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Unformatted text preview: Lecture 11
Chapter 11: Carbohydrates
Overview: • mono- and disaccharides -structure -nomenclature -reactions • polysaccharides -structure -nomenclature -chemical properties • glycoconjugates Carbohydrates: introduction • carbohydrates ‘carbon hydrates’ -(CH2O)n or C•(H2O) n where n ! 3 -many carbons attached to oxygen • highly abundant class of biological molecules • multiple functions in cell -energy source -structural roles -component of DNA & RNA -involved in cellular interactions & recognition events • many proteins in eukaryotic cells are glycosylated (sugars are covalently attached to their surfaces) • cell surfaces display a variety of sugar-based structures polyhydroxy aldehyde or ketone HW: 11.2, 11.10, 11.12, 11.13, 11.14 Carbohydrate nomenclature monosaccharide: simple sugar; cannot be broken down into simpler sugars oligosaccharide: covalently linked chain of monosaccharides (2-10) polysaccharide: polymer of simple sugars aldose: contains aldehyde functional group ketose: contains ketone functional group triose, tetrose, etc. denotes # of carbons monosaccharides ‘sugars’ aldose & ketose deoxy sugar Stereochemical depictions starch: glucose polymer aldotetrose Do not memorize structures/names Know general names aldotriose Do not memorize structures/names aldopentose aldohexose ketotriose ketotetrose Do not memorize structures/names Stereochemistry enantiomers: mirror images diastereomers: opposite conﬁguration at one or more chiral centers D vs. L deﬁned using highest number chiral center D and L do not specify the rotation of plane-polarized light Two sugars that differ in conﬁguration at only one chiral center are epimers. Stereochemistry
e na ntio mer s e pime r s Epimers most (but not all) natural sugars occur in the D forms C HO H H H H OH OH OH OH C H2O H HO HO HO HO C HO H H H H C H2O H H HO HO HO C HO OH H H H C H2O H H H HO HO C HO OH OH H H C H2O H D -a llos e L -a llos e L -a ltr os e L -ma n no se d ias ter eo me rs Hemiketal & hemiacetal formation Hemiacetal (pyranose) formation cyclic forms of D-glucose ! vs. " anomers For D sugars: !: OH down ": OH up How is ring numbered? Pyranoses & furanoses Haworth projections Which is hemiacetal? Which is hemiketal? draw the reaction schemes and show equilibrium mixtures of the 5 open/closed forms of D-glucose. Hemiketal (furanose) formation How is ring numbered? Pyranoses & furanoses
C H2O H O H H H OH OH OH C H2O H D-psicose H O H2C H O H2C H H OH OH OH H O C H2O H H H OH OH H C H2O H OH !-D-psicofurnaose H O H2C H H H H H OH O H O H (or C H2OH) C H2O H (or OH) H OH C H2O H H OH OH "-D-psicofurnaose H O H2C OH OHOHO H H H C H2O H Pyranoses & furanoses
CHO HO H H H H OH OH OH C H2O H D-altrose
O H H O H2C H H OH CHO OH OHOH H C H2O H C H2O H H H OH OH H H OH OH O H OH OH H !-D-altropyranose C H2O H H H H OH OH H "-D-altropyranose O OH H OH H H H O OH H OH O OH OH ! (or ")-D-psicopyranose Four ring structures of fructose Pentoses of RNA and DNA How are the rings numbered? Cyclic conformers of pyranose Cyclic conformers of pyranose In general, the chair is preferred over the boat conformer, and large substituents prefer to occupy the equatorial positions. Envelope conformations of !-D-ribose Monosaccharides are joined to alcohols and amines through glycosidic bonds D-glucose hydroxyl + methanol: sugar acetal formation at C-1 position: glycoside (contains Oglycosidic bond) Reaction with amine produces Nglycoside Differing reactivities of methyl glucopyranoside and parent glucose due to aldehyde functionality Fehling’s solution: glucose test reactions with reducing sugars Modiﬁed monosaccharides Condensation of monosaccharides Phosphorylated sugars play key roles as intermediates in energy generation and biosynthesis Condensation of monosaccharides Common disaccharides Polysaccharides Glycogen Starch: a storage polysaccharide in plants -amylose (10-30%) & amylopectin (70-90%) -branches in amylopectin every 24-30 residues Glycogen: stored energy (polymer of glucose) -10% of liver mass -1-2% of muscle mass -branches differ from starch -!(1"6) branch every 8-12 residues Starch !(1"4) and !(1"6) linkages Cellulose most abundant natural polymer support and strength to trees and plants #(1"4) linkage Chitin chitin - exoskeletons of crustaceans, insects, and spiders similar to cellulose, but C-2s are N-acetyl cellulose strands are parallel; chitin strands are parallel or antiparallel H-bonding networks make stable sheets provide strength Conformation of glycosidic bonds Repeating units in glucosaminoglycans Conformation of glycosidic bonds Conformation of glycosidic bonds Speciﬁc enzymes are responsible for oligosaccharide assembly Oligosaccharide linkages in glycoproteins N- or O-linked glycoproteins Glycosyltransferases: catalyze glycosidic bond formation; sugar nucleotides are important intermediates - will be revisited in Ch. 16 and Ch. 21 Oligosaccharides attached to erythropoietin Blood serum protein: glycoprotein hormone erythropoietin (EPO) (165 amino acids); 40% carbohydrate by weight; enhances protein stability in blood. N-glycosylated at three sites and O-glycosylated at one site: linkages to three asparagines and one serine. Secreted by kidneys and stimulates red blood cell production. Important for treatment of anemia. Protein glycosylation: takes place in the lumen of the ER and golgi complex Lectins: proteins that read the sugar code; mediate biological processes Lectins: proteins that read the sugar code; mediate biological processes carbohydrates on the cell surface: oligosaccharide components of glycolipids on the cell surfaces have multiple roles ...
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