Chapter 7

Carbohydrates and Glycobiology

Carbohydrates

Carbohydrates The most abundant biomolecules on Earth Many carbohydrates: (CH2O)n

Polyhydroxy aldehydes or ketones Glycoconjugates

Carbohydrate polymers attached to proteins or lipids

1. Energy source 2. ECM 3. Lubricant 4. Protection barrier 5. Cell-cell communication 6. Intracellular trafficking

Functional group: Aldose and Ketose

# of carbons: triose (C3)tetrose (C4) pentose (C5) hexose (C6)heptose (C7)

Carbohydrates

Classes of carbohydrates Monosaccharides (simple sugar) Oligosaccharides

Disaccharides Polysaccharides

7.1 Monosaccharides and Disaccharides

Aldoses and Ketoses

Monosaccharide

Unbranched carbon chain with 3 to 7 carbons Triose, tetrose, pentose, hexose, heptose

One carbonyl C & other carbons with hydroxyl group Aldose : carbonyl group is at the end aldehyde group Ketose : carbonyl group is not at the end ketone

Chiral Centers of Monosaccharides

One or more asymmetric (chiral) carbon centers Optically active isoforms; optical isomers or enantiomers # of stereoisomers; 2n (n = # of chiral centers)

Chiral Centers of Monosaccharides

L- or D-isomer Depending on the configuration of the reference carbon D-isomer

Reference carbon configuration is the same as D-glyceraldehyde -OH on the reference carbon; the right in the projection formula most hexoses of living organism

Chiral Centers of Monosaccharides

Chiral Centers of Monosaccharides

Numbering C from the C nearest the carbonyl group Nomenclature of 4- and 5-carbon ketoses

Inserting ‘ul’ into the name of a corresponding aldose Ribose ribulose

Epimer Two sugars that differ only in the configuration around one C

Cyclic Structure of Monosaccharides

Formation of cyclic structure Monosaccharides with 5 or more carbons

(+ aldotetrose) Formation of hemiacetals or hemiketals

Reaction between alcohol and aldehyde or ketone

Cyclic Structure of Monosaccharides

Generation of or isomers Anomers

Isomers differ only in the configuration of hemiacetal or hemiketal carbon

Mutarotation Interconversion of and anomers

in aqueous solution D-glucose

1/3 -D-glucose 2/3 -D-glucose Very small amount of linear and

glucofuranose

Types of ring structure Pyranose : 6-membered ring Furanose : 5-membered ring

Conformation and Configuration

Conformation of 6-membered pyranose Not planar 2 “chair” conformations Interconvertible without the breakage of covalent bonds

Hexose Derivatives

Glucosamine, galactosamine, mannosamine C-2 OH is replaced with amino group

N-acetylglucosamine, N-acetylmuramic acid Bacterial cell wall structure

C-6 deoxy sugars L-galactose L-fucose (glycoproteins, glycolipids) L-mannose L-rhamnose (plant polysaccharides)

Acidic sugars Aldonic acid : oxidation of the carbonyl carbon

Gluconic acid gluconate (ionization at pH 7) Uronic acid : oxidation of the other end of the C chain

Glucuronic acid, galacturonic acid, mannuronic acid Lactone

Stable intramolecular ester formation of aldonic and uronic acids N-acetylneuraminic acid (sialic acid)

Component of many glycoproteins and glycolipids in animals Phosphorylated sugar

Trap the sugar inside the cell Activation of sugars for subsequence chemical transformation

Hexose Derivatives

Monosaccharide are Reducing Agents

Reducing sugar Sugars capable of reducing Fe3+ or Cu2+

Oxidation of carbonyl to carboxyl group Only linear forms can be oxidized

Detection of glucose levels Fehling’s reaction

Measuring the amount of oxidizing agent reduced by reducing sugar

Using glucose oxidase

Red cuprous oxide precipitate under alkaline conditions

Disaccharides

Disaccharide (maltose, lactose, sucrose)

Glc (14)Glc

O-glycosidic bond Covalent bond between

monosaccharides Reaction of –OH (alcohol) with

anomeric carbon of another sugar (hemiacetal) to form acetal

Hydrolysis by acid

N-glycosyl bond Anomeric carbon joined to N

Reducing end A free anomeric carbon

Disaccharides

Reducing Disaccharides

Maltose, lactose Nonreducing disaccharides

(glycosides)

Sucrose Major intermediate product of

photosynthesis

Trehalose Major constituent of the

circulating fluid (hemolymph) of insects energy-storage

7.2 Polysaccharides

1. monosaccharide unit2. chain length3. type of bond4. degree of branching

Polysaccharides (Glycan)

Polysaccharides (Glycan)

