Chapter 7: Outline

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Chapter 7 Outline
Monosaccharides Monosaccharide
stereoisomers Cyclic structures Reactions Examp
les and derivatives Di and oligosaccharides Polysa
ccharides Homo and heteropolysaccharides Glycocon
jugates
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• Originally, carbohydrates were those compounds
  having the formula Cn(H2O)n. Only
  monosaccharides or simple sugars fit the formula.
  As more complex carbohydrates were discovered,
  the term came to mean compounds associated with
  polyhydroxy aldehydes and ketones.

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7.1 Monosaccharides
3triose 4tetrose 5pentose 6hexose
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Monosaccharides generic names

• The generic name for a simple sugar begins with
  the carbonyl prefix aldo or keto and ends with
  the term for the number of carbons.
• An aldose with three carbons is called an
  aldotriose. A ketose with three carbons is a
  ketotriose.

What is the name for a six carbon aldehyde
sugar?
aldohexose
What is the name for a five carbon ketone sugar?
ketopentose
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Dihydroxyacetone

• Dihydroxyacetone

Is a ketotriose
Is a ketotriose
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Glyceraldehyde
Glyceraldehyde exists in two stereoisomeric
forms because the starred carbon is a
stereocenter it has four different groups
attached.
Glyceraldehyde
Is an aldotriose
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• The stereoisomers of glycer-aldehyde are
  designated D or L. The D isomer has the OH on
  the stereocenter to the right. The L isomer has
  the OH on the left .

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Glyceraldehyde 3

• The stereoisomeric forms of glycer-aldehyde are
  enantiomers nonsuper-imposable mirror image
  molecules.
• Perspective drawings of the two enantiomers of
  glyceraldehyde are on the next slide. A stereo
  view is on slide 11.
• Remember, barred bonds ( ) recede behind the
  plane of the screen and wedge ( ) bonds
  project in front of the plane.

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Perspective View
10
View with blue lens on the left eye.
Carbonyl group
OH group
H atom
CH2OH group
Stereoscopic view of glyceraldehyde
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Fischer Projections

• In a Fischer projection, the sugar molecule is
  oriented so that the most oxidized carbon is to
  the top. The stereocenter carbons are arranged
  so that the groups not part of the main chain
  project horizontally toward the viewer.
• The molecule is in the all eclipsed form.

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• Monosaccharides are drawn in Fischer projections
  with the most oxidized carbon closest to the top.
  The carbons are numbered from the top. If the
  the stereocenter with the highest number has the
  OH to the right, the sugar is D. If the OH is to
  the left, the sugar is L.
• Most common sugars are in the D form.
• Note Fisher projections represent an all
  eclipsed conformation.

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1
2
1
2
3
3
4
4
5
5
6
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These diastereomers are also epimers, they
differ in configuration at only one
stereo- center (colored dot).
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Cyclic forms for sugars

• Most simple sugars of four or more carbons exist
  in the cyclic (hemiacetal or hemiketal) form.
• A hydroxy group in the sugar reacts with the
  carbonyl group.
• The new OH bearing carbon is now a stereo center
  and is called an anomeric carbon.
• If the OH on the ring is up the carbon is b, if
  the OH is down it is a.

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Cyclic forms for sugars-2

• Fischer projections for a D glucose

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Cyclic forms for sugars-3 Haworth

• 1. Draw a five- or six-membered ring

2. Anomeric C to right of O. Place OH


up
or down. Left on Fischer, up on
ring.
3. In D- sugars, the last C is always up.
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Cyclic forms for sugars-4 Haworth
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Cyclic forms for sugars-5 Glucose
Pyranose ring form
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Cyclic forms for sugars-6

• The alpha and beta forms of cyclic sugars are
  said to be anomers. They differ in configuration
  about the hemiacetal or hemiketal carbon.

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Cyclic forms for sugars-7

• Ribose also exists mainly in the cyclic form.

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Oxidation of Monosaccharides
Aldoses react with Tollens reagent (Ag(NH3)2)
to give a lactone (cyclic ester). The silver
ion plates out as a mirror.
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Oxidation of Monosaccharides-2

• Aldehyde oxidn
• ?aldonic acid

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Oxidation of Monosaccharides-3

• Aldehyde term CH2OH oxidn ?
• aldaric acid

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Reduction of Monosaccharides

• The most important reduced sugar is deoxyribose.
  (In DNA)

• When the carbonyl of a sugar is reduced to an
  alcohol, alditols are produced. The two shown
  above are used to sweeten nonsugar gum.

