino acids in the polypeptide, which is bound to chondroitin sulfate and keratan sulfate. Biblicane, decorin, fibromodulin and keratocan belong to this group. Another major group of proteoglycans contains heparan sulfate, most of them are free in the extracellular matrix, such as perlecan and agrine, and a few of them, such as syndecan and glypican, are inserted in the plasma membrane. A diverse group of proteins, such as CD44 receptor, amyloid precursor protein, and some collagen (IX, XII, XIV, XVIII), may or may not contain sulfated glycosaminoglycans, i.e. they are part-time glycosaminoglycans.
The distinct function of the different types of proteoglycans mostly relies on their glycosaminoglycan content. These functions are hydration, resistance to tensile forces, lubrication, help in cell physiology and motility, etcetera. For example, the mechanical role is essential in cartilage and bones. Furthermore they act as anchoring points for cell attachment, either because the proteoglycan is part of the plasma membrane, or because they are recognized by cell adhesion molecules, such as integrins.
Arrangement of cellulose molecules and how they interact between each other by hydrogen bridges (red lines).
Cellulose is the main component of cell walls, the extracellular matrix of plants. Some authors do not include cell wall as a type of extracellular matrix, but here it will be studied as a highly specialized extracellular matrix. Cellulose is a polysaccharide made up of glucose molecules (more than 500 per cellulose molecule) linked by β(1-4) bonds (see figure on the left). It allows plant tissues to resist pressure, as well as stretching forces. But it also allows the presence of aerial parts in plants, working as a "skeleton". About 50 cellulose molecules join together by hydrogen bonds and van der Waals forces to form a crystal structures known as microfibrils, which are oriented with the same polarity. Cellulose microfibrils are linked together by hemicellulose and pectins to form cellulose fibrils, and these form cellulose fibers. Fibers can be observed with light microscope.
In the same way as hyaluronan, cellulose is synthesized in the plasma membrane by cellulose synthase, a transmembrane protein. This enzyme takes glucose units from cytosol, makes them to cross the plasma membrane, and chemically links them in the extracellular space. One interesting aspect of cellulose synthesis is how the nascent chain is left over the plasma membrane, because the orientation of cellulose molecules will determine the orientation of cellulose fibers. During plant growth, the increase in size of cells is mainly along the axis perpendicular to cellulose fibers. The orientation of cellulose molecules over the plasma membrane is determined by cytoplasmic cortical microtubules, which are located just under the plasma membrane. In this way, cells can control the direction of their own growth, which will affect the growth of different organs, as the growth of stems toward a light source.
Bosman FT, Stamenkovic I . Functional structure and composition of the extracellular matrix. 2003. Journal of pathology. 200:423-428.
Lamoureux F. Proteoglycans: key partners in bone cell biology. 2007. BioEssays. 29:758-771.
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Updated: 2016-03-21. 18:39
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo