09-Carbohydrates

09-Carbohydrates - What are carbohydrates? Carbohydrates...

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What are carbohydrates? Carbohydrates Function energy stores, fuels, and metabolic intermediates ribose/deoxyribose sugar part of RNA/DNA structural elements in cell walls of bacteria and plants frequently linked to proteins and lipids to mediate cellular interactions Carbohydrates aldehyde or ketone compounds with multiple hydroxyl groups, aka sugars or saccharides (poly- or mono-) built from monosaccharides (usually 3-9 carbons w/ various stereo centers, formula (CH 2 O) n ) structurally diverse
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Simple Monosaccharides: Trioses Ketose: ketone Aldose: aldehyde Glyceraldehyde is chiral dihydroxyacetone is achiral
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D -Tetroses, Pentoses, and Hexoses D-Ribose: component of RNA
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D -Ketoses D -Fructose is most prevalent
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Aldohexoses Cyclize in Solution Aldehydes can react with alcohols to form hemiacetals. For aldohexoses such as glucose, the C1 aldehyde reacts with the C5 hydroxyl to form an intramolecular cyclic hemiacetal, called a pyranose (similar to pyran, 6-membered ring). Formation of the hemiacetal introduces another asymmetric center (C1 for glucose). α ( β ) designates that the hydroxyl group at C1 is below (above) the plane of the ring. The new asymmetric carbon is the anomeric carbon, and the two forms are anomers. Glucopyranose: 35% α , 65% β
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Aldohexoses Cyclize in Solution Glucopyranose: 35% α , 65% β OH H H HO OH H CH 2 OH OH H O H H OH H OH H OH H OH OH O H OH OH H H OH H OH CH 2 OH H OH H H OH OH H HOH 2 C OH H O H O H O H OH H HO H H HO HOH 2 C H OH O HO OH HO HO HOH 2 C OH H OH H OH H OH CH 2 OH H O H OH H OH H OH H OH CH 2 OH H H O O H OH H OH H OH H OH CH 2 OH H O HO OH H HO H H HO HOH 2 C H H O OH HO HO HOH 2 C OH β α ανδ ΟΗ Η Η ΗΟ ΟΗ Η ΧΗ 2 ΟΗ ΟΗ Η Ο Η
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Aldohexoses Cyclize in Solution
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Aldopentoses Cyclize in Solution D-Ribose and 2-deoxy-D-ribose form furanose rings, the sugar component of RNA and DNA, respectively.
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Ketohexoses Cyclize in Solution Ketones can react with alcohols to form hemiketals. For ketohexoses such as fructose, the C2 ketone can react with the C6 hydroxyl for form a hemiketal pyranose, or react with the C5 hydroxyl to from a hemiketal furanose (similar to furan, 5- membered ring). Pyranose form dominates for fructose.
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Conformation of Pyranose Rings Pyranoses adopt chair (more favored) or boat (less favored) conformations. In the chair conformation, substituents can be axial (perpendicular to ring plane) or equitorial (nearly parallel to ring plane). Axial substituents can be unfavorable due to 1,3-diaxial interactions. Equitorial substituents are less sterically challenged. β - D -glucopyranose is more stable than the α form.
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Furanoses adopt envelope conformations (four atoms coplanar). For ribofuranoses, either C2 (C2-endo) or C3 (C3-endo) is
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This note was uploaded on 11/10/2010 for the course CHE Biochem taught by Professor Disney during the Spring '10 term at SUNY Buffalo.

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09-Carbohydrates - What are carbohydrates? Carbohydrates...

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