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Isomerization4

• Isomerization of mono-
• saccharides occurs
• through an enediol.

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Esters of Monosaccharides

• The OH groups of sugars can react with phosphoric
  acid to give phosphate esters.

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Glycosides

• The anomeric OH can react with another OH on an
  alcohol or sugar. Water is lost to form an
  acetal/ketal

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Important Monosaccharides
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Amino Sugars
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Amino Sugars-2
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7.2 Disaccharides Sucrose

• Sucrose is formed by linking a D-glucose with b
  D-fructose to give a 1,2 glycosidic link.

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Disaccharides Lactose

• Lactose is formed by joining b D-galactose to a
  D-glucose to give a 1,4 glycoside

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Disaccharides Maltose

• Maltose is formed by linking two a-D-glucose
  molecules to give a 1,4 glycosidic link.

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DisaccharidesCellobiose

• Cellobiose is formed by linking two b D-glucose
  molecules to give a 1,4 glycosidic link. It comes
  from hydrolyzed cellulose.

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7.3 Polysaccharides Cellulose

• Cellulose is the major structural polymer in
  plants. It is a liner homopolymer composed of b
  D-glucose units linked b-1,4. The repeating
  disaccharide of cellulose is b-cellobiose.
• Animals lack the enzymes necessary to hydrolyze
  cellulose. The bacteria in ruminants (eg. cows)
  can digest cellulose so that they can eat grass,
  etc.

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Polysaccharides Starch

• Starches are storage forms of glucose found in
  plants.
• They are polymers of a linked glucose.
• If the links are only 1,4, the polymer is linear
  and is called amylose. (Figure on next slide.)
  Amylose usually assumes a helical configuration
  with six glucose units per turn.
• If the links are both 1,4 and 1,6, the polymer is
  branched and is called amylopectin. (Figure on
  next slide.

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Polysaccharides amylose/amylopectin
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Polysaccharides glycogen

• The storage carbohydrate in animals is glycogen.
  It is a branched chain polymer like amylopectin
  but it has more frequent branching (about every
  10 residues). Glycogen is stored in liver and
  muscle cells.

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Polysaccharides chitin

• Chitin is a linear homopolysaccharide of
  N-acetyl-b-D-glucosamine and provides structural
  support for the exoskeleton (shell) of
  invertibrates. (eg. Insects, lobsters, shrimp)
• The polymer is
• linked as
• b-1,4-units.

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Polysaccharides

• Bacterial cell walls have heteropolysaccharides
  as major components. The polymers consist of
  chains of alternating N-acetyl-b-D-glucosamine
  and N-acetylmuramic acid (next slide). The
  parallel chains are linked by short peptide
  residues.

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Polysaccharides
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Glycosaminoglycans (GAGs)

• GAGs are linear polymers with disaccharide
  repeating units. A GAG which may be familiar is
  chondroitin sulfate, a component of cartilage.

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7.4 Glycoconjugates

• Glycoconjugates are compounds that covalently
  link carbohydrates to proteins and lipids.
• Proteoglycans and glycoproteins are two kinds of
  glycoconjugates that contain protein.

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Proteoglycans

• Proteoglycans have a very high carbohydrate to
  protein ratio, often 955, and are found in the
  extracellular matrix.
• GAG chains are linked to core proteins by N- and
  O-glycosidic links.

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Glycoproteins

• These materials contain carbohydrate residues on
  protein chains. Very important examples of these
  materials are antibodies-chemicals which bind to
  antigens and immobilize them.
• The carbohydrate part of the glycoprotein plays a
  role in determining the part of the antigen
  molecule to which the antibody binds.

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Glycoproteins 2

• The human blood groups A, B, AB, and O depend on
  the oligosaccharide part of the glycoprotein on
  the surface of erythrocyte cells. The terminal
  monosaccharide of the glycoprotein at the
  nonreducing end determines blood group.

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Glycoproteins 3

• Type Terminal sugar
• A N-acetylgalactosamine
• B a-D-galactose
• AB both the above
• O neither of the above
• O is the universal donor
• AB is the universal acceptor

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Glycoprotein Functions
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Glycoprotein Functions Recognition

• Cell-molecule
• Insulin receptor
• Cell-virus
• Gp120 is the target binding site for HIV
• Cell-cell
• Play a role in glycocalyx (cell coat) adhesion
  between cells.