chapter 7 carbohydrates and glycobiology -...
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CHAPTER 7 Carbohydrates and Glycobiology
– Structures and names of monosaccharides – Open-chain and ring forms of monosaccharides – Structures and properties of disaccharides – Biological function of polysaccharides – Biological function of glycoconjugates
Key topics about carbohydrates
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Carbohydrates
• Named so because many have formula Cn(H2O)n
• Produced from CO2 and H2O via photosynthesis in plants
• Range from as small as glyceraldehyde (Mw = 90 g/mol) to as large as amylopectin (Mw = 200,000,000 g/mol)
• Fulfill a variety of functions including – energy source and energy storage
– structural component of cell walls and exoskeletons
– informational molecules in cell-cell signaling
• Can be covalently linked with proteins to form glycoproteins and proteoglycans
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Aldoses and Ketoses • An aldose contains an aldehyde functionality • A ketose contains a ketone functionality
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Enantiomers
• Enantiomers: Stereoisomers that are nonsuperimposable mirror images
• In sugars that contain many chiral centers, only the one that is most distant from the carbonyl carbon is designated as D (right) or L (left)
• D and L isomers of a sugar are enantiomers – For example, L and D glucose have the same water solubility
• Most hexoses in living organisms are D stereoisomers. Some simple sugars occur in the L-form, such as L-arabinose
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Drawing Monosaccharides
• Chiral compounds can be drawn using perspective formulas
• However, chiral carbohydrates are usually represented by Fischer projections
• Horizontal bonds are pointing toward you; vertical bonds are projecting away from you
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Diastereomers
• Diastereomers: stereoisomers that are not mirror images
• Diastereomers have different physical properties – For example, water solubilities of threose and erythrose are different
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Epimers
• Epimers are two sugars that differ only in the configuration around one carbon atom
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Structures to Know
• Ribose is the standard five-carbon sugar • Glucose is the standard six-carbon sugar • Galactose is an epimer of glucose • Mannose is an epimer of glucose • Fructose is the ketose form of glucose
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Hemiacetals and Hemiketals • Aldehyde and ketone carbons are electrophilic • Alcohol oxygen atom is nucleophilic • When aldehydes are attacked by alcohols, hemiacetals form • When ketones are attacked by alcohols, hemiketals form
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Cyclization of Monosaccharides
• Pentoses and hexoses readily undergo intramolecular cyclization • The former carbonyl carbon becomes a new chiral center, called
the anomeric carbon • The former carbonyl oxygen becomes a hydroxyl group; the
position of this group determines if the anomer is α or β • If the hydroxyl group is on the opposite side (trans) of the ring as
the CH2OH moiety the configuration is α • If the hydroxyl group is on the same side (cis) of the ring as the
CH2OH moiety, the configuration is β
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Pyranoses and Furanoses
• Six-membered oxygen-containing rings are called pyranoses
• Five-membered oxygen-containing rings are called furanoses
• The anomeric carbon is usually drawn on the right side
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Chain-Ring Equilibrium and Reducing Sugars
• The ring forms exist in equilibrium with the open-chain forms • Aldehyde can reduce Cu2+ to Cu+ (Fehling’s test) • Aldehyde can reduce Ag+ to Ag0 (Tollens’ test) • Allows detection of reducing sugars, such as glucose
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Colorimetric Glucose Analysis
• Nowadays, enzymatic methods are used to quantify reducing sugars such as glucose
– Glucose oxidase catalyzes the conversion of glucose to gluconolactone and H2O2
– H2O2 oxidizes organic molecules into highly colored compounds
– Concentrations of such compounds is measured
• Electrochemical detection is used in portable glucose sensors
O
OH
OHOH
OH
CH2OHO
OH
OH O
OH
CH2OH
NH2
NH2
OCH3
OCH3
NH
NH
OCH3
OCH3
β-D-Glucose δ-D-Gluconolactone
Glucose oxidase
O2
H2O2
Peroxidase
2 H2O
Reducedo-dianisidine(faint orange)
Oxidizedo-dianisidine
(bright orange)
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Important Hexose Derivatives
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The Glycosidic Bond
• Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon
• The glycosidic bond (an acetal) between monomers is less reactive than the hemiacetal at the second monomer – Second monomer, with the hemiacetal, is reducing – Anomeric carbon involved in the glycosidic linkage is nonreducing
• The disaccharide formed upon condensation of two glucose
molecules via 1 → 4 bond is called maltose
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Nonreducing Disaccharides
• Two sugar molecules can be also joined via a glycosidic bond between two anomeric carbons
• The product has two acetal groups and no hemiacetals • There are no reducing ends, this is a nonreducing sugar • Trehalose is a constituent of hemolymph of insects
– Provides protection from drying – Resurrection plant (> 15 yrs)
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Polysaccharides
• Natural carbohydrates are usually found as polymers • These polysaccharides can be
– homopolysaccharides – heteropolysaccharides – linear – branched
• Polysaccharides do not have a defined molecular weight. – This is in contrast to proteins because unlike proteins,
no template is used to make polysaccharides
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Glycogen
• Glycogen is a branched homopolysaccharide of glucose – Glucose monomers form (α1 → 4) linked chains – Branch-points with (α1 → 6) linkers every 8–12 residues – Molecular weight reaches several millions – Functions as the main storage polysaccharide in animals
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Starch
• Starch is a mixture of two homopolysaccharides of glucose • Amylose is an unbranched polymer of (α1 → 4) linked
residues • Amylopectin is branched like glycogen but the branch-
points with (α1 → 6) linkers occur every 24–30 residues • Molecular weight of amylopectin is up to 200 million
• Starch is the main storage polysaccharide in plants
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Glycosidic Linkages in Glycogen and Starch
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Mixture of Amylose and Amylopectin in Starch
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Metabolism of Glycogen and Starch
• Glycogen and starch often form granules in cells
• Granules contain enzymes that synthesize and degrade these polymers
• Glycogen and amylopectin have one reducing end but many nonreducing ends
• Enzymatic processing occurs simultaneously in many nonreducing ends
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Cellulose
• Cellulose is a branched homopolysaccharide of glucose – Glucose monomers form (β1 → 4) linked chains – Hydrogen bonds form between adjacent monomers – Additional H-bonds between chains – Structure is now tough and water-insoluble – Most abundant polysaccharide in nature – Cotton is nearly pure fibrous cellulose
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Hydrogen Bonding in Cellulose
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Cellulose Metabolism
• The fibrous structure and water-insolubility make cellulose a difficult substrate to act on
• Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose
• Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (β1 →4) linkages
• Ruminants and termites live symbiotically with microorganisms that produces cellulase
• Cellulases hold promise in the fermentation of biomass into biofuels
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Chitin
• Chitin is a linear homopolysaccharide of N-acetylglucosamine
– N-acetylglucosamine monomers form (β1 → 4)-linked chains
– Forms extended fibers that are similar to those of cellulose
– Hard, insoluble, cannot be digested by vertebrates
– Structure is tough but flexible, and water-insoluble
– Found in cell walls in mushrooms, and in exoskeletons of insects, spiders, crabs, and other arthropods
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Chitin
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Agar and Agarose
• Agar is a complex mixture of hetereopolysaccharides containing modified galactose units
• Agar serves as a component of cell wall in some seaweeds
• Agarose is one component of agar • Agar solutions form gels that are commonly used in the
laboratory as a surface for growing bacteria • Agarose solutions form gels that are commonly used in the
laboratory for separation DNA by electrophoresis
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Agar and Agarose
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Glycosaminoglycans • Linear polymers of repeating disaccharide units • One monomer is either
– N-acetyl-glucosamine or – N-acetyl-galactosamine
• Negatively charged – Uronic acids (C6 oxidation) – Sulfate esters
• Extended hydrated molecule – Minimizes charge repulsion
