25, Carbohydrates
Jan. 2008
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Overview
Monosaccharides Ring structures Glycosidic Bond Disaccharides Polysaccharides
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Sugars are conformationally variable
Five- and six-membered hexose rings are more stable
Stability depends on interactions between substituents on the ring
Haworth formulas are just models and not what the rings really look like naturally Atoms in the rings are sp3 hybridized so tetrahedrally shaped
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Conformations of a cyclohexane ring
Substituents are eclipsed
e = equatorial a = axial 4 a and e are interconvertable
The two alternative chair conformations of -D-glucopyranose.
Predominant conformation Bulky substituents are equatorial e = equatorial a = axial
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Monosaccharides are reducing sugars
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Some Hexose Derivatives
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The reversible oxidation of Lascorbic acid to Ldehydroascorbic acid.
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Glycosidic Bond
Links monsaccharides
disaccharides polysaccharides
Glycosidic bond: anomeric hydroxyl group of a sugar reversibly condenses with an alcohol of another sugar Carb analog of the peptide bond Glycosidases hydrolyze this bond Bond is acid catalyzed Stable in basic and neutral conditions in absence of glycosidase
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Disaccharides
Two monosaccharides linked by a glycosidic bond Several common disaccharides
Sucrose, lactose, maltose Described by:
Component monosaccharides Ring types Anomeric forms linkage
May or may not be reducing
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Maltose.
Reducing Sugar
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Lactose.
Milk sugar (0 to 7% of milk) Reducing sugar because free anomeric C on glucose is present
Most Lactase
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Sucrose
abundant Found in plant kingdom Table sugar Not a reducing sugar Hydrolysis leads to a change in optical rotation from dextro to levo and this is referred to as invert sugar Invertase (alpha-D-glucosidase)
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Homo- and Heteropolysaccharides
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Structural Polysaccharides
Cellulose
Primary structural component of plant cell walls Accounts for over half of C in biosphere Predominately in plants Also found in tunicates (invertebrates)
Chitin
Primary structural component of
Exoskeletons: Crustaceans, insects, spiders Cell walls: Fungi, algae
Almost as abundant as cellulose
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The primary structure of cellulose
Linear polymer of up to 15,000 D-glucose residues Beta(1-->4) glycosidic bonds No defined size Vertebrates cannot hydrolzye beta(1-->4) glycosidic bond, but microbes and termites can using cellulase
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Electron micrograph of cellulose fibers
The fibers are held together by a matrix of polysaccharides In wood the matrix is called lignin (plasticlike phenolic polymer) Cellulose fibers are held in position by intra- and interchain H-bonds Cotton balls are 100% cellulose
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Structure of chitin
Homopolymer of N-acetyl-D-glucosamine, 1-->4 linkage
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Storage Polysaccharides
Starch
Glycogen Plant synthesized Animal synthesized Food reserve More branched than starch Deposited in plant cell cytoplasm Present in all cells but most as insoluble granules prevalent in skeletal muscle and liver Main carb source in human diet -amylose, amylopectin
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