Chapter 20 Carbohydrates

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Chapter 20 Carbohydrates
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Carbohydrates

• Carbohydrates (or saccharides) consist of only
  carbon, hydrogen and oxygen.
• Carbohydrates come primarily from plants, however
  animals can also biosynthesize them
• The Carbon Cycle describes the processes by
  which carbon is recycled on our planet
• - Energy from the sun is stored in plants, which
  use photosynthesis to convert carbon dioxide and
  water to glucose and oxygen
• - In the reverse process, energy is produced
  when animals oxidize glucose during respiration

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Simplified Carbon Cycle
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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. Simple
  sugars. Cant be split into smaller carbohydrate
  units
• - Examples glucose, fructose, galactose,
  ribose
• Monosaccharides 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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Carbs

• Disaccharides are two monosaccharides bonded
  together
• - Can be split into two monosaccharides using an
  acid or enzyme catalyst
• - Examples sucrose (table sugar), lactose
  (milk sugar)
• Polysaccharides are polymers of monosaccharides
• - Used for storage of carbohydrates
• - Can be split into many monosaccharides with
  acid or enzymes
• - Examples starch, cellulose, glycogen

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Types of Carbs
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Monosaccharides

• Are chiral! The suffix -ose indicates that a
  molecule is a carbohydrate.
• The prefixes tri-, tetra, penta, and so forth
  indicate the number of carbon atoms in the chain.
• Those containing an aldehyde group are classified
  as aldoses.
• Those containing a ketone group are classified as
  ketoses.
• There are only two trioses
• Often aldo- and keto- are omitted and these
  compounds are referred to simply as trioses.
• Although triose does not tell the nature of the
  carbonyl group, it at least tells the number of
  carbons.

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Monosacharides

• Glyceraldehyde, the simplest aldose, contains
  one stereocenter and exists as a pair of
  enantiomers.

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Monosaccharides

• Fischer projection A two-dimensional
  representation for showing the configuration of
  tetrahedral stereocenters.
• Horizontal lines represent bonds projecting
  forward from the stereocenter.
• Vertical lines represent bonds projecting to the
  rear.
• Only the stereocenter is in the plane.

Terminal carbonyl groups (aldehydes CHO and
carboxylic acids COOH)
are written vertically on Fisher diagram
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Monosacharides

• In 1891, Emil Fischer made the arbitrary
  assignments of D- and L- to the enantiomers of
  glyceraldehyde.
• D-monosaccharide the -OH on its penultimate
  carbon is on the right in a Fischer projection.
• L-monosaccharide the -OH on its penultimate
  carbon is on the left in a Fischer projection.

Fischer got lucky. For sugars, DR and rotates
light clockwise () LS and counter (-). BUT,
does the Fisher projection for D appear to be R
or S? EXPLAIN!
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Get a Model Kit

• Before building the molecule, draw the D-
  Glyceraldehyde enantiomer (last slide) on your
  paper as both a wedge diagram and a Fisher
  diagram. Answer the following
• 1- Based solely on the Fisher diag, assign
  priority to the groups (Ch 15 p.427). Reading the
  order of the groups, what direction would you
  assign,clockwise or counterclockwise?__
• 2- Build the molecule, based on the Fisher. Is
  the carbonyl C group pointing toward you or away
  from you?____ What about the H and OH? _______
• 3- Now, rotate the molecule, putting the lowest
  ranked group away from you. Approximately, how
  many degrees did you turn the molecule?____Where
  is the OH after the rotation?
• 4- Is the direction now clockwise, or
  counter?_____________
• 5- In terms of ONLY seeing a (similar) Fisher
  diagram, what will you have to do (in your head)
  before determining D or L?

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D,L-Monosaccharides

• The most common D-tetroses and D-pentoses are
• The three most common D-hexoses are

You do NOT have
To memorize these!
See table 20.1 and 20.2
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Sugars (Monosaccharides)

• All other sugars are classified based on the
  position of the hydroxyl group farthest away from
  the carbonyl, but not the one on the end.
• Penultimate carbon- Point of reference that
  refers to the next to last C atom in the chain.
• See Table 20.1 and 20.2 (penultimate C is in red)

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Three Important Monosaccharides

