CHAPTER 19: POLYSACCHARIDES AND CARBOHYDRATE STRUCTURES

Polysaccharides: These macromolecules occur in nature with molecular weights exceeding one million. Their biologic activity is determined largely by the sequence and three-dimensional structure in which their various monosaccharides are arranged. Glucose is the most prevalent monosaccharide unit in polysaccharides, but polysaccharides of mannose, fructose, galactose, xylose, and arabinose are also found in nature. Some polysaccharides are present in glycoproteins and in glycolipids.

Three important polysaccharides to animals that consist entirely of glucose are glycogen, starch, and cellulose. Glycosaminoglycans are important mucopolysaccharides, chitin is an important structural polysaccharide of invertebrates, and inulin, a fructose polymer of plants, is sometimes used to evaluate kidney function.

Glycogen, the form in which animal tissues store glucose, has a basic structure that consists of numerous a-1,4 linked D-glucose molecules, and is sometimes referred to as "animal starch". Once in every 11 to 18 linkages, a branching point occurs via an a-1,6 linkage, which yields a tree-like structure. Glycogen can account for up to 6-8% of the weight of the canine liver; however, this amount may decrease to less than 1% between meals (e.g., overnight). The glycogen content of muscle is normally around 1% by weight, a value that can be similarly decreased following prolonged exercise. Then, if a normal diet is consumed, glycogen contents of both liver and muscle can be replenished.

Glycogen is stored in both liver and muscle in granules of varying size. The enzymes required for both the synthesis and degradation of glycogen, as well as several factors required for control of these processes, are also found in glycogen granules\.

Starch, the glucose store of plants, is an important food source of carbohydrate, and is found in cereals, potatoes, legumes, and other vegetables. Starch contains 15 to 20% amylose, and 80-85% amylopectin. While both glycogen and starch consist totally of D glucose, amylose is nonbranched, and amylopectin contains about one a-1,6 branch per 30 a-1,4 linkages. Dextrins are substances formed in the course of hydrolytic breakdown of starch and glycogen in the digestive tract, by the action of salivary and pancreatic amylase. Limit dextrins are the first formed products as hydrolysis reaches a certain degree of branching.

Storage polysaccharides such as starch and glycogen are usually deposited in the form of large granules in the cytoplasm of cells. These granules can sometimes be isolated from cell extracts through differential centrifugation. In times of glucose surplus, glucose units are stored by undergoing enzymatic linkage to the ends of starch or glycogen chains; in times of metabolic need, these glucose units are released enzymatically for use as fuel.

Inulin is a fructose polymer found in tubers and roots of dahlias, artichokes, and dandelions. Since it is hydrolyzable to individual fructose units, it is properly referred to as a fructosan. This polysaccharide, unlike plant starch, is easily soluble in warm water, and has been used in clinical evaluation of kidney function. Inulin is freely filtered by the kidneys, it is not reabsorbed or secreted by the nephron, it is not metabolized following injection into blood, it is nontoxic, and therefore its clearance from the circulation closely approximates the glomerular filtration rate (GFR).

Cellulose is insoluble in water, and consists of long, non-branched b-1,4 linked glucose units strengthened by cross-linked hydrogen bonds. Due to b-linkages, cellulose is nondigestible by enzymes of mammalian origin (i.e., constitutive enzymes). However, in the digestive tracts of herbivores, cellulase-containing microbes attack the blinkages, thus making the glucose units of cellulose available as major metabolic entities for the microbes. In return, the microbes make volatile fatty acids available for the host. This process also takes place (to a limited extent) by microbes in the colons of omnivores and carnivores. Fungi and protozoa secrete cellulases, and the digestion of wood by termites depends on protozoa in their digestive tract. Cellulose, containing over 50% of the carbon in plant material, is the most abundant organic compound on earth.

Chitin is a key structural polysaccharide of invertebrates found, for example, in the exoskeletons of crustaceans and insects. It consists of N-acetyl-D-glucosamine units joined by the same (undigestible) b-1,4 glycosidic linkages found in cellulose. Thus, chitin is similar to cellulose except that the C-2 moiety is an acetylated amino group rather than a hydroxyl group.