• Forms meshwork with fibrous proteins to form extracellular matrix – Connective tissue – Lubrication of joints
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Heparin and Heparan Sulfate
• Heparin is linear polymer, 3–40 kDa • Heparan sulfate is heparin-like polysaccharide but
attached to proteins • Highest negative charge density biomolecules • Prevent blood clotting by activating protease
inhibitor antithrombin • Binding to various cells regulates development and
formation of blood vessels • Can also bind to viruses and bacteria and decrease
their virulence
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Glycoconjugates: Glycoprotein
• A protein with small oligosaccharides attached – Carbohydrate attached via its anomeric carbon – About half of mammalian proteins are glycoproteins – Carbohydrates play role in protein-protein recognition – Only some bacteria glycosylate few of their proteins – Viral proteins heavily glycosylated; helps evade the immune
system
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Glycoconjugates: Glycolipids
• A lipid with covalently bound oligosaccharide – Parts of plant and animal cell membranes – In vertebrates, ganglioside carbohydrate
composition determines blood groups – In gram-negative bacteria, lipopolysaccharides cover
the peptidoglycan layer
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Bacterial lipopolysaccharides. lipopolysaccharide of the outer membrane of Salmonella typhimurium. Kdo is 3-deoxy-D-manno-octulosonic acid ; Hep is L-glycero-D-manno-heptose; AbeOAc is abequose (a 3,6-dideoxyhexose) acetylated on one of its hydroxyls. Different bacterial species have in common a lipid region (lipid A), a core oligosaccharide also known as endotoxin, and an O-specific chain, which is the principal determinant of the serotype of the bacterium
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Glycoconjugates: Proteoglycans
• Sulfated glycosaminoglycans attached to a large rod-shaped protein in cell membrane – Syndecans: protein has a single transmembrane
domain – Glypicans: protein is anchored to a lipid membrane – Interact with a variety of receptors from
neighboring cells and regulate cell growth
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GlcNS (N-sulfoglucosamine) with a sulfate ester at C-6 GlcA and IdoA with a sulfate ester at C-2
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Proteoglycans
• Different glycosaminoglycans are linked to the core protein
• Linkage from anomeric carbon of xylose to serine hydroxyl
• Our tissues have many different core proteins; aggrecan is the best studied
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A typical tetrasaccharide linker connects a glycosaminoglycan (chondroitin 4-sulfate) to a Ser residue in the core protein. The xylose residue at the reducing end of the linker is joined by its anomeric carbon to the hydroxyl of the Ser residue
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Proteoglycan Aggregates
• Hyaluronan and aggrecan form huge (Mr > 2•108) noncovalent aggregates
• Hold lots of water (1000× its weight); provides lubrication • Very low friction material • Covers joint surfaces: articular cartilage
– Reduced friction – Load balancing
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62 Proteoglycan aggregate of the extracellular matrix
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Extracellular Matrix (ECM)
• Material outside the cell • Strength, elasticity, and physical barrier in tissues • Main components
– Proteoglycan aggregates – Collagen fibers – Elastin (a fibrous protein)
• ECM is a barrier for tumor cells seeking to invade new tissues – Some tumor cells secrete heparinase that degrades ECM
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Interaction of the Cells with ECM
• Some integral membrane proteins are proteoglycans – Syndecans
• Other integral membrane proteins are receptors for extracellular proteoglycans – Integrins
• These proteins link cellular cytoskeleton to the ECM and transmit signals into the cell to regulate – cell growth – cell mobility – apoptosis – wound healing
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Oligosaccharides in Recognition
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Glycoconjugates: Analysis
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Chapter 7: Summary
• structures of some important monosaccharides • structures and properties of disaccharides • structures and biological roles of polysaccharides • functions of glycosylaminoglycans as structural components of
the extracellular matrix • functions glycoconjugates in regulating a variety of biological
functions
In this chapter, we learned about
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