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Chapter 5

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Lovaye Kajiura

Biology Chapter 5: An Introduction to Carbohydrates Bernard Ho September 20, 2010 Sugars as Monomers − Monosaccharide  Simple sugar − Carbonyl group in monosaccharides can be found at the end of the molecule, forming an aldehyde sugar (aldose) or within the carbon chain, forming a ketone sugar (ketose) − Presence of a carbonyl group along with multiple hydroxyl groups provides an array of functional groups in sugars − By convention, carbons in monosaccharide are numbered consecutively, starting with the end nearest the carbonyl group − Ribose (five carbons) is a pentose and present in RNA and glucose (six carbons) is a hexose and present in DNA − Sugars do not usually exist in the form of linear chains, but form ring structures in aqueous solution − In glucose, carbon 1 forms a bond with an oxygen atom and with a hydroxyl group that can be oriented in two different ways (forms α-glucose and β-glucose) − Glucose is present in starch, cellulose, glycogen − Galactose is present in cartilage − Many distinct monosaccharides exist because so many aspects of their structure are variable o Alternative ring forms o Optical isomers with different arrangements of hydroxyl groups in space o Different mirror-image forms o Variation in carbon number o Aldose or ketose placement of carbonyl group − Distinguishing between the two ring forms of glucose o A-glucose, OH group at C #1 is below plane of ring o B-glucose, OH group at C #1 is above plan of ring Structure of Polysaccharides − Polymers that form when monosaccharides are linked together − Two sugars  disaccharide − Simple sugars polymerize when a condensation reaction occurs between two hydroxyl groups, resulting in a covalent bond called a glycosidic linkage − Analogous to peptide bonds and phosphodiester bonds o One important difference o Peptide bonds and phosphodiester bonds always form at the same location, but since glycosidic linkages form between hydroxyl groups and because every monosaccharide contains at least two hydroxyl groups, location and geometry of glycosidic linkages can vary widely among polysaccharides − Maltose o α-glucose + α-glucose o α-1,4 glycosidic linkage − Lactose o β-galactose + β-glucose o β-1,4 glycosidic linkage o RWE  Lactose intolerance • Adults do not produce lactase which splits lactose into glucose and galactose • Take lactase supplements • Autosomal recessive condition • Lactase is produced only during infancy, when primary source of nutrition is mother’s milk • Lactase production ends as these individuals mature • This pattern has been observed in most human cultures where traditional diets did not include cow’s milk − Starch o α-glucose + α-glucose o α-1,4 glycosidic linkage o Used for energy storage in plant cells o Angle of linkages between carbons 1 and 4 causes the chain of glucose subunits to coil into a helix o A mixture of two polysaccharides  Unbranched molecule called amylose, which contains only α-1,4 glycosidic linkages  Other is a branched molecule called amylopectin • Branching occurs when glycosidic linkages form between carbon 1 of a glucose monomer on one strand and carbon 6 of a glucose monomer on another strand • Branches occur in about one out of every 30 monomers − Glycogen o α-glucose + α-glucose o α-1,4 glycosidic linkage o Used for energy storage in animal cells o Stored in liver and muscles o When you start exercising, enzymes begin breaking glycogen into glucose molecules, which are processed in muscle cells to supply energy o Polymer of α-glucose and nearly identical to branched form of starch o Instead of an α-1,6 glycosidic linkage occurring in about one of every 30 monomers, a branch occurs in about one out of every 10 glucose subunits − Cellulose o β-glucose + β-glucose o β-1,4 glycosidic linkage o Used for structural support cell walls in plants and many algae o Major component of the cell wall in plants o Each glucose monomer in the chain is flipped in relation to the adjacent monomer o Arrangement increases stability of cellulose strands because the flipped orientation makes it possible for multiple hydrogen bonds to form between adjacent and parallel strands of cellulose o Often occurs in long, parallel strands that are joined by hydrogen bonds o Linked cellulose fibres are strong and provide the cell with structural support − Chitin o β-1,4 glycosidic linkage o Used for structural support in the cell walls of fungi and the external skeletons of insects and crustraceans o Stiffens the cell walls of fungi o Similar to cellulose, but instead of consisting of glucose monomers, the monosaccharide involved is called N-acetylglucosamine (NAc) o NAc are joined by β-1,4 glycosidic linkages o Geometry of these bonds results in every other residue being flipped in orientation o Subunits of NAc form hydrogen bonds between adjacent strands, which results in
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