Polysaccharides include a number of physiologically important carbohydrates. They can provide one of the three functions (1) structural or mechanical protection, (2) energy storage, and(3) bind water to resist dehydration.

Starch is a homopolymer of glucose forming an α-glucosidic chain, called a glucosan or glucan. It is the most important dietary carbohydrate in cereals, potatoes, legumes, and other vegetables. The two main constituents are amylose (13-20%), which has a nonbranching helical structure, and amylopectin (80-87%), which consists of branched chains, consisting of 24 to 30 glucose residues with α1 → 4 linkages in the chains and by α1 → 6 linkages at the branch points (Figure 1).

Fig1. The structure of starch and glycogen. Amylose is a linear polymer of glucose residues linked α1→4, which coils into a helix. Amylopectin and glycogen consist of short chains of glucose residues linked α1→4 with branch points formed by α1→6 glycoside bonds. The glycogen molecule is a sphere ~21 nm in diameter that can be seen in electron micrographs. It has a molecular mass of ~107 Da and consists of polysaccharide chains, each containing about 13 glucose residues. The chains are either branched or unbranched and are arranged in 12 con centric layers. The branched chains (each has two branches) are found in the inner layers and the unbranched chains in the outermost layer. The blue dot at the center of the glycogen molecule is glycogenin, the primer molecule for glycogen synthesis.

The extent to which starch in foods is hydrolyzed by amylase is determined by its structure, the degree of crystallization or hydration (the result of cooking), and whether it is enclosed in intact (and indigestible) plant cell walls. Individuals with diabetes or prediabetes have trouble controlling their glucose concentration and are encouraged to eat complex carbohydrates that do not rapidly increase glucose levels on ingestion. The glycemic index of a starchy food is based on the extent to which it raises the blood concentration of glucose compared with an equivalent amount of glucose or a reference food such as white bread or boiled rice. An important determinant of the glycemic index is its digestibility. Glycemic index ranges from 1 (or 100%) for starches that are readily hydrolyzed in the small intestine to 0 for those that are not hydrolyzed at all.

Glycogen is the energy storage polysaccharide in animals and is sometimes called animal starch. It is a more highly branched structure than amylopectin, with chains of 12 to 15 α-d-glucopyranose residues (in α1 → 4 glucosidic linkage) with branching by means of α1 → 6 glucosidic bonds. Muscle glycogen granules (β-particles) are spherical and contain up to 60,000 glucose residues; in liver there are similar granules and also rosettes of glycogen granules that appear to be aggregated β-particles. Glycogen can be readily mobilized by the liver to support glucose needs of peripheral tissues.

Inulin is a polysaccharide of fructose (a fructosan) found in tubers and roots of dahlias, artichokes, and dandelions. It is not hydrolyzed by intestinal enzymes, so has no nutritional value.

It is used clinically to assess kidney function. When infused it is excreted by the kidney. The ability of the kidney to remove inulin is a reflection of the glomerular filtration rate. Dextrins are intermediates in the hydrolysis of starch. Cellulose is the chief constituent of plant cell walls. It is insoluble and consists of β-d-glucopyranose units linked by β1 → 4 bonds to form long, straight chains strengthened by cross-linking hydrogen bonds. Mammals lack any enzyme that hydrolyzes the β1 → 4 bonds, and so cannot digest cellulose. It is the major component of dietary fiber. Microorganisms in the gut of ruminants and other herbivores can hydrolyze the linkage and ferment the products to short-chain fatty acids as a major energy source. Recent data suggest that the availability of these short-chain fatty acids, which is determined by the microbial composition in the intestine, can impact metabolic health in at-risk individuals (diabetes, obesity, irritable bowel disease). By manipulating the microbial composition using prebiotics, one can shift the microbial population and possibly improve metabolic health. There is some bacterial metabolism of cellulose in the human colon. Chitin is a structural polysaccharide in the exoskeleton of crustaceans and insects, and also in mushrooms. It consists of N-acetyl-d-glucosamine units joined by β1 → 4 glycosidic bonds. Pectin occurs in fruits; it is a polymer of galacturonic acid linke α1 → 4, with some galactose an/or arabinose branches, and is partially methylated (Figure 2).

Fig2. The structures of some important nonstarch polysaccharides.

Glycosaminoglycans (mucopolysaccharides) are complex carbohydrates containing amino sugars and uronic acids. They may be attached to a protein molecule to form a proteoglycan. Proteoglycans provide the ground or packing substance of connective tissue . They hold large quantities of water and occupy space because of the large number of —OH groups an negative charges on the molecule, which, by repulsion, keep the carbohydrate chains apart. They serve to cushion or lubricate other structures, such as connective tis sue in a joint and cartilage. Examples are hyaluronic acid and chondroitin sulfate. Heparin is an important anticoagulant (Figure 3).

Fig3. Structure of some complex polysaccharides and glycosaminoglycans.

Glycoproteins (also known as mucoproteins) are proteins containing branched or unbranched oligosaccharide chains (Table 1), including fucose (Figure 4). They occur in cell membranes  and many proteins are glycosylated. The sialic acids are N- or O-acyl derivatives of neuraminic acid (see Figure 4). Neuraminic acid is a nine-carbon sugar derived from mannosamine (an epimer of glucosamine) and pyruvate. Sialic acids are constituents of both glycoproteins and gangliosides.

Proteins can undergo glycation, which is the nonenzymatic addition of a saccharide (eg, glucose) to a protein. Glycation increases as glucose levels increase. A glycated form of hemoglobin (hemoglobin A1c) increases in individuals with diabetes. It is used for the diagnosis of diabetes. It is also monitored during medical treatment of diabetes to determine how well glucose is managed.

Fig 4. β-l-Fucose (6-deoxy-β-l-galactose) and N-acetylneuraminic acid, a sialic acid.

Table1. Saccharides Found in Glycoproteins

مواضيع ذات صلة

الأهمية التشخيصية للإنزيمات

التمائم الإنزيمية Coenzymes

نوعية الإنزيمات Enzyme Specificity

طلائع الإنزيمات Proenzymes

الأدوية المضادة للصفيحات

تفعيل (تنشيط) الصفيحات والتخثر

بروتينات حل الخثرات Fibrinolysis Proteins

ضبط مستويات الثرومبين الجوال

Collagen illustrates the role of posttranslational processing in protein maturation

شلال التخثر Coagulation cascade

بروتينات الإرقاء والخثار Hemostasis & Thrombosis Proteins

جملة المتممة Complement system