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Carbohydrates

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Title: Carbohydrates


1
Chapter 22
  • Carbohydrates

2
Carbohydrates
  • Fun Facts
  • Photosynthesis converts more than 100 billion
    metric tons of CO2 and H20 into carbohydrates
    annually.
  • Non-photosynthetic cells can make there own
    glucose from amino acids, fats and other
    breakdown products.

3
Carbohydrates
  • Fun Facts 2
  • Mole Ratios 1C, 2H, 1O
  • Empirical Formula CH2O
  • monosaccharides have from 3 to 8 carbons
  • aldose linear sugar with an aldehyde group
  • ketose linear sugar with a ketone group

4
Carbohydrates
  • Fun Facts 3
  • Three classes of carbohydrates
  • Monosaccharides
  • 3 to 8 carbons with carbonyl and alcohol FG
  • Disaccharides
  • 2 monosaccharides connected with a ketal or
    acetal connection
  • Polysaccharides
  • Multiple ketal or acetal connections

5
Monosaccharides
  • Monosaccharides are classified by their number of
    carbon atoms

6
Monosaccharides
  • And they differ by the type of carbonyl present
  • Aldehyde
  • Ketone

7
Monosaccharides
  • There are only two trioses
  • often aldo- and keto- are omitted and these
    compounds are referred to simply as trioses

8
Monosaccharides
  • Glyceraldehyde, the simplest aldose, contains a
    stereocenter and exists as a pair of enantiomers

9
Monosaccharides
  • 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

10
D,L Monosaccharides
  • Emil Fischer decided on of D- and L- assignments
    for the enantiomers of glyceraldehyde
  • D-monosaccharide the -OH is on the right
  • L-monosaccharide the -OH is on the left

11
D,L Monosaccharides
  • the most common D-tetroses and D-pentoses

12
D,L Monosaccharides
  • the three common D-hexoses

13
Amino Sugars
  • Amino sugars contain an -NH2 group in place of an
    -OH group

14
Cyclic Structure
  • Aldehydes and ketones react with alcohols to form
    hemiacetals
  • cyclic hemiacetals form readily as five- or
    six-membered ring

15
Haworth Projections
  • D-Glucose forms these cyclic hemiacetals

16
Haworth Projections
  • a five- or six-membered cyclic hemiacetal is
    represented as a planar ring
  • groups lie either above or below the plane
  • the new carbon stereocenter is called an anomeric
    carbon
  • stereoisomers that differ in configuration only
    at the anomeric carbon are called anomers
  • the anomeric carbon of an aldose is C-1 that of
    the most common ketoses is C-2

17
Haworth Projections
  • Terminology of carbohydrate chemistry,
  • b means that the anomeric -OH is on the same side
    of the ring as the terminal -CH2OH
  • a means that the anomeric -OH is on the side of
    the ring opposite the terminal -CH2OH
  • a six-membered hemiacetal ring is called a
    pyranose, and a five-membered hemiacetal ring is
    called a furanose

18
Haworth Projections
  • aldopentoses also form cyclic hemiacetals
  • the most prevalent forms of D-ribose and other
    pentoses in the biological world are furanoses

19
Haworth Projections
  • D-fructose also forms a five-membered cyclic
    hemiacetal

20
Mutarotation
  • Mutarotation the equilibrium interconversion of
    a- and b-anomers in aqueous solution

21
Chair Conformations Pg 475
  • Lets leave this out. I will be very happy if you
    can draw Fisher and Hayworth forms.

22
Physical Properties
  • Monosaccharides are colorless crystalline solids,
    very soluble in water
  • sweetness relative to sucrose

23
Chemical Properties
  • Monosaccharides
  • Hemiacetal into acetal glycosidic bond
  • A glycosidic bond slows mutarotation to snails
    pace.
  • Acid is needed to break acetal or ketal
  • Aldoses reduce Cu2, Fe3, and cold MnO4-
  • Only works with the linear aldehyde form
  • Hemiacetals are in equilibrium with aldehyde form
  • Called reducing sugars
  • Glycosides cannot reduce these
  • Carbonyl can be reduced

24
Formation of Glycosides
  • Treatment of a monosaccharide with an alcohol
    gives an acetal

25
Glycosides
  • a cyclic acetal derived of a monosaccharide is
    called a glycoside
  • the bond from the anomeric carbon to the -OR
    group is called a glycosidic bond
  • mutarotation is VERY SLOW in a glycoside
  • glycosides are stable in water and aqueous base,
    but like other acetals, are hydrolyzed in aqueous
    acid to an alcohol and a monosaccharide

26
Oxidation to Aldonic Acids
  • the aldehyde group of an aldose is oxidized under
    basic conditions to a carboxylate anion
  • the oxidation product is called an aldonic acid
  • reducing sugar (it reduces the oxidizing agent)