• D-Glucose (aldose) is the most common
  monosaccharide
• - Primary fuel for our cells, required for many
  tissues
• - Main sources are fruits, vegetables, corn
  syrup and honey
• - Blood glucose is maintained within a fairly
  small range
• - Some glucose is stored as glycogen, excess is
  stored as fat
• D-Galactose (aldose) comes from hydrolysis of the
  disaccharide lactose
• - Used in cell membranes of central nervous
  system
• - Converted by an enzyme into glucose for
  respiration (lack of this enzyme causes
  galactosemia, which can cause retardation in
  infants if not treated by complete removal from
  diet)
• D-Fructose (ketose) is the sweetest carbohydrate
• - Converted by an enzyme into glucose for
  respiration
• - Main sources are fruits and honey
• - Also obtained from hydrolysis of the
  disaccharide sucrose

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Structures of Glucose, D-Galactose and D-Fructose

• Note that in nature, only the D enantiomers of
  sugars are used
• What is the relationship between D-glucose and
  L-glucose?
• What is the relationship between D-glucose and
  D-galactose?
• What is the relationship between D-glucose and
  D-fructose?
• (use these choices for each question
  enantiomers, diastereomers or constitutional
  isomers?)

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Amino Sugars

• Amino sugars contain an -NH2 group in place of
  an -OH group.
• Only three amino sugars are common in nature
  D-glucosamine, D-mannosamine, and D-galactosamine.

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Cyclic Structure

• Aldehydes and ketones react with alcohols to form
  hemiacetals (Chapter 17).
• Cyclic hemiacetals form readily when the hydroxyl
  and carbonyl groups are part of the same molecule
  and their interaction can form a five- or
  six-membered ring.

AlcoholAldehyde (or alcoholketone)?Hemiacetal
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Cyclic Structures of Monosaccharides

• Recall that an alcohol can react with an aldehyde
  or ketone to form a hemiacetal
• If the alcohol and aldehyde or ketone are in the
  same molecule, a cyclic hemiacetal is formed
• Monosaccharides in solution are in equilibrium
  between the open-chain and ring forms, and exist
  primarily in the ring form

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Haworth Projection

• A Haworth projection is a common way of
  representing the cyclic structure of
  monosaccharides with a simple three-dimensional
  perspective

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Drawing Haworth Structures for Cyclic Forms

• Step 1 Number the carbons in the chain and turn
  the Fischer projection of the open-chain form
  clockwise 90 degrees
• - Hydroxyl groups that were on the right are now
  on the bottom, and hydroxyl groups that were on
  the left are now on the top (they will stay on
  bottom or top in the Haworth structure)
• Step 2 Rotate around so that C-6 sticks up from
  C-5, and the hydroxyl group on C-5 points towards
  C-1
• Step 3 Form the cyclic hemiacetal by bonding
  the hydroxyl O to the carbonyl C and moving the
  hydroxyl H to the carbonyl O
• Note For C-6 aldose sugars, the cyclic
  hemiacetal has a new chiral carbon at C-1
• - The two possible stereoisomers are called
  anomers
• - The alpha anomer has the hydroxyl group down
• - The beta anomer has the hydroxyl group up

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Haworth Projections

• D-Glucose forms these two cyclic hemiacetals.

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Haworth Projections

• Groups bonded to the carbons of the ring then lie
  either above or below the plane of the ring.
  (Remember your stereoisomer lab!)
• Stereoisomers that differ in configuration only
  at the anomeric carbon are called anomers.
• In a ring, the OH on the alpha anomer is down
  and axial Beta is up and equatorial. Most
  Glucose in our bodies is Beta because its
  farther away and more stable
• The anomeric carbon of an aldose is C-1 that of
  the most common ketoses is C-2 (because there is
  a branch off of the first C and the C in the
  branch is 1). See next slide.

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Haworth Projections

• D-Fructose (a 2-ketohexose) also forms a
  five-membered cyclic hemiacetal.

For test, know that 6 C ketose sugar (fructose)
has anomeric C at 2
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Haworth Projections

• A six-membered hemiacetal ring is called a
  pyranose, and a five-membered hemiacetal ring is
  called a furanose because these ring sizes
  correspond to the heterocyclic compounds furan
  and pyran. (DO NOT have to know furanose for
  TEST)

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Chair Conformations

• For pyranoses, the six-membered ring is more
  accurately represented as a chair conformation.

Which is equatorial and which is axial?
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Chair Conformations

• In both Haworth projections and chair
  conformations, the orientations of groups on
  carbons 1- 5 of b-D-glucopyranose are up, down,
  up, down, and up.

