Carbohydrates Lecture 1

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CarbohydratesLecture 1
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Why Carbohydrates ?
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Objectives

• Define what a carbohydrate molecule is
• Recognise and classify carbohydrate molecules
• Explain why carbohydrates are important
• Name other molecules that interact with
  carbohydrates and explain how and why these
  interactions occur

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Lecture Outline

• Lecture 1 Monosaccharides
• Definition
• Nomenclature
• Fischer projections
• Lecture 2 Monosaccharides
• Haworth projections
• Furanose Pyranose rings
• Modified Sugars
• Sugars in Nucleic Acids

• Lecture 3 Dissaccharides Energy storage
• Glycosidic bond
• Glycolysis
• Lecture 4 Complex Carbohydrates Function
• Starch glycogen
• Cellulose
• Blood groups
• Glycoproteins

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Outline

• Importance of carbohydrates
• Definition of a carbohydrate
• Monosaccharides nomenclature
• Stereochemistry in carbohydrates
• Enantiomers
• Drawing sugars - Fischer Projections

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Objectives

• Explain why carbohydrates are important
• Define what a carbohydrate molecule is
• Recognise and classify carbohydrate molecules
• Define stereochemistry and explain its importance
  in the structures of carbohydrates

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Importance of Carbohydrates

• Primary energy store in biosphere
• Energy distribution
• Glycoproteins addresses for intracellular
  traffic
• Antigenic ABO blood groups
• Structural and mechanical components of cell
  walls in plants and insects, cartilage in
  vertebrates

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Importance of Carbohydrates

• Most organisms that live in air obtain energy
  from the oxidation of carbohydrates
• Glucose is most common simple carbohydrate used
  as fuel
• Important to consider structure of carbohydrates
• Differences in molecular structure are often
  small but critical to their function

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What is a Carbohydrate?

• Aldehyde or ketone compounds with multiple
  hydroxyl groups (OH)
• Ketones both groups attached to carbonyl group
  are carbon
• Aldehydes one carbon and one hydrogen attached
  to carbonyl group

Acetaldehyde
Acetone
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What is a Carbohydrate?

• Simplest carbohydrate molecule is a
  monosaccharide (C-H20)n carbon hydrate
• Monosaccharides have 3 to 7 carbons
• Either aldehyde or ketone group and hydroxyl (OH)
  groups on nearly every carbon
• Polyhydroxyaldehydes or Polyhydroxyketones

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Polyhydroxyaldehydes
Glucose
Galactose
Ribose
R
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Polyhydroxyketone
R
R
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Hexose aldo keto
R
R
R
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Monosaccarides
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Monosaccharide Nomenclature
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Which classification fits this sugar?
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Which classification fits this sugar?
Dihydroxyacetone
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Which classification fits this sugar?
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Which classification fits this sugar?
Galactose
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Which classification fits this sugar?
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Which classification fits this sugar?
Glyceraldehyde
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Which classification fits this sugar?
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Which classification fits this sugar?
D-Threose
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Which classification fits this sugar?
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Which classification fits this sugar?
Ketoheptose
Sedoheptulose
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Stereochemistry

• Stereochemistry study of arrangement of atoms
  in 3D space
• Carbohydrates with identical functional groups
  linked in the same order but with different
  spatial orientation
• Identical molecules can be made to superimpose by
  rotating and flipping
• Different compounds cannot be made to superimpose

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Are these compounds the same?
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Nonsuperimposable mirror images
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Different Compounds

• Nonsuperimposable so different compounds
• One carbon atom is chiral (not symmetric)
• Nonsuperimposable mirror images are called
  enantiomers

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Enantiomers

• Stereoisomers are molecules that have the same
  bonds connecting the same atoms but different
  relative orientations of the bonds
• Enantiomer one type of stereoisomer
• Nonsuperimposable mirror images

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Enantiomers are different compounds

• Different relative orientations of bonds in a
  pair of enantiomers cannot be changed by rotation
  about the bonds
• Only be changed by breaking bonds and remaking
  them
• One enantiomer can only be changed into another
  enantiomer by chemical reaction

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Stereoisomers important implications

• Starch and cellulose both polymers of glucose
  units linked together
• C4 of one glucose bonded to C1 of next glucose
• Only difference is the direction of the bond
• Cellulose bond points up, in starch bond points
  down
• Humans can digest starch but not cellulose

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Chiral Carbon Atom
Single asymmetric carbon
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Glyceraldehyde Enantiomers

• Groups attached to an asymmetric carbon atom take
  either of 2 fixed positions with respect to other
  carbon atoms
• -OH group can extend either right or left of the
  carbon chain with reference to the CHO and CH2OH
  groups

R
L
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Glyceraldehyde Enantiomers

• Orient molecule so carbonyl is at top
• If hydroxyl (OH) on highest numbered chiral
  carbon
• to the right D-glyceraldehyde
• to the left L-glyceraldehyde
• Dexter right in latin
• Laevus left in latin

R
L
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Fischer Projections

• Drawing 2D images representing 3D configuration
  of molecule is difficult but possible by using
  set of rules
• Acyclic compounds drawn so that vertical bonds
  represent bonds pointing back and horizontal
  bonds are bonds pointing forward

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Fischer Projections

• Using these rules the distinguishing features of
  the 3D structure of stereoisomers can easily and
  accurately represented with 2D drawings
• Fischer projection named after Emil Fischer who
  established the molecular structures of many
  sugars

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Fischer Projections
galactose
Glucose
Galactose
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Fischer Projections
galactose
C
C
C
C
C
Glucose
Galactose
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Fischer Projections
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Fischer Projections
C
D-ribose
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Fischer Projections
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Fischer Projections
C
D-glyceraldehyde
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Lecture 1 Summary

• Why are carbohydrates important?
• Primary energy store
• Structural properties
• Antigenic properties
• What is a carbohydrate?
• Aldehyde or ketone compounds with multiple
  hydroxyl groups (OH)
• Stereochemistry
• Carbohydrates have identical functional groups
  linked in the same order but with different
  spatial orientation
• Enantiomers
• Drawing sugars
• Fischer projections
• Stereo chemistry