Chapter 1 Carbohydrates

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Chapter 1 Carbohydrates
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Carbohydrates

• Carbohydrate a polyhydroxyaldehyde or
  polyhydroxyketone, or a substance that gives
  these compounds on hydrolysis
• Monosaccharide a carbohydrate that cannot be
  hydrolyzed to a simpler carbohydrate
• they have the general formula CnH2nOn, where n
  varies from 3 to 8
• aldose a monosaccharide containing an aldehyde
  group
• ketose a monosaccharide containing a ketone group

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Monosaccharides

• Monosaccharides are classified by their number of
  carbon atoms

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Monosaccharides

• There are only two trioses
• aldo- and keto- are often omitted and these
  compounds are referred to simply as trioses
  although this designation does not tell the
  nature of the carbonyl group, it at least tells
  the number of carbons

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Monosaccharides

• Glyceraldehyde contains a stereocenter and exists
  as a pair of enantiomers

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Fischer Projections

• Fischer projection a two dimensional
  representation for showing the configuration of
  tetrahedral stereocenters
• horizontal lines represent bonds projecting
  forward
• vertical lines represent bonds projecting to the
  rear
• the carbon atom at the intersection of the
  horizontal and vertical lines is not shown

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D,L Monosaccharides

• According to the conventions proposed by Fischer
• D-monosaccharide a monosaccharide that, when
  written as a Fischer projection, has the -OH on
  its penultimate carbon on the right
• L-monosaccharide a monosaccharide that, when
  written as a Fischer projection, has the -OH on
  its penultimate carbon on the left

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The four aldotetroses

• Enantiomers stereoisomers that are mirror images
• example D-erythrose and L-erythrose are
  enantiomers
• Diastereomers stereoisomers that are not mirror
  images
• example D-erythrose and D-threose are
  diastereomers

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D,L Monosaccharides

• Following are the two most common D-aldotetroses
  and the two most common D-aldopentoses

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D,L Monosaccharides

• and the three most common D-aldohexoses. Note
  that the third of these is an amino sugar
• also shown is the most common 2-keto-D-hexose

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Cyclic Structure

• Monosaccharides have -OH and CO groups in the
  same molecule and exist almost entirely as five-
  and six-membered cyclic hemiacetals
• anomeric carbon the new stereocenter resulting
  from cyclic hemiacetal formation
• anomers carbohydrates that differ in
  configuration only at their anomeric carbons

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Haworth Projections

• Haworth projections
• five- and six-membered hemiacetals are
  represented as planar pentagons or hexagons, as
  the case may be, viewed through the edge
• most commonly written with the anomeric carbon on
  the right and the hemiacetal oxygen to the back
  right
• the designation ?-?means that -OH on the anomeric
  carbon is cis to the terminal -CH2OH ?- means
  that it is trans

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Haworth Projections
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Haworth Projections

• a six-membered hemiacetal ring is shown by the
  infix -pyran-
• a five-membered hemiacetal ring is shown by the
  infix -furan-

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Conformational Formulas

• five-membered rings are so close to being planar
  that Haworth projections are adequate to
  represent furanoses

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Conformational Formulas

• for pyranoses, the six-membered ring is more
  accurately represented as a strain-free chair
  conformation

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Conformational Formulas

• if you compare the orientations of groups on
  carbons 1, 2, 3, 4, and 5 in the Haworth and
  chair projections of ?-D-glucopyranose, you will
  see that in each case they are up-down-up-down-up
  respectively

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Ascorbic Acid (Vitamin C)

• L-Ascorbic acid (vitamin C) is synthesized both
  biochemically and industrially from D-glucose

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Ascorbic Acid (Vitamin C)

• L-Ascorbic acid is very easily oxidized to
  L-dehydroascorbic acid
• both are physiologically active and are found in
  most body fluids

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Oxidation

• Reducing sugar one that reduces an oxidizing
  agent
• oxidation of a cyclic hemiacetal form gives a
  lactone
• when the oxidizing agent is Tollens solution, Ag
  precipitates as a silver mirror

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Reduction

• The carbonyl group of a monosaccharide can be
  reduced to an hydroxyl group by a variety of
  reducing agents, including H2/M and NaBH4
• reduction of the CO group of a monosaccharide
  gives a polyhydroxy compound called an alditol

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Phosphoric Esters

• Phosphoric esters are particularly important in
  the metabolism of sugars
• phosphoric esters are frequently formed by
  transfer of a phosphate group from ATP

