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Published byPearl Craig Modified over 9 years ago
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Carbohydrates
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Most abundant class of biological molecules on Earth Originally produced through CO 2 fixation during photosynthesis
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Roles of Carbohydrates Energy storage (glycogen,starch) Structural components (cellulose,chitin) Cellular recognition Carbohydrate derivatives include DNA, RNA, co-factors, glycoproteins, glycolipids
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Carbohydrates Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions Oligosaccharides = "a few" - usually 2 to 10 Polysaccharides are polymers of the simple sugars
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Monosaccharides Polyhydroxy ketones (ketoses) and aldehydes (aldoses) Aldoses and ketoses contain aldehyde and ketone functions, respectively Ketose named for “equivalent aldose” + “ul” inserted Triose, tetrose, etc. denotes number of carbons Empirical formula = (CH 2 O) n
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Monosaccharides are chiral Aldoses with 3C or more and ketoses with 4C or more are chiral The number of chiral carbons present in a ketose is always one less than the number found in the same length aldose Number of possible steroisomers = 2 n (n = the number of chiral carbons)
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Stereochemistry Enantiomers = mirror images Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers Epimers = Two sugars that differ in configuration at only one chiral center
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Cyclization of aldose and ketoses introduces additional chiral center Aldose sugars (glucose) can cyclize to form a cyclic hemiacetal Ketose sugars (fructose) can cyclize to form a cyclic hemiketal
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Glucopyranose formation
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Fructofuranose formation
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Monosaccharides can cyclize to form Pyranose / Furanose forms = 64% = 36% = 21.5% = 58.5% = 13.5% = 6.5%
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Haworth Projections Anomeric carbon (most oxidized) -OH up = beta -OH down = alpha 1 2 3 4 5 6 For all non-anomeric carbons, -OH groups point down in Haworth projections if pointing right in Fischer projections
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Conformation of Monosaccharides Pyranose sugars not planar molecules, prefer to be in either of the two chair conformations.
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Reducing Sugars When in the uncyclized form, monosaccharides act as reducing agents. Free carbonyl group from aldoses or ketoses can reduce Cu 2+ and Ag + ions to insoluble products
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Derivatives of Monosaccharides
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Sugar Phosphates
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Deoxy Acids
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Amino Sugars
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Sugar alcohols
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Monosaccharide structures you need to know 1)Glucose 2)Fructose 3)Ribose 4)Ribulose 5)Galactose 6)Glyceraldehyde
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Carbohydrates Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions Oligosaccharides = "a few" - usually 2 to 10 Polysaccharides are polymers of the simple sugars
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Glycosidic Linkage
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Disaccharides maltose cellobiose lactose sucrose -D-glucosyl-(1->4)- -D-glucopyranose) -D-glucosyl-(1->4)- -D-glucopyranose) -D-galactosyl-(1->4)- -D-glucopyranose) -D-glucosyl-(1->2)- -D-fructofuranose)
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Higher Oligosaccharides
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Oligosaccharide groups are incorporated in to many drug structures
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Polysaccharides Nomenclature: homopolysaccharide vs. heteropolysaccharide Starch and glycogen are storage molecules Chitin and cellulose are structural molecules Cell surface polysaccharides are recognition molecules
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Starch A plant storage polysaccharide Two forms: amylose and amylopectin Most starch is 10-30% amylose and 70-90% amylopectin Average amylose chain length 100 to 1000 residues Branches in amylopectin every 25 residues (15- 25 residues) -1->6 linkages Amylose has -1->4 links, one reducing end
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Amylose and Amylopectin
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Starch Amylose is poorly soluble in water, but forms micellar suspensions In these suspensions, amylose is helical
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Glycogen Storage polysaccharide in animals Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass Glycogen is stored energy for the organism Only difference from starch: number of branches Alpha(1,6) branches every 8-12 residues Like amylopectin, glycogen gives a red-violet color with iodine
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Dextrans If you change the main linkages between glucose from alpha(1,4) to alpha(1,6), you get a new family of polysaccharides - dextrans Branches can be (1,2), (1,3), or (1,4) Dextrans formed by bacteria are components of dental plaque Cross-linked dextrans are used as "Sephadex" gels in column chromatography These gels are up to 98% water!
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Dextrans
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Cellulose Cellulose is the most abundant natural polymer on earth Cellulose is the principal strength and support of trees and plants Cellulose can also be soft and fuzzy - in cotton
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Cellulose vs Amylose Glucose units rotated 180 o relative to next residue cellulose amylose
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Cellulose Beta(1,4) linkages make all the difference! Strands of cellulose form extended ribbons Interchain H-bonding allows multi-chain interactions. Forms cable like structures.
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Chitin exoskeletons of crustaceans, insects and spiders, and cell walls of fungi similar to cellulose, but instead of glucose uses N-acetyl glucosamine (C-2s are N- acetyl instead of –OH) -1->4 linked N-acetylglucosamine units cellulose strands are parallel, chitins can be parallell or antiparallel
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Chitin vs Cellulose
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Peptidoglycan N-acetylglucosamine and N-acetylmuramic acid groups linked -1->4 Heteroglycan linked to a tetrtapeptide (Ala- IsoGlu-Lys-Ala) Gram (-) have petanta- glycine linker to next strand Gram (+) have directly cross links to next strand
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Peptidoglycan
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Peptidoglycan is target of antibacterial agents Lysozyme = enzyme that cleaves polysaccharide chain of peptidoglycan Penicillin = inhibits linking of peptidoglycan chains. Inhibits bond formation between terminal alanine and pentaglycine linker Penicillian looks like an Ala-Ala
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Peptidoglycan and Bacterial Cell Walls Composed of 1 or 2 bilayers and peptidoglycan shell Gram-positive: One bilayer and thick peptidoglycan outer shell Gram-negative: Two bilayers with thin peptidoglycan shell in between Gram-positive: pentaglycine bridge connects tetrapeptides Gram-negative: direct amide bond between tetrapeptides
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Glycoproteins May be N-linked or O-linked N-linked saccharides are attached via the amide nitrogens of asparagine residues O-linked saccharides are attached to hydroxyl groups of serine, threonine or hydroxylysine
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O-linked Glycoproteins Function in many cases is to adopt an extended conformation These extended conformations resemble "bristle brushes" Bristle brush structure extends functional domains up from membrane surface
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O-linked Glycoproteins
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N-linked Glycoproteins Oligosaccharides can alter the chemical and physical properties of proteins Oligosaccharides can stabilize protein conformations and/or protect against proteolysis Cleavage of monosaccharide units from N-linked glycoproteins in blood targets them for degradation in the liver Involved in targeting proteins to specific subcellular compartments
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