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Chapter 7 CARBOHYDRATES Outline 1. 
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What is a carbohydrate? Monosaccharides Cycliza=on of monosaccharides Glycosidic linkages Common disaccharides Polysaccharides –  Starch and glycogen –  Cellulose and chi=n 7.  Glycoproteins Ring conforma=ons •  Configura)on vs. conforma)on •  Furanose rings –  Sugar pucker –  C2-­‐endo and C3-­‐endo favored in nucleic acids Pseudorotational cycle for nucleoside furanose ring puckers.
Maderia M et al. Nucl. Acids Res. 2007;35:1978-1991
© 2007 The Author(s)‫‏‬
Ring conforma=ons •  Configura)on vs. conforma)on •  Furanose rings –  Sugar pucker –  C2-­‐endo and C-­‐3 endo favored in nucleic acids •  Pyranose rings –  Chair and boat conforma=ons –  Chair is more stable (crowding at axial posi=ons in boat form) Pyranose ring conforma=ons Pyranose ring conforma=ons Configura4onal and conforma4onal isomers Configura=onal: Enan=omers-­‐ Stereoisomers that are mirror images; D-­‐ or L-­‐ based on configura=on at highest numbered chiral carbon. Configura=onal: Diastereomers-­‐ Stereoisomers that are not mirror images; may have same D-­‐ or L-­‐
configura=on (or not). Configura=onal: Anomers-­‐ Stereoisomers that differ in configura=on at the anomeric carbon (formerly the carbonyl C). Conforma=onal isomers-­‐ Possess same stereochemical configura=on, but differ in three-­‐
dimensional conforma=on. Monosaccharide derivatives
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Phosphate esters Acids and lactones Alditols Amino sugars Glycosides Monosaccharide derivates- phosphate esters
•  Phosphate esters are important intermediates in the
metabolism of sugars to provide energy
–  frequently formed by transfer of a phosphate group
from ATP
–  fairly acidic (pKa1~ 1-2; pKa2~ 6-7)
The hydrolysis of
phosphate esters is
thermodynamically
favored
For ATP: ΔG°’= -­‐31 kJ/mol Monosaccharide derivatives- acids/lactones •  Oxida=on of carbon to carboxyl –  Glucose à gluconic acid –  Oxida=on at C6à uronic acids –  Aldonic and uronic acids can form stable lactones Monosaccharide derivatives- alditols
•  Reduc=on of the C=O group of a monosaccharide gives a polyhydroxy compound called an alditol Monosaccharide derivatives- amino sugars
•  Amino sugars have a hydroxyl of the parent monosaccharide replaced by an amino group Monosaccharide abbrevia=ons Monosaccharide derivatives- glycosides
•  Glycoside: a carbohydrate in which the -­‐OH of the anomeric carbon is replaced by -­‐OR •  those derived from furanoses are furanosides; those derived from pyranoses are pyranosides •  glycosidic bond: the bond from the anomeric carbon to the -­‐OR group •  This is the basis for the forma=on polysaccharides/
oligosaccharides Glycosidic bond formation
Monosaccharide derivatives- glycosides
•  Cyclic monosaccharide + X-­‐OH à O-­‐glycoside •  Cyclic monosaccharide + X-­‐NH3à N-­‐ glycoside Two different disaccharides of α-D-glucose
•  Glycosidic linkages can take various forms; the anomeric
carbon of one sugar to any of the -OH groups of another
sugar to forma an α- or β-glycosidic linkage
Forma=on of maltose, a glucose-­‐based disaccharide