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Pitt BIOSC 1000 - Carbohydrates and Glycobiology
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Lecture 9Outline of Last Lecture1.Properties of Enzymes2.Michaelis-Menten Equation3.Lineweaver-Burk Double-Reciprocal Plot4.Enzyme Inhibition5.Pre-Steady State kineticsOutline of Current Lecture1. Carbohydrates2. Monosaccharides3. Disaccharides4. Polysaccharides1. Carbohydrates & GlycobiologyMolecular Organization: sugars& various polymers of sugarsMain sugars: starch and celluloseCarbohydrate: sugars or molecules whose hydrolysis produces sugarsGlucose is probably the most abundant monosaccharide in nature, although it is usually polymeric as starch or celluloseGlucose regulation – constant testing of the blood, administrating insulinBiological Roles of Carbohydrates in PlantsStructural polysaccharides most abundant biomolecules on earthPhotosynthesis: CO2 + H2O + light energy  (CH2O)nInsoluble polymers of sugars serve structural and protective roles including cell walls of bacteria, fungi, and plantsStructural component of trees and arthropod shells (chitin)Biological roles of CarbohydratesSugar and starch: dietary staple for most world populationOxidation of carbohydrates: main energy-yielding pathway for tissues and organismsConnective tissue in animalsCHO polymers lubricate sketeal jointsCHOpolymers participate in recognition and cell adhesionCHO polymers participate in cell recognition and signalingPlant structural material and arthropod exoskeltonStructure of Simple CarbohydratesEmpirical Formula: (CH2O)nSome also contain N, P, S3 major classes of CHO: monosaccharides and disaccharides (simple sugars)Oligosaccharides – polymers of 2-20 monomeric unitsPolysaccharides: > 20 monomeric units2. Monosaccharides – simple sugars – single polyhydroxyl aldehyde or ketone unitUsually white or colorless, water solubleEasily form crystalline solids, sweet tasteSugars are classified by position of carbonylNaming:TrioseTetrosePentoseHexoseHeptose(KNOW THE STRUCTURE OF glyceraldehyde, erythrose, ribose, arabinose, xylose, glucose, mannose, galatose)some important moneclature rulescarbons are numbered from the end of chain closest to carbonylD or L configuration specifies chiral carbon most DISTAL from carbonyl:D sugars show –OH on RIGHT and H—on leftL sugars show –OH on LEFT and H—on RIGHT in Fischer ProjectionD-isomers Versus L-isomers of sugars – be able to draw compounds in Fischer ProjectionsA molecule with n chircal centers can have 2n stereoisomersEpimers- 2 stereoisomers that have different configurations at one asymmetric center in a molecule of 2 or more asymmetric centers (Glucose and Mannose)Solution structures are usually cyclicStable – for sugars with 5 or more Carbons, cyclic or ring structures usually more stable in solution than linear aldehyde formsBoth cyclic forms and linear aldehyde rapidly exchange at equlibrium in solutionResulting linkage has two forms: stereoisomer α (minority) & β (majority) called anomersFormed by the attack of alcohol on carbonyl carbonHemiacetal linkageHaworth ProjectionsPyranose – 6-membered ringFuranose – 5-membered ring64% exists as β-D-glucose in cyclic form36% exists as α-D-glucose in cyclic formonly trace amount exists as linear form (reactions that target aldehyde only react with <1% linear form)anomer interconversion reaction is also called “mutarotation”bonding configurations for cyclic sugarsaxial points along the axisequatorial points away from the axis (or along the equator)most stable configuration has most of the larger & bulkier groups pointed out (equatorial)3. Disaccharides contain an O-glycosidic bondBond formed between hydroxyl (alcohol) of one sugar and the hemiacteal of another (condensation reaction) – why is anomeric carbon attacked?This yields an acetalDisaccharides: very large structural and functional diversity generated by great variety of O-glycosidic linkages possible with 2 monomersTrisaccharides: even greater structural and functional diversity with 3 monomers and various patterns of linkageTrehalose—non reducing sugar α linkage – opposite from C6 (same side as 2-OH)Lactose – β- linkage – same side as C6 (opposite of 2-OH)Common DisaccharidesLactose: D-galactose & D-glucose primary sugar found in milkLactose-intolerate individuals do not have lactase to hydrolyze β 14 linkageLactaidSucrose: D-glucose & D-fructose – common table sugar (cane sugar, beet sugar)Non-reducingNot very reactive – no free aldehydeTrehalose: D-glucose “dimmer” major constitutent of circulating fliud in insect (hemolymph) energy storage compound, antifreeze compoundNon-reducingSucralose: sugar substitute4. Polysaccharides (glycans)Repeating units of monosaccharides – many glycosidic linkagesAmylose – repeating glucose units (HOMO- polysaccharides)Other examples: starch, glycogen, cellulose, chitinCan be branched or unbranchedHeteropolysaccharides – different sugar molecules in the chainSometimes common repeating pattern or sometimes branched complex patternPeptidoglycan (bacterial cell wall)Extracellular matrix (polysaccharides in animals of several types)Do not have definite molecule weightsNo template guides polymerization of polysaccharidesChains may be branched by using multiple different glycosidic linkagesStorage Polymers: Starch (plants) Glycogen (animals)For energy storage, what characteristics are desirable?High energy density (store as many monomers as possible)Compact (cells have limited space)Easy to mobilize when sugar is needed for fuelStarchContains 2 types of glucose polymersAmyloseunbranchedD-glucose polymerConnected by α 1-4 linkagesLong chains, vary in lengthMW from 100 monomers to mil monomersAmylopectinBranched structureD-glucose polymerConnected by α 1-4 links with branch points as α1-6Such branches occur every 24-30 residuesMW up to 100 millionSecondary structure is helixWhy Branch?More ends allow faster sugar releaseIncrease ability to store more sugar compactlyHighly insolubleGlycogenMain storage form of glucose in animal cells structurally similar to amylopectinHighly branched polymer of D-glucoseMain chain linkages are α 1 4Branches points are 16Branches every 8-12 residuesBranching fits more monomers units into small volume7% mass of liver2% mass of musclestored as large granules with clusters of smaller graules and the enzymes for glycogen synthesis and degradation (glycogen synthesis and degradation covered later)enzymes that hydrolyse the sugar polymers are specific α-amylases – α 1 4 linkagesdebranching enzyme: α1 6


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Pitt BIOSC 1000 - Carbohydrates and Glycobiology

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