Carbohydrates: Glycogen, Dextrins, Inulin, Cellulose

Glycogen (Animal Sugar or Animal Starch)

• Glycogen serves as the storage form of carbohydrates in animals and humans
• It is primarily found in the liver and muscle tissue
• Glycogen possesses a highly branched structure, more so than amylopectin, comprised of hundreds or thousands of units
• Alpha-D-glucose units are linked by α 1-4 and α 1-6 (at branch points) glycosidic bonds
• Glycogen gives a red color when combined with iodine
• Glycogen can be hydrolyzed by the amylase enzyme

Dextrins

• Dextrins are hydrolytic products of starch

Inulin (Fructosan)

• Inulin (Fructosan) is found in tubers and roots of dahlia and artichokes
• Inulin is a fructosan that is hydrolyzable to fructose
• It is also easily soluble in warm water
• It is used for determination of glomerular filtration rate (GFR), excreted completely by the renal tubules

Cellulose

• Cellulose is the main constituent of plant cell walls
• Sources for cellulose are all plants and fruits
• It is glucosan consisting of beta-D-glucose units linked by β 1-4 glucosidic bonds
• It is non-branched and insoluble in water
• Humans can't hydrolyze cellulose in the intestine due to the absence of cellulase
• Cellulose can prevent constipation by increasing stool bulk stimulating intestinal peristalsis

Chitin

• Chitin is the polysaccharide found in invertebrates
• It forms the exoskeleton of crustaceans and insects

Agar

• Agar is derived from algae
• Its structure is galactosan
• It serves as a solid medium for bacterial growth

Heteropolysaccharides (or glycans)

• Definition: Consists of repeating units with more than one type of simple sugar units.
• Two known types are:
  • Proteoglycans (containing Glycosaminoglycans or Mucopolysaccharides chains)
  • Glycoproteins

Proteoglycans vs. Glycoproteins

Functions of Glycoproteins

• Structural: Found in cell walls, collagen, elastin, fibrin, and bone matrix.
• Lubricant and protective: Mucins and mucous secretions.
• Immunologic: Immunoglobulin and interferone.
• Catalytic: Proteases, nucleases, glycosidases, hydrolases, and clotting factors.
• Cell attachment: Recognition sites (cell receptors) on cell-cell, virus-cell, bacterium-cell and hormone receptors.
• Hormonal: Chorionic gonadotropin and thyroid stimulating hormone (TSH).

Glycosaminoglycans (GAGs)

• These are heteropolysaccharides composed of repeating disaccharide units of amino sugars and uronic acids

• When these chains link to a protein molecule, the compound is known as a proteoglycan

• GAGs are compressible due to negative charges which repel, allowing water to interpose; compression squeezes out the water, reducing size

• Because of compressibility, it's present in connective tissue (CT); vitreous humor; synovial fluids; bone; elastin and ligaments which form tissues

• Carboxyl groups and sulfate groups give them a strongly negative nature

Types of GAGs

• Hyaluronic acid
• Chondroitin 4 (A) and 6 (C) sulfate
• Keratan sulfates I and II
• Heparin
• Heparan sulfate
• Dermatan sulfate

GAG Composition

Hyaluronic Acid

• Found in synovial fluid, vitreous humor of the eye, and loose connective tissue
• Primary function of Hyaluronic acid is to make extracellular matrix loose
• It permits cell migration (wound repair, morphogenesis & tumour spread)
• Helps make cartilage compressible and acts as lubricant in joints
• It is hydrolyzed by hyaluronidase (spreading factor)
  • Secreted by certain bacteria
  • Found in acrosomal cap of sperm
  • Hydrolyzes Hyaluronic acid in CT to helps spread of infection, called a "spreading factor"
  • Aids fertilization by hydrolyzing Hyaluronic acid around the ovum

Chondroitin Sulfates (4 and 6)

• Present in cartilage, tendons, ligaments, and bones
• Also found in Aorta, skin, cornea, and umbilical cord
• Binds collagen and holds fibers in a strong network, which helps with cartilage compressibility during weight bearing
• Aids in maintaining skeletal muscles shape

Keratan Sulfates I and II

• Keratan sulfate I is abundant in the cornea - with an important role corneal transparency
• Type II is found in loose connective tissue and cartilage

