Carbohydrates - the biochemistry of sugars (Part III).pdf

Carbohydrates (Part III) Polysaccharides Heparin segment Dr Annie Godwin ([email protected])  CHAPTER 7 Carbohydrates and Glycobiology  Checklist: what we need to know Carbohydrates: Part III...

Carbohydrates (Part III) Polysaccharides Heparin segment Dr Annie Godwin ([email protected])  CHAPTER 7 Carbohydrates and Glycobiology  Checklist: what we need to know Carbohydrates: Part III Key terminology 1. Introduction to monosaccharides: Carbohydrate: A sugar understanding the chemistry of (monosaccharide) or one of its dimers (disaccharide) or polymers sugars, carbonyl groups and chiral (polysaccharide). centres. 2. Open-chain and ring forms of Monosaccharide: A carbohydrate consisting of a single sugar unit. monosaccharides, disaccharides & the glycosidic bond. Disaccharide: A carbohydrate consisting of two covalently joined 3. Understand the structure of linear monosaccharide units. and branched homo- and heteropolysaccharides. Polysaccharide (Glycan): A linear 4. Consider the biological function of or branched polymer of monosaccharide units linked by homo- and heteropolysaccharides. glycosidic bonds. Polysaccharides Polysaccharide (Glycan): A Natural carbohydrates are usually found as linear or branched polymers. polymer of These polysaccharides can be monosaccharide units linked by glycosidic bonds. – homopolysaccharides – heteropolysaccharides Homopolysaccharide: A – linear polysaccharide made up – branched of one type of monosaccharide unit. Polysaccharides do not have a defined molecular weight. Heteropolysaccharide: A – This is in contrast to proteins and DNA, since polysaccharide containing more than one type of no template is used to make polysaccharides. sugar. Amylose Polysaccharide structure Polysaccharides may be composed of 1, 2 or several different monosaccharides in a straight or branched chains of varying length. Homopolysaccharide: starch, glycogen, collagen, chitin. Heteropolysaccharide: agarose, petidoglycans, glycosaminoglycan. Starch (glucose: amylose + amylopectin) Starch is a mixture of two homopolysaccharides of D-glucose. Amylose is an unbranched polymer of (a1 ® 4) linked residues (MW varies few 1000 to > 1 million): Amylopectin is branched like glycogen but the branch-points with (a1 ® 6) linkers occur every 24–30 residues. Molecular weight of amylopectin is up to 200 million! Starch is the main storage polysaccharide in plants. Heavily hydrated due to number of exposed -OH. Hydrolyzed by a -amalyses. Glycogen (glucose) Glycogen is a branched homopolysaccharide of glucose. – Glucose monomers form (a1 ® 4) linked chains. – Branch-points with (a1 ® 6) linkers every 8–12 residues (more compact than starch). – Molecular weight reaches several millions. – Functions as the main storage polysaccharide in animals. Abundant in the liver and skeletal muscle. ‘n’+1 nonreducing ends and 1 reducing end Glucose units are removed from the nonreducing end. Glycosidases hydrolyze simultaneous branches speeding up the conversion of the polymer to monosaccharides. Glycogen (glucose) Glycogen is a branched homopolysaccharide of glucose. Whymonomers – Glucose store polysaccharides form (a1 ®like glycogen? 4) linked chains. – Branch-points with (a1 ® 6) linkers every 8–12 residues (more compact than starch). – Molecular weight reaches several millions. – Functions as the main storage polysaccharide in animals. Abundant in the liver and skeletal muscle. ‘n’+1 nonreducing ends and 1 reducing end Glucose units are removed from the nonreducing end. Glycosidases hydrolyze simultaneous branches speeding up the conversion of the polymer to monosaccharides. Dextrans Dextrans are bacterial and yeast polysaccharides. – Poly-D-glucose form (a1 ® 6) linked chains. – All have branch-points with (a1 ® 3) and some have (a1 ® 2) and (a1 ® 4) linkers. – Dental plaque on the surface of teeth is rich in dextrans. – Synthetic dextrans are commercially sold. These products are chemically cross-linked to form insoluble materials with various porosities. Cellulose Cellulose is an unbranched homopolysaccharide of glucose. – D-glucose monomers form (b1 ® 4) linked chains (10,000 – 15,000). – Hydrogen bonds form between adjacent monomers. – Additional H-bonds between chains. – Structure is now tough and