Carbohydrate Derivatives: Several classes of compounds derived from carbohydrates are metabolically significant. These include sugar alcohols, where the carbonyl group is reduced to a hydroxyl group such as glycerol. Glycerol is a sugar alcohol and a gluconeogenic precursor in the liver. It also serves as the backbone of many lipids (to include tri-, di-, and monoglycerides, as well as phospholipids). Another important sugar alcohol is inositol (or myoinositol), which is formed from glucose 6-phosphate by many different organisms. Inositol is a six-membered polyhydroxy alcohol, that exists in animals primarily as the common triphosphoester isomer, inositol triphosphate (IP3), and in plants as a hexophosphoester. Most IP3 in animals is generated from membrane-bound phospholipid (phosphatidyl inositol), and participates in the intracellular Ca++ second messenger system.

Sugar acids are another important group of compounds involved with both structural polysaccharides (e.g., hyaluronate), and with clearing metabolites from the body. Uronic acid anions, such as glucuronate, result from the oxidation of the non-reducing terminal carbon of UDP-glucose from CH2OH to COO– . Glucuronates are conjugated (i.e., linked covalently) to certain metabolites (such as bilirubin and steroids), and drugs by liver enzymes to form hepatic glucuronides. These conjugates have increased water solubility for excretion in either bile or urine. Glucuronides are also used in the formation of certain glycoproteins, heparin, and vitamin C.

Aldonic acids result from the oxidation of the aldehyde (-CHO) of aldoses, as in the formation of D-gluconic acid. The phosphate ester of this acid is an important intermediate in the metabolism of glucose via the hexose monophosphate shunt (HMS). Ascorbic acid (vitamin C) is a lactone of a hexonic acid, and an essential nutrient for primates, guinea pigs, flying mammals, fish, and songbirds. This vitamin is an antioxidant participating in oxidation-reduction reactions, and in the addition of hydroxyl groups to proline and lysine in the formation of collagen. It also assists in the hepatic formation of bile acids from cholesterol, and in dopamine hydroxylation during catecholamine biosynthesis.

Deoxysugars lack one hydroxyl group, with perhaps the most important being 2-deoxyribose, the pentose moiety of DNA. Aminosugars are important components of glycoproteins, membranes, and bacterial cell walls.

Glycosaminoglycans (mucopolysaccharides) consist of chains of complex carbohydrates characterized by their content of amino sugars and uronic acids. When these chains are attached to a protein, the compound is known as a proteoglycan. The amount of carbohydrate in a proteoglycan is usually much greater than that found in a glycoprotein, and may comprise up to 95% of its weight. As ground substance, proteoglycans are associated with structural elements of tissues such as bone, elastin, and collagen. Their property of holding large quantities of water and occupying space, thus cushioning or lubricating other structures, is assisted by the large number of negatively charged groups on their molecules, which, by repulsion, keep the carbohydrate chains apart. Examples are hyaluronate, chondroitin sulfate, keratan sulfate, and heparin. Keratan sulfate, hyaluronate, and chondroitin sulfate are found in the extracellular matrix of cartilage, where they assist in helping it to cushion compressive forces. Hyaluronate is also found in synovial fluid, the vitreous body of the eye, and in bacteria.

Heparin is an anticoagulant found in the granules of mast cells, which are found most often in the skin, but also in the liver, lungs, and stomach. Mast cell tumors are common in dogs and cats, and affected animals develop bleeding disorders and secondary infections. Since mast cell granules also contain histamine, these tumors are also associated with gastric ulceration secondary to increased HCl secretion. Heparin can also bind to lipoprotein lipase (LPL; also called clearing-factor lipase) present in capillary walls, causing release of this enzyme into the circulation. The protein molecule of the heparin proteoglycan is unique, consisting exclusively of serine and glycine residues.

Heparan sulfate, which lacks the anticoagulant properties of heparin, has fewer N- and O sulfated groups yet more N-acetyl groups than heparin, and is found on many outer cell surfaces as a proteoglycan. Heparan sulfate is negatively charged, and participates in cell growth and cell-cell communication. It is also found in the basement membrane of the glomerulus, where it plays a major role in determining the charge selectiveness of the renal glomerular filtration barrier.