Heparin

• Found in granules of mast cells in liver, lung, and skin (only intracellular GAG).
• Secreted by mast cells beside blood vessels (liver, lungs, heart, spleen).
• Functions as a natural anticoagulant
• Activates lipoprotein lipase (clearing factor) that hydrolyzes blood lipids

Heparan Sulfate

• This proteoglycan is present on many cell surfaces as receptors, with a role in adhesion
• Also present in the basement membrane of kidney determining charge selectivity

Dermatan Sulfate

• It is present mainly in cornea, sclera, and skin
• Functions to play an important role with keratan sulphate in corneal transparency
• Maintains eye shape

Importance of Carbohydrates

• Glucose is the primary fuel of mammalian tissues.
• Oxidation of glucose gives glucuronic acid for liver conjugation
• This transforms insoluble compounds into soluble forms (hormones, drugs).
• Oligo- and polysaccharides are important constituents of cell membranes
• Cellulose forms plant cell walls, is important in preventing constipation
• Mucopolysaccharides are important components of connective tissues
• Heparin is the most important natural anticoagulant
• Carbohydrates combine with lipids or proteins by forming glycolipids or glycoproteins

Disaccharides

• Formed from 2 monosaccharide units linked by a glycosidic bond
• Physiologically important disaccharides: maltose, lactose, and sucrose.
• Types:
  • Reducing: Has a free aldehyde group (anomeric carbon)
  • Non-reducing: Has no free aldehyde group (anomeric carbon)

Maltose (Malt Sugar)

• A disaccharide of 2 α-D-glucose units linked by α 1-4 glycosidic bond.
• Found in germinating cereals and malt.
• Produced during hydrolysis of starch by amylase

Lactose (Milk Sugar)

• A disaccharide formed by condensation of β-D-galactose and β-D-glucose linked together by β-1-4 galactosidic bond.
• Found in milk; may appear in urine during pregnancy.
• Hydrolyzed by enzyme lactase.
• Lactase deficiency leads to lactose malabsorption, diarrhea, and flatulence

Sucrose (Cane, Table, or Beet Sugar)

• It is a disaccharide formed by the condensation of α-D-glucose and β-D-fructose which are connected by an α-β-1-2-glycosidic bond
• Bond is either α 1-2 glucosidic or β 2-1 fructosidic bond.
• Found in cane and beet sugar, sorghum, pineapple, carrots and roots
• Non-Reducing: It has sugar due to its glycosidic linkage involving carbonyl groups of fructose and glucose therefore there is no free functional group
• It displays a dextrorotatory property
• Hydrolysis: Upon being subjected to the invertase (or sucrase) enzyme, it produces an equal mixture of glucose and fructose, resulting in a levrorotatory mixture named invert sugar. Strong levorotatory of fructose inverts the previous action of sucrose

Polysaccharides

• These are sugars that results from the condensation of polymers (>10) of monosaccharide units or its derivatives.
• These exhibit non-reducing sugar behaviour because the condensation involves the carbonyl groups of the sugars, leaving only one free carbonyl group at the end of a big molecule
• Polysaccharides are classified into 2 groups based on their structure
  • Simple or homopolysaccharides
  • Mixed or heteropolysaccharides

Homopolysaccharides

• These are polysaccharides made up of one type of sugar unit
• Pentosans: Composed of pentoses (e.g., pectin in fruits)
• Hexosans: Composed of hexoses (e.g., glucosans, fructosans, galactosans, and mannosans)
• Hexosans include:
  • Starch (Plant sugar)
  • Glycogen (animal sugar or animal starch)
  • Dextrins
  • Inulin (Fructosan)
  • Cellulose
  • Chitin
  • Agar (galactosan)

Starch (Plant Sugar)

• The storage form of carbohydrates in plants.
• Found in cereals, potatoes, legumes and vegetables.
• Glucosan with α-D glucose units linked by α 1-4 and α 1-6 glycosidic bonds.
• The Chief constituents of the starch granuole
  • Inner layer (amylose)
  • Outer layer (amylopectin)

Amylose vs. Amylopectin

Explore Glycogen as the animal carbohydrate storage, found in liver and muscle. Understand Dextrins as starch hydrolytic products. Learn about Inulin (Fructosan) in tubers and roots, used for GFR determination. Discover Cellulose, the plant cell wall's main part.