Polysaccharides Flashcards
Polysaccharides definition
Also called glycans
Consist of more than 10 monosaccharide units and/or their derivatives joined together by glycosidic linkage
Polysaccharides are classified into
- Homopolysaccharides (homoglycans): contain only one type of monosaccharides
Examples:
Starch, Glycogen, Dextran, Dextrin, Cellulose, Inulin - Heteropolysaccharides (heteroglycans): contain more than one type of monosaccharides
Examples:
Glucosaminoglycans (GAGs, mucopolysaccharides), agar
Polysaccharides are also classified
According to their function:
1. Storage polysaccharides:
Starch, glycogen, dextran, and Inulin
2. Structural polysaccharides:
cellulose and agar
Starch
Structure:
- It is a glucosan (alpha D-glucose units)
- Consists of two layers:
1. Inner linear non-branching layer called amylose
- Glucose units are linked up together by alpha 1-4 glycosidic bonds
- iodine reacts with alpha amylose to give blue color
2. Outer highly branched layer called amylopectin
- the branch points occur every thirty linkages forming alpha 1-6 glycosidic bonds
- iodine reacts with amylopectin to give red-violet color
Similarity: both amylose and amylopectin are rapidly hydrolyzed by salivary and pancreatic amylase to yield maltose and dextrins
Function of starch: most common storage polysaccharide in plants
Alpha-Dextrin
Structure:
-Starch is partially hydrolyzed by the action of acids or enzymes into dextrins
-made of several glucose units
-hydrolyzed by alpha-dextrinase
Function:
Dextrin is used as mucilage
Dextran
Structure:
-It is formed of alpha-glucose units (glucosan)
-it is a highly branched molecule with most links are alpha 1-6 glycosidic bonds but rarely alpha 1-4 glycosidic bonds
Function:
-it is a storage polysaccharide produced by microorganisms (yeast and bacteria)
-it is used for intravenous infusion as plasma volume expander for treatment of hypovolemic shock
N.B.
Dextrose: is D-glucose, a term used in bedside medicine
Example: dextrose drip
Dextrin: is the partially digested product of starch
Dextran: is a high molecular weight carbohydrate, synthesized by bacteria
Glycogen
Animal starch
Structure:
-formed of alpha glucose units (glucosan)
-it is a highly branched molecule (branches every 10 glucose unit; more branches than amylopectin)
-it yields a red-violet color with iodine
Function:
-The major form of storage polysaccharide in animals and human body
-it is found mainly in the liver (about 6% of liver mass) and skeletal muscle (about 1% of muscle mass)
-during fasting: glycogen is hydrolyzed to glucose for maintenance of blood glucose level
Inulin
Structure:
-it is formed of fructose (fructosan)
Function:
-it is hydrolyzed by the enzyme inulinase in plants. It has no dietary importance in human beings as inulinase in absent in human
-it is used in Inulin clearance test to determine the rate of glumerular filtration. It can be used as a diet for diabetics
Cellulose
Structure:
-formed of beta-D-glucose units
Function:
-most abundant natural polymer found in the world, it is the structural component of the cell walls of nearly all plants
-cellulose is extremely resistant to hydrolysis whether by acid or by the digestive tract amylases. So, it can simulate peristaltic movement and prevent constipation
-the bacteria that live in the gut of ruminant animals secrete cellulase enzyme (beta-glucosidase) which is effective in the hydrolysis of cellulose
Glycoaminoglycans (GAGs)
-Structure: are long linear unbranched chains composed of repeating disaccharides units (acid sugar-amino sugar)
-The acid sugar may be either glucuronic or iduronic
-the amino sugar may be either D-glucosamine or D-galactosamine and the amino group is usually acetylated (sometimes sulfated)
Proteoglycans
All of GAGs except hyaluronic acid are found covalently attached to proteins, forming proteoglycan monomers
Heparin GAG
Sugar acid: L-iduronic acid
Amino sugar: glucosamine (non acetylated)
Sulfate group: more sulfated
Site: mast cells lining arteries of lungs, liver, and skin
Main function: anticoagulant
Heparan sulfate
Acid sugar: D-glucuronic acid
Amino sugar: glucosamine(acetylated)
Sulfate group: less sulfated
Site: extracellular receptors in cell membrane, basement membranes, components of cell surfaces
Main function: component of cell surfaces
Hyaluronic acid
Not proteoglycan
Sugar acid: D-glucuronic acid
Amino sugar: glucosamine (acetylated)
Sulfate group: non sulfated
Site: synovial fluid of joints, loose connective tissue, vitreous humor of eye, skin
Main function: shock absorbing and lubricant in joint cavities
Keratan sulfate
Sugar acid: galactose(not sugar acid)
Amino sugar: glucosamine(acetylated)
Sulfate group:sulfated
Site: type 1 is abundant in cornea
Type 2 is found in mucid connective tissue (bone,cartilage, tendons,ligaments and joints)
Main function: corneal transparency, support and protection
Chondroitin sulfate
Sugar acid:D-glucuronic acid
Amino sugar: galactosamine (acetylated)
Sulfate group: sulfated
Site: mucid connective tissue (bone, cartilage, tendons, ligaments, and joints)
Main function: elasticity and comprehensibility of cartilage in weight bearing. Support and protection
Dermatan sulfate
Sugar acid: L-iduronic acid
Amino sugar: galactosamine (acetylated)
Sulfate group: sulfated
Site: wide distribution, skin,blood vessels and heart valves (found in all organs in our body)
Main function: the main GAG in skin, blood coagulation , wounds repair and resistance to infection
Agar
Not found in our body
-a polysaccharide isolated from marine red algae
-composed of agarose and agaropectin
-agarose gel is used in gel chromatography and gel electrophoresis
-nutrient agar is used in preparation of culture media in microbiology