SUMMARY

Chapter 19 discusses polysaccharides and carbohydrate derivatives. Polysaccharides are macromolecules found in nature with molecular weights exceeding one million. They are composed of various monosaccharides arranged in a sequence and three-dimensional structure. Glucose is the most common monosaccharide unit in polysaccharides, but other monosaccharides like mannose, fructose, galactose, xylose, and arabinose are also found. Important polysaccharides in animals include glycogen, starch, and cellulose. Glycogen is the form in which animal tissues store glucose, while starch is the glucose store in plants. Cellulose is insoluble in water and is the most abundant organic compound on earth. Chitin is a structural polysaccharide found in invertebrates. Carbohydrate derivatives include sugar alcohols, sugar acids, aldonic acids, deoxysugars, aminosugars, and glycosaminoglycans. Glycosaminoglycans are chains of complex carbohydrates attached to proteins, known as proteoglycans. They are associated with structural elements of tissues and have properties like water retention and cushioning. Examples of glycosaminoglycans include hyaluronate, chondroitin sulfate, keratan sulfate, and heparin. Heparin is an anticoagulant found in mast cells and is also associated with gastric ulceration. Heparan sulfate, another glycosaminoglycan, is involved in cell growth and communication and is found in the basement membrane of the glomerulus.

OUTLINE

I. Polysaccharides

A. Occur in nature with molecular weights exceeding one million

B. Biologic activity determined by sequence and three-dimensional structure

C. Glucose is the most prevalent monosaccharide unit, but others are also found

D. Some polysaccharides are present in glycoproteins and glycolipids

E. Important polysaccharides in animals:

1. Glycogen - glucose store in animals, consists of a-1,4 linked D-glucose molecules with branching points

2. Starch - glucose store in plants, contains amylose and amylopectin

3. Cellulose - structural polysaccharide in plants, non-digestible by mammalian enzymes

4. Chitin - structural polysaccharide in invertebrates, similar to cellulose but with acetylated amino group

II. Carbohydrate Derivatives

A. Sugar alcohols

1. Glycerol - gluconeogenic precursor, backbone of lipids

2. Inositol - polyhydroxy alcohol, involved in intracellular Ca++ second messenger system

B. Sugar acids

1. Uronic acids - result from oxidation of non-reducing terminal carbon, involved in conjugation and excretion of metabolites

2. Aldonic acids - result from oxidation of aldehyde group, important intermediate in glucose metabolism

QUESTIONS

Flashcard 1:

Question: What are polysaccha

C. Ascorbic acid (vitamin C) - lactone of hexonic acid, antioxidant and essential nutrient

D. Deoxysugars - lack one hydroxyl group, important in DNA structure

E. Aminosugars - components of glycoproteins, membranes, and bacterial cell walls

F. Glycosaminoglycans (mucopolysaccharides

1. Chains of complex carbohydrates with amino sugars and uronic acids

2. Proteoglycans - carbohydrate chains attached to a protein

3. Examples: hyaluronate, chondroitin sulfate, keratan sulfate, heparin

G. Heparin - anticoagulant found in mast cells, also binds to lipoprotein lipase

)

rides?

Answer: Polysaccharides are macromolecules that occur in nature with molecular weights exceeding one million. Their biologic activity is determined by the sequence and three-dimensional structure in which their various monosaccharides are arranged.

Flashcard 2:

Question: What are three important polysaccharides that consist entirely of glucose?

Answer: The three important polysaccharides that consist entirely of glucose are glycogen, starch, and cellulose.

Flashcard 3:

Question: What is the basic structure of glycogen?

Answer: The basic structure of glycogen consists of numerous a-1,4 linked D-glucose molecules, with branching points occurring via an a-1,6 linkage.

Flashcard 4:

Question: What is the glucose store of plants?

Answer: Starch is the glucose store of plants and is found in cereals, potatoes, legumes, and other vegetables.

Flashcard 5:

Question: What is cellulose and why is it nondigestible by enzymes of mammalian origin?

Answer: Cellulose is a polysaccharide consisting of long, non-branched b-1,4 linked glucose units. It is nondigestible by enzymes of mammalian origin due to the b-linkages.

Flashcard 6:

Question: What is chitin and where is it found?

Answer: Chitin is a key structural polysaccharide found in the exoskeletons of crustaceans and insects. It consists of N-acetyl-D-glucosamine units joined by b-1,4 glycosidic linkages.

Flashcard 7:

Question: What are sugar alcohols and give an example?