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Mutarotation

• Mutarotation The change in specific rotation
  that accompanies the equilibration of a- and
  b-anomers in aqueous solution.
• Example When either a-D-glucose or b-D-glucose
  is dissolved in water, the specific rotation of
  the solution gradually changes to an equilibrium
  value of 52.7, which corresponds to 64 beta
  and 36 alpha forms.

ONLY have to know that most glucose in body is
Beta
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Disaccharides

• Formation occurs by dehydration between 2 OHs
  of 2 monosaccharide monomers. One monomer removes
  OH and the other removes H. The O that was not
  removed forms a bond between the anomeric Cs.
  The new bond is called a Glycosidic bond. The
  process is commonly called dehydration synthesis
  or condensation.
• Maltose is glucoseglucose
• Sucrose is glucosefructose
• Lactose is glucosegalactose

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Disaccharides

• Disaccharide a carbohydrate containing two
  monosaccharide units joined by a glycosidic bond.
• Sucrose (table sugar)
• Sucrose is the most abundant disaccharide in the
  biological world it is obtained principally from
  the juice of sugar cane and sugar beets. A water
  molecule is removed (not shown)

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Disaccharides

• Maltose
• Present in malt, the juice from sprouted barley
  and other cereal grains.
• Maltose consists of two units of D-glucopyranose
  joined by an a-1,4-glycosidic bond.

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Disaccharides

• Lactose
• Lactose is the principal sugar present in milk
  it makes up about 5 to 8 percent of human milk
  and 4 to 6 percent of cow's milk.
• It consists of D-galactopyranose bonded by a
  b-1,4-glycosidic bond to carbon 4 of
  D-glucopyranose.

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Physical Properties

• Monosaccharides are colorless crystalline
  solids, very soluble in water, but only slightly
  soluble in ethanol.
• Sweetness relative to sucrose

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Polysaccharides

• Polysaccharide A carbohydrate consisting of
  large numbers of monosaccharide units joined by
  glycosidic bonds.
• Starch A polymer of D-glucose.
• Starch can be separated into amylose and
  amylopectin.
• Amylose is composed of unbranched chains of up to
  4000 D-glucose units joined by a-1,4-glycosidic
  bonds.
• Amylopectin contains chains up to 10,000
  D-glucose units also joined by a-1,4-glycosidic
  bonds at branch points, new chains of 24 to 30
  units are started by a-1,6-glycosidic bonds.

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Polysaccharides

• Figure 20.3 Amylopectin, a branched polymer of
  approximately 10,000 units of D-glucose joined by
  ?-1,4-glycosidic bonds.

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Polysaccharides

• Glycogen is the energy-reserve carbohydrate for
  animals.
• Glycogen is a branched polysaccharide of
  approximately 106 glucose units joined by a-1,4-
  and a-1,6-glycosidic bonds.
• The total amount of glycogen in the body of a
  well-nourished adult human is about 350 g,
  divided almost equally between liver and muscle.

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Polysaccharides

• Cellulose is a linear polysaccharide of
  D-glucose units joined by b-1,4-glycosidic bonds.
• It has an average molecular weight of 400,000
  g/mol, corresponding to approximately 2200
  glucose units per molecule.
• Cellulose molecules act like stiff rods and align
  themselves side by side into well-organized
  water-insoluble fibers in which the OH groups
  form numerous intermolecular hydrogen bonds.
• This arrangement of parallel chains in bundles
  gives cellulose fibers their high mechanical
  strength.
• It is also the reason why cellulose is insoluble
  in water.

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Polysaccharides

• Figure 20.4 Cellulose is a linear polymer
  containing as many as 3000 units of D-glucose
  joined by b-1,4-glycosidic bonds.

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Polysaccharides

• Cellulose (contd)
• Humans and other animals can not digest cellulose
  because their digestive systems do not contain
  b-glycosidases, enzymes that catalyze the
  hydrolysis of b-glycosidic bonds.
• Termites have such bacteria in their intestines
  and can use wood as their principal food.
• Ruminants (cud-chewing animals) and horses can
  also digest grasses and hay.
• Instead, we have only a-glucosidases hence, the
  polysaccharides we use as sources of glucose are
  starch and glycogen.
• Many bacteria and microorganisms have
  b-glucosidases.

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Chapter 20 Carbohydrates

• End
• Chapter 20