water-insoluble. – Most abundant polysaccharide in nature. – Cotton & wood is nearly pure fibrous cellulose. Hydrogen bonding in Cellulose: Two units of a cellulose chain. Cellulose Metabolism The fibrous structure and water-insolubility make cellulose a difficult substrate to act on (found cell wall of plants). – Fungi, bacteria, and protozoa secrete cellulases, which allow them to use wood as source of glucose. – Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (b1 ®4) linkages. Ruminants and termites live symbiotically with microorganisms that produce cellulases. Cellulases hold promise in the fermentation of biomass into biofuels. The Gribble (Limnoria quadripunctata) Gribble is a wood-boring marine isopod. Historically damaged hulls of ships. Continues to damage wooden piers and docks in coastal communities. Modified cellulase with an extended substrate-binding motif and surface charges. Resulting from evolution in an ‘extreme’ marine environment? Could marine cellulases provide a more effective ways of converting wood to liquid fuel? Fungi, Ruminants and Cellulose Wood fungus growing on Oak: Fungi use cellulases to break or hydrolyse the cellulose (b1à4) glycosidic bonds. The wood then becomes a source of metabolizable sugar (glucose) for the fungi. Ruminants are the only vertebrates able to utilize cellulose as a food/energy Trichonympha source. Rumen: bacteria and protists secreting cellulase. Chitin Chitin is a linear homopolysaccharide of N-acetylglucosamine (NAG). – N-acetylglucosamine monomers form (b1 ® 4)-linked chains. – Forms extended fibers that are similar to those of cellulose (differ in loss of hydroxyl group at C-2 for acetylated amino group). – Hard (tough but flexible), water-insoluble, cannot be digested by vertebrates. – Found in cell walls in mushrooms, and in exoskeletons of insects, spiders, crabs, lobsters and other arthropods (making it 2nd most abundant polysaccharide). Agar and Agarose Experimental biology PCR practical 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 with fewest charged groups. 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. Glycosaminoglycans Extracellular space in multicellular tissues is filled with the extracellular matrix (a gel like substrate). ECM holds cells together and facilitates diffusion of nutrients and oxygen. Basement membrane is a specialized Hyaluronan ECM: collagens, laminin and heteropolysaccharides (glycosaminoglycans). Glycosaminoglycans are linear polymers with repeating disaccharide units – unique to animals and bacteria. Contain sulfate groups. ….or N-acetylgalactosamine Structure and role of Polysaccharides Carbohydrates part III: The finale! 1. Polysaccharides (glycans) serve as stored fuel and structural components of cell walls/ extracellular matrix. 2. The homopolysaccharides starch and glycogen are stored fuels, consist of D-glucose (a 1à4) linkage and branching. 3. The homopolysaccharides cellulose, chitin and dextran serve structural roles. 4. Homopolysaccharides fold in 3D. Starch and glycogen can form helical structures with interchain hydrogen bonding, cellulose and chitin form long straight strands that interact with neighbouring strands. 5. Bacterial and algae cell walls strengthened by heteropolysaccharides: peptidoglycan and agar. 6. Glycosaminoglycans are extracellular heteropolysaccharides, usually with a high-density of negative charge, which provide support and aid diffusion. Overview of Carbohydrates 1. Monosaccharides: the chemistry of sugars, carbonyl group and chiral centers. 2. Monosaccharides: Open-chain and ring forms of monosaccharides (hemiacetals and hemiketals). 3. Structures and nomenclature for monosaccharides 4. Structures and properties of disaccharides & the glycosidic bond. 5. Biological function of linear and branched homo- and heteropolysaccharides in nature. Heparin segment Polysaccharides (Quiz) 1. Which homopolysaccharide is an energy store in plants? 2. Which homopolysaccharide is an energy store in animals? 3. Which of these are polymers of a -glucose? (Starch, Glycogen, Cellulose, Chitin) 4. Name a polymer of b -glucose? 5. Name a homopolysaccharide which only has unbranched chains? 6. True or False: Agarose is a homopolysaccharide. 7. True or False: Glycosaminoglycans are polymers with repeating disaccharide units?