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Formation of 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 grou

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Amino Sugars
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Disaccharides

• Sucrose
• table sugar obtained from the juice of sugar
  cane and sugar beet
• one unit of D-glucose and one unit of D-fructose
  joined by an a-1,2-glycosidic bond

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Disaccharides

• Lactose
• about 5 - 8 in human milk, 4 - 5 in cows
  milk
• one unit of D-galactose and one unit of D-glucose
  joined by a b-1,4-glycosidic bond

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Disaccharides

• Maltose
• two units of D-glucose joined by an
  a-1,4-glycosidic bond

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Polysaccharides

• Cellulose the major structural component of
  plants, especially wood and plant fibers
• a linear polymer of approximately 2800 D-glucose
  units per molecule joined by b-1,4-glycosidic
  bonds
• fully extended conformation with alternating
  180 flips of glucose units
• extensive intra- and intermolecular hydrogen
  bonding between chains

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Polysaccharides

• Starch is used for energy storage in plants
• a polymers of a-D-glucose units
• amylose continuous, unbranched chains of up to
  4000 a-D-glucose units joined by ?-1,4-glycosidic
  bonds
• amylopectin a highly branched polymer consisting
  of 24-30 units of D-glucose joined by
  ?-1,4-glycosidic bonds and branches created by
  ?-1,6-glycosidic bonds
• amylases catalyze hydrolysis of ?-1,4-glycosidic
  bonds
• debranching enzymes catalyze the hydrolysis of
  ?-1,6-glycosidic bonds

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Polysaccharides

• Figure 13.22 Branching in amylopectin and
  glycogen

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Polysaccharides

• Chitin the major structural component of the
  exoskeletons of invertebrates, such as insects
  and crustaceans also occurs in cell walls of
  algae, fungi, and yeasts
• composed of units of N-acetyl-?-D-glucosamine
  joined by ?-1,4-glycosidic bonds

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Polysaccharides

• Bacterial cell walls prokaryotic cell walls are
  constructed on the framework of the repeating
  unit NAM-NAG joined by b-1,4-glycosidic bonds

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Bacterial Cell Walls

• The NAM-NAG polysaccharide is in turn
  cross-linked by small peptides
• in Staphylococcus aureus, the cross link is a
  tetrapeptide
• this tetrapeptide is unusual in that it contains
  two amino acids of the D-series, namely D-Ala and
  D-Gln
• each tetrapeptide is cross linked to an adjacent
  tetrapeptide by a pentapeptide of five glycine
  units

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Bacterial Cell Walls
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Bacterial Cell Walls

• Figure 13.24(d) The peptidoglycan of a bacterial
  cell wall

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Plant Cell Walls

• consist largely of cellulose
• also contain pectin which functions as an
  intercellular cementing material
• pectin is a polymer of D-galacturonic acid joined
  by ?-1,4-glycosidic bonds
• the major nonpolysaccharide of cell walls,
  especially in woody plants, is lignin (next
  screen)

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Plant Cell Walls - Lignin

• Figure 13.25 Lignin

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Polysaccharides

• Glycosaminoglycans polysaccharides based on a
  repeating disaccharide where one of the monomers
  is an amino sugar and the other has a negative
  charge due to a sulfate or carboxylate group
• heparin natural anticoagulant
• hyaluronic acid a component of the vitreous
  humor of the eye and the lubricating fluid of
  joints
• chondroitin sulfate and keratan sulfate
  components of connective tissue

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Hyaluronic Acid
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Heparin
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Glycoproteins

• Glycoproteins contain carbohydrate units
  covalently bonded to a polypeptide chain
• antibodies are glycoproteins
• carbohydrates play a role as antigenic
  determinants, the portions of the antigenic
  molecule that antibodies recognize and to which
  they bond

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Blood Group Substances

• Membranes of animal plasma cells have large
  numbers of relatively small carbohydrates bound
  to them
• these membrane-bound carbohydrates act as
  antigenic determinants
• among the first antigenic determinants discovered
  were the blood group substances
• in the ABO system, individuals are classified
  according to four blood types A, B, AB, and O
• at the cellular level, the biochemical basis for
  this classification is a group of relatively
  small membrane-bound carbohydrates

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ABO Blood Classification

• in type A, the nonreducing end is NAGal
• in type B it is Gal
• in type AB, both types are present
• in Type O, neither of these terminal residues is
  present

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L-Fucose

• L-fucose is synthesized biochemically from
  D-mannose