Answer: Sugar alcohols are compounds derived from carbohydrates where the carbonyl group is reduced to a hydroxyl group. An example is glycerol.

Flashcard 8:

Question: What are glycosaminoglycans and what are they associated with?

Answer: Glycosaminoglycans are chains of complex carbohydrates characterized by their content of amino sugars and uronic acids. When attached to a protein, they form proteoglycans and are associated with structural elements of tissues such as bone, elastin, and collagen.

Flashcard 9:

Question: What is heparin and where is it found?

Answer: Heparin is an anticoagulant found in

Mind Map: Polysaccharides and Carbohydrate Derivatives

Polysaccharides

  • Glucose is the most prevalent monosaccharide unit in polysaccharides
  • Polysaccharides of mannose, fructose, galactose, xylose, and arabinose also found in nature
  • Polysaccharides present in glycoproteins and glycolipids
  • Important polysaccharides:
    • Glycogen
    • Starch
    • Cellulose
    • Glycosaminoglycans
    • Chitin
    • Inulin

Glycogen

  • Form in which animal tissues store glucose
  • Basic structure consists of a-1,4 linked D-glucose molecules
  • Branching points occur via a-1,6 linkage
  • Stored in liver and muscle in granules
  • Enzymes for synthesis and degradation found in glycogen granules

Starch

  • Glucose store of plants
  • Found in cereals, potatoes, legumes, and vegetables
  • Contains amylose (nonbranched) and amylopectin (branched)
  • Hydrolytic breakdown forms dextrins and limit dextrins

Cellulose

  • Insoluble in water
  • Consists of b-1,4 linked glucose units
  • Nondigestible by mammalian enzymes
  • Digestible by cellulase-containing microbes in herbivores
  • Fungi and protozoa secrete cellulases
  • Most abundant organic compound on earth

Glycosaminoglycans

  • Chains of complex carbohydrates with amino sugars and uronic acids
  • Attached to proteins, known as proteoglycans
  • Found in tissues like bone, elastin, and collagen
  • Examples: hyaluronate, chondroitin sulfate, keratan sulfate, heparin

Chitin

  • Key structural polysaccharide of invertebrates
  • Found in exoskeletons of crustaceans and insects
  • Similar to cellulose but with acetylated amino group at C-2

Inulin

  • Fructose polymer found in tubers and roots of plants
  • Soluble in warm water
  • Used in clinical evaluation of kidney function

Carbohydrate Derivatives

  • Sugar alcohols:
    • Glycerol

Study Plan: Chapter 19: Polysaccharides and Carbohydrate Derivatives

Day 1:

  • Read and understand the introduction to polysaccharides and their biologic activity.
  • Take notes on the different monosaccharides found in polysaccharides.
  • Study the presence of polysaccharides in glycoproteins and glycolipids.
  • Familiarize yourself with the three important polysaccharides in animals: glycogen, starch, and cellulose.

Day 2:

  • Focus on understanding the structure and function of glycogen.
  • Learn about the branching points and tree-like structure of glycogen.
  • Study the storage and distribution of glycogen in the liver and muscle.
  • Take note of the enzymes and factors involved in the synthesis and degradation of glycogen.

Day 3:

  • Shift your focus to starch, the glucose store of plants.
  • Understand the composition of starch, including amylose and amylopectin.
  • Differentiate between glycogen and starch in terms of structure and branching.
  • Learn about the hydrolytic breakdown of starch and glycogen, forming dextrins and limit dextrins.

Day 4:

  • Study the storage and deposition of starch and glycogen in cells.
  • Understand the enzymatic linkage and release of glucose units for fuel.
  • Focus on inulin, a fructose polymer, and its role in evaluating kidney function.
  • Learn about the solubility and clearance of inulin in the body.

Day 5:

  • Shift your focus to cellulose, the most abundant organic compound on earth.
  • Understand the structure and digestibility of cellulose.
  • Learn about the role of cellulase-containing microbes in breaking down cellulose.
  • Study chitin, a structural polysaccharide found in invertebrates, and its similarities to cellulose.

Additional Tips:

  • Review the key concepts and takeaways from each day's study session.
  • Create flashcards or summaries for important terms and definitions.
  • Practice answering questions or solving problems related to the chapter.
  • Seek clarification from your instructor or classmates if needed.
  • Take breaks and ensure you get enough rest to optimize learning and retention.