Chapter 17: Carbohydrates

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Chapter 17Carbohydrates
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• IMPORTANT FUNCTIONS OF CARBOHYDRATES
• To provide energy through their oxidation
• To supply carbon for the synthesis of cell
  components
• To serve as a stored form of chemical energy
• To form a part of the structural elements of some
  cells and tissues
• Biomolecule a general term referring to organic
  compounds essential to life
• Biochemistry a study of the compounds and
  processes associated with living organisms

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• CARBOHYDRATES
• Carbohydrates are polyhydroxy aldehydes or
  ketones, or substances that yield such compounds
  upon hydrolysis.
• Example

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• CLASSIFICATION OF CARBOHYDRATES
• Carbohydrates are classified according to size
• Monosaccharide a single polyhydroxy aldehyde or
  ketone unit
• Disaccharide composed of two monosaccharide
  units
• Polysaccharide very long chains of linked
  monosaccharide units

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• STEREOCHEMISTRY
• Many carbohydrates exist as enantiomers
  stereoisomers that are mirror images.

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• A chiral object cannot be superimposed on its
  mirror image.
• A chiral carbon is one that has four different
  groups attached to it.

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• The presence of a single chiral carbon gives rise
  to stereoisomerism.
• If a carbon atom is attached to four different
  groups, it is chiral.
• If any two groups are identical, it is not
  chiral.

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• Compounds can have more than one chiral
  carbon The maximum number of
  stereoisomers is 2n where n number of chiral
  carbon atoms.
• Therefore, this compound with two chiral carbon
  atoms has 22 or 4 stereoisomers.
• The compound on the previous slide with four
  chiral carbon atoms has 24 or 16 stereoisomers.

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• FISCHER PROJECTIONS
• Fischer projections depict three-dimensional
  shapes for chiral molecules, with the chiral
  carbon represented by the intersection of two
  lines.

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• Fischer projections of carbohydrates have the
  carbonyl (CO) at the top. It is projecting away
  from the viewer behind the plane in which it is
  drawn.
• The hydroxyl group on the chiral carbon farthest
  from the CO group determines whether the
  carbohydrate is D (OH on right) or L (OH on
  left). The two horizontal bonds are coming
  toward the viewer out of the plane in which they
  are drawn.

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• D and L enantiomers rotate polarized light in
  opposite directions.

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• The enantiomer that rotates polarized light to
  the left is the levorotatory or (-) enantiomer.
• The enantiomer that rotates it to the right is
  the dextrorotatory or () enantiomer.
• The D and L designations do not represent
  dextrorotatory and levorotatory.
• In some instances only the D or L enantiomers are
  found in nature. They are rarely found together
  in the same biological system.
• For example, humans can only metabolize the
  D-isomers of monosaccharides.

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• MONOSACCHARIDE CLASSIFICATION
• Is the monosaccharide an aldehyde (aldose) or
  ketone (ketose)?
• How many carbon atoms are in the monosaccharide?

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• COMBINING MONOSACCHARIDE CLASSIFICATIONS

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• PHYSICAL PROPERTIES OF MONOSACCHARIDES
• Most are called sugars because they taste sweet.
• Because of the many OH groups, they form
  hydrogen bonds with water molecules and are
  extremely water soluble.

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• MONOSACCHARIDE REACTIONS
• All monosaccharides with at least five carbon
  atoms exist predominantly as cyclic hemiacetals
  and hemiketals.
• A Haworth structure can be used to depict the ?
  (-OH on the anomeric carbon pointing down) and ?
  (-OH on the anomeric carbon pointing up) anomers
  of a monosaccharide.
• Anomers are stereoisomers that differ in the 3-D
  arrangement of groups at the anomeric carbon of
  an acetal, ketal, hemiacetal, or hemiketal group.

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• SIX-MEMBERED PYRANOSE RING SYSTEM

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• FIVE-MEMBERED FURANOSE RING SYSTEM

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• MONOSACCHARIDE REACTIONS, cont.
• The OH groups of monosaccharides can behave as
  alcohols and react with acids (especially
  phosphoric acid) to form esters.

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• MONOSACCHARIDE REACTIONS, cont.
• Cyclic monosaccharide hemiacetals and hemiketals
  react with alcohols to form acetals and ketals,
  referred to as glycosides.

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• IMPORTANT MONOSACCHARIDES
• Ribose and Deoxyribose Used in the synthesis of
  DNA and RNA
• Glucose Most nutritionally important
  monosaccharide Sometimes called dextrose or
  blood sugar

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• IMPORTANT MONOSACCHARIDES, cont.
• Galactose A component of lactose (milk sugar)
• Fructose The sweetest monosaccharide Sometimes
  called levulose or fruit sugar

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• DISACCHARIDES
• Two monosaccharide units linked together by
  acetal or ketal glycosidic linkages
• A glycosidic linkage is identified by
• the numbers associated with the carbon atoms
  joined?? together by the linkage
• the configuration of the linkage for any
  anomeric carbon?atom joined by the linkage

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• IMPORTANT DISACCHARIDES
• Maltose Two glucose units linked ?(1?4) Formed
  during the digestion of starch to glucose Found
  in germinating grain Hemiacetal means maltose is
  a reducing sugar

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• IMPORTANT DISACCHARIDES, cont.
• Lactose Galactose and glucose units linked
  ?(1?4) Found in milk Hemiacetal means lactose
  is a reducing sugar
• Sucrose Fructose and glucose units Found in
  many plants (especially sugar cane, sugar
  beets) Not a reducing sugar

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• POLYSACCHARIDES
• StarchA polymer consisting of glucose units Has
  two forms
• Unbranched amylose (10-20)
• Branched amylopectin (80-90)
• Amylose complexes with iodine to form a dark
  blue color

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• THE STRUCTURE OF AMYLOSE

The molecular conformation of starch and the
starch-iodine complex (a) the helical
conformation of the amylose chain and (b) the
starch-iodine complex.
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• AMYLOPECTIN STRUCTURE

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• POLYSACCHARIDES, cont.
• Glycogen (animal starch)
• A polymer of glucose unitsUsed to store
  glucose, especially in the liver and
  musclesStructurally similar to amylopectin with
  ?(1?4) and
• ?(1?6) linkages, but more highly branched

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• POLYSACCHARIDES, cont.
• CelluloseA polymer of glucose unitsThe most
  important structural polysaccharideFound in
  plant cell wallsLinear polymer like amylose, but
  has ? (1?4) glycosidic linkagesNot easily
  digested, a constituent of dietary fiber

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• CELLULOSE STRUCTURE