Heteropolysaccarides
Heteropolysaccarides contain two or more different kind of monosaccharides. Usually they provide extracellular support for organisms of all kingdoms: the bacteria cell envelope, or the matrix that holds individual cells together in animal tissues, and provides protection, shape and support to cells, tissues and organs.
Heteropolysaccharides provide extracellular support to very different organisms, from bacteria to humans; together with fibrous proteins, like collagen, elastin, fibronectin, laminin and others, heteropolysaccharides are the most important components of the extracellular matrix. Hyaluronic acid, condroitin sulfates and dermatan sulfates are important heteropolysaccharides in the extracellular matrix. These heteropolysaccharides usually are formed by the repetition of a disaccharide unit of an aminosugar and an acid sugar.
Other common constituents are sulfate groups linked to certain monosaccharides. Usually heteropolysaccharides are associated with proteins forming proteoglycans, glycosaminoglycans or mucopolysaccharides (since they are abundant in mucous secretions). As a group, they perform diverse functions: structural, water metabolism regulation (as a reservoir of water), cellular cement, biological sieve, biological lubricant, docking sites for growth factors, among other functions.
Established specific functions of some glycosaminoglycans are:
Hyaluronic Acid (Hyaluronate): It is a lubricant in the synovial fluid of joints, give consistency to vitreous humor, contributes to tensile strength and elasticity of cartilages and tendons (Answer to C-O6)
Chondroitin Sulfates: contributes to tensile strength and elasticity of cartilages, tendons, ligaments and walls of aorta. Dermatan sulfate (former chondroitin sulfate B) is found mainly in skin, but also is in vessels, heart, lungs. It may be related to coagulation and vascular diseases and other conditions.
Keratan sulfate: Present in cornea, cartilage bone and a variety of other structures as nails and hair.
Digestion and absorption
All carbohydrates absorbed in the small intestine must be hydrolyzed to monosaccharides prior to absorption. The digestion of starch begins with the action of salivary alpha-amylase/ptyalin, although its activity is slight in comparison with that of pancreatic amylase in the small intestine. Amylase hydrolyzes starch to alpha-dextrin, which are then digested by gluco-amylase (alpha-dextrinases) to maltose and maltotriose. The products of digestion of alpha-amylase and alpha dextrinase, along with dietary disaccharides are hydrolyzed to their corresponding monosaccharides by enzymes (maltase, isomaltase, sucrase and lactase) present in the brush border of small intestine. In the typical Western diet, digestion and absorption of carbohydrates is fast and takes place usually in the upper small intestine. However, when the diet contains carbohydrates not easily digestible, digestion and absorption take place mainly in the ileal portion of the intestine.
Digestion of food continues while simplest elements are absorbed. The absorption of most digested food occurs in the small intestine through the brush border of the epithelium covering the villi(small hair-like structure). It is not a simple diffusion of substances, but is active and requires energy use by the epithelial cells.
During the phase of carbohydrate absorption, fructose is transported into the intestinal cell’s cytosol, glucose and galactose competes with other Na + transporter required for operation. From the cytosol, monosaccharides pass into the capillaries by simple or facilitated diffusion.
Carbohydrates that are not digested in the small intestine, including resistant starch foods such as potatoes, beans, oats, wheat flour, as well as several nonpolysaccharides oligosaccharides and starch, are digested in a variable when they reach the large intestine. The bacterial flora metabolize these compounds in the absence of oxygen. This produces gases (hydrogen, carbon dioxide and methane) and short-chain fatty acids (acetate, propionate, butyrate). The gases are absorbed and excreted by breathing or through the anus. Fatty acids are rapidly metabolized. Thus butyrate, used mainly in the colonic, is an important nutritional source for these cells and regulates their growth, acetate into the blood and taken up by the liver, muscle and other tissues, and propionate, which is an important precursor of glucose in animals, it is not so in humans.
Are polymers made up of two to ten monosaccharide units joined together by glycosidic linkages. Oligosaccharides can be classified as di-, tri-, tetradepending upon the number of monosaccharides present. Among these the most abundant are the disaccharides, with two monosaccharide units.
QUESTIONS
1. Compounds having same structural formula but differing in spatial configuration are known as ………………
2. This change in rotation with time is called ………………
3. Equimolecular mixture of glucose and fructose thus formed is called ………………
4. Types of Polysaccharides are ……………… & ………………