Types of polysaccharide Homopolysaccharides Contain single type of monomer Storage of monomer : glycogen, starch Structural elements : cellulose, chitin

Heteropolysaccharides Contain two or more types of monomer Extracellular support

Bacterial cell wall Extracellular matrix of animal

Synthesis of polysaccharides Enzymatic polymerization without

template No specific stopping point

Storage polysaccharide

Starch (plant) and glycogen (animal) Exist as intracellular clusters or granules Heavily hydrated

Why do cells store glucose in the form of glycogen? Glycogen in hepatocyte : 0.01M equivalent to 0.4M glucose Little contribution to the osmolarity of cytosol Problem with glucose uptake

Homopolysaccharides as Stored Forms of Fuel

Glycogen and starch

Starch Amylose; D-glucose connected by 14 linkages Amylopectin; 14 chains with 16 branches

Glycogen 14 chains with extensive 16 branches More compact than starch Stored in liver (7% of wet weight) and skeletal muscle Degradation from nonreducing ends

Homopolysaccharides Playing Structural Roles

Cellulose Component of plant cell wall Fibrous, tough, water-insoluble Linear D-glucose chains (unbranched)

10,000 to 15,000 glc units 14 linkages; different structural and physical properties from amylose

Cellulase Produced by Trichonympha living in termites, wood-rot fungi, bacteria Animals produce only -amylase

Chitin Exoskeleton of arthropods Linear N-acetylglucosamine chains connected by 14 linkages

Structural difference between cellulose and amylose

Folding of Homopolysaccharides

Factors affecting folding of homopolysaccharides Weak interactions

Hydrogen bonding Rotation of glycosidic bonds

Restriction by steric hindrance

Folding of Homopolysaccharides

Stable structure Starch and glycogen Tightly coiled helix stabilized by H bond Amylose

Helical structure with 6 residues/ turn

Cellulose Straight extended chain Extensive intra- and inter-chain H bond

Stable fiber of great tensile strength Low water content

Heteropolysaccharides of Bacterial Cell Walls (Peptidoglycan)

Bacterial cell wall Alternating N-acetylglucosamine and

N-acetylmuramic acid; 14

Polysaccharide chains are linked by bacterial specific peptide linkage

Prevention of cell wall formation Lysozyme : hydrolysis of 14

linkage Tears, bacterial viruses

Penicillin : preventing peptide-mediated cross-linking

Heteropolysaccharides of Bacterial Cell Walls

Heteropolysaccharides of Algal Cell Walls

Cell walls of red algae Agar

A mixture of sulfated heteropolysaccharides

- D-glactose and L-glactose derivatives (3,6-ether link) Agarose (Mr ~ 120,000)

Unbranched polymer of D-Gal(14)3,6-anhydro-L-Gal2S Formation of gel after heating-cooling in water

Applications of agar and agarose Surface to grow bacteria Capsule for drug delivery Used as matrix for DNA electrophoresis

Glycosaminoglycans

Extracellular matrix Gel-like material filling the extracellular space in the tissues Functions

Holding cells together Providing porous pathways for nutrients and signaling

molecules Composition

Heteropolysaccharide: glycosaminoglycan Fibrous proteins: collagen, elastin, fibronectin, laminin

Glycosaminoglycan Linear polymer of repeating disaccharides

N-acetylglucosamine or N-acetylgalactosamine Uronic acid : D-glucuronic or L-iduronic acid

Unique to animal and bacteria (not plant) Addition of sulfate groups - High density of negative charges extended conformation - Specific recognition by protein ligands - Attached to extracellular protein proteoglycan

Repeating disaccharides of Glycosaminoglycans

Hyaluronic acid : up to 50,000 repeat Clear, highly viscous solutions

Lubricants in joints Vitreous humor of vertebrate eye ECM of cartilage and tendons

tensile strength and elasticity

Hyaluronidase Hydrolysis of glycosidic linkage of

hyaluronate Pathogenic bacteria or sperm invasion

Chondroitin sulfate Tensile strength of cartilage, tendons,

ligaments, and the walls of the aorta Dermatan sulfate

Pliability of skin, blood vessel, heart valves

GlcA in chondroitin IodA (iduronate)

Hyaluronate, ~ 50,000

Chondroitin 4-sulfate, 20-60

Glycosaminoglycans

Keratan sulfate Cornea, cartilage, bone Horny structures : horn, hair, hoofs,

nails, claws Heparin

Anticoagulant made in mast cells Binding to antithrombin

Causes antithrombin to bind to and inhibit thrombin (protease) prevent blood clotting

Highest negative charge density in biological macrobmolecules

Keratan sulfate, ~ 25

Heparin, 15-90