Disaccharides

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== Lactose and Cellobiose ==
== Lactose and Cellobiose ==
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Lactose (β-D-galactopyranosyl (1→4) D-glucopyranose) and cellobiose (β-D-glucopyranosyl (1→4) D-glucopyranose) are disaccharides whose monomeric units are connected by β(1→ 4) glycosidic bonds. The second glucose unit (contains green anomeric carbon) of lactose has been rotated 180° as was described for cellobiose above. As can be seen from their chemical names, the structural difference in the two disaccharides is that lactose contains a galactopyranose whereas both monomeric units in cellobiose are glucopyranose. Since the structures of glucose and galactose are very similar, the structures of these two disaccharides are different at only one stereogenic (chiral) carbon. Determine the one stereogenic center that is different in the two disaccharides. Do not be distracted by differences that are present in the location of hydroxyl groups that are on nonchiral carbons and by hydrogens that project differently from oxygens on which they are bonded. (Hint: Remeber that glucose and galactose are epimers.) When lactose is in an aqueous solution, like cellobiose and maltose, it is a reducing sugars because the open-chain aldehyde is in equilbrium with the α and β anomers of the glucopyranose.
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<scene name='Disaccharides/Lactose/1'>Lactose</scene> (β-D-galactopyranosyl (1→4) D-glucopyranose) and <scene name='Disaccharides/Cellobiose/4'>cellobiose</scene> (β-D-glucopyranosyl (1→4) D-glucopyranose) are disaccharides whose monomeric units are connected by β(1→ 4) glycosidic bonds. The second glucose unit (contains green anomeric carbon) of lactose has been rotated 180° as was described for cellobiose above. As can be seen from their chemical names, the structural difference in the two disaccharides is that lactose contains a galactopyranose whereas both monomeric units in cellobiose are glucopyranose. Since the structures of glucose and galactose are very similar, the structures of these two disaccharides are different at only one stereogenic (chiral) carbon. Determine the one stereogenic center that is different in the two disaccharides. Do not be distracted by differences that are present in the location of hydroxyl groups that are on nonchiral carbons and by hydrogens that project differently from oxygens on which they are bonded. (Hint: Remember that glucose and galactose are epimers.) When lactose is in an aqueous solution, like cellobiose and maltose, it is a reducing sugars because the open-chain aldehyde is in equilbrium with the α and β anomers of the glucopyranose.
Even though both of these sugars have β(1→4) glycosidic bonds, lactose is the only one of the two that is catalytically hydrolyzed by lactase. Lactase distingishes between the two because of the difference in the one stereogenic center in the pyranosyl unit mentioned above.
Even though both of these sugars have β(1→4) glycosidic bonds, lactose is the only one of the two that is catalytically hydrolyzed by lactase. Lactase distingishes between the two because of the difference in the one stereogenic center in the pyranosyl unit mentioned above.
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<Structure load='Lactose.pdb' size='400' frame='true' align='left' caption='' scene='Disaccharides/Lactose/1' />
 
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<Structure load='Cellobiose.pdb' size='400' frame='true' align='right' caption='' scene='Disaccharides/Cellobiose/3' />
 
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Revision as of 09:24, 7 April 2013

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