27
Oxidation to Uronic Acids
  • Enzyme-catalyzed oxidation of the primary alcohol
    at C-6 of a hexose yields a uronic acid
  • enzyme-catalyzed oxidation of D-glucose, for
    example, yields D-glucuronic acid

28
Reduction to Alditols
  • The carbonyl group can be reduced to a hydroxyl
    group by NaBH4 and H2/Pd
  • the reduction product is called an alditol

29
Reduction to Alditols
  • sorbitol is found in the plant world in many
    berries and in cherries, plums, pears, apples,
    seaweed, and algae
  • it is about 60 percent as sweet as sucrose
  • these three alditols are also common in the
    biological world

30
D-Glucuronic Acid
  • D-glucuronic acid exists in the plants and
    animals
  • in humans, it is an important component of the
    acidic polysaccharides of connective tissues
  • it is used to detoxify foreign phenols and
    alcohols in the liver, these compounds are
    converted to glycosides of glucuronic acid and
    excreted in the urine

31
Phosphate Esters
  • Mono- and diphosphoric esters are intermediates
    in metabolism of monosaccharides
  • the first step in glycolysis is conversion of
    D-glucose to a-D-glucose 6-phosphate

32
Disaccharides
  • Sucrose
  • most abundant disaccharide
  • sucrose is a nonreducing sugar (why)

33
Disaccharides
  • Lactose
  • lactose is the principal sugar present in milk
  • it consists of D-galactopyranose bonded by a
    b-1,4-glycosidic bond to carbon 4 of
    D-glucopyranose
  • lactose is a reducing sugar (why)

34
Disaccharides
  • Maltose
  • present in malt
  • two D-glucopyranose joined by an a-1,4-glycosidic
    bond
  • maltose is a reducing sugar (Why)

35
Polysaccharides
  • Polysaccharide lots of monosaccharide units
  • Also called glycans
  • Can be a or b linked anomers
  • One we can digest a
  • The other we cannot b

36
Polysaccharides - a
  • Starch an energy storage polymer of D-glucose
    found in plants
  • starch can be separated into amylose and
    amylopectin
  • amylose is D-glucose units joined by a-1,4-bonds
  • Amylopectin - D-glucose units joined by a-1,4
    bonds at branch points, new chains every 24 to
    30 units are started by a-1,6-glycosidic bonds

37
Polysaccharides - a
  • Glycogen is the energy-reserve carbohydrate for
    animals
  • glycogen - glucose units joined by a-1,4- and
    a-1,6-glycosidic bonds (branches occur every 8 to
    12 residues - more compact than starch)
  • 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

38
Polysaccharides - a
  • Why Store sugar as starch or glycogen?
  • Osmolarity
  • Individual sugars would be 0.4 M
  • Polymers (mostly insoluable) 10-8 M
  • Cells would burst with water running into the to
    equilibrate osmotic pressure!

39
Polysaccharides - b
  • 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, approximately 2200 glucose units
  • cellulose molecules act like stiff rods and align
    themselves side by side into well-organized
    water-insoluble fibers in which the OH groups
    hydrogen bond with each other rather than water.
  • 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

40
Polysaccharides - b
  • Cellulose (contd)
  • animals cannot digest cellulose
  • no contain b-glucosidases, enzymes that catalyze
    hydrolysis of b-glucosidic bonds
  • we have only a-glucosidases hence we can digest
    starch and glycogen
  • many bacteria and microorganisms have
    b-glucosidases and can digest cellulose
  • termites have such bacteria in their intestines
    and can use wood as their principal food

41
Acidic Polysaccharides
  • Acidic polysaccharides contain carboxyl groups
    and/or sulfuric ester groups
  • play important roles in the structure and
    function of connective tissues
  • there are a large number of highly specialized
    forms of connective tissue
  • such as cartilage, bone, synovial fluid, skin,
    tendons, blood vessels, intervertebral disks, and
    cornea
  • most connective tissues are made up of collagen,
    a structural protein, in combination with a
    variety of acidic polysaccharides

42
Acidic Polysaccharides
  • Hyaluronic acid
  • Found in embryonic tissues, synovial fluid,
    lubricant of joints in the body, and the vitreous
    of the eye

43
Acidic Polysaccharides
  • Heparin a heterogeneous mixture of variably
    sulfonated polysaccharide chains, ranging in
    molecular weight from 6,000 to 30,000 g/mol

44
Acidic Polysaccharides
  • Heparin (contd)
  • heparin is synthesized and stored in mast cells
    of various tissues, particularly the liver,
    lungs, and gut
  • the best known and understood of its biological
    functions is its anticoagulant activity
  • it binds strongly to antithrombin III, a plasma
    protein involved in terminating the clotting
    process

45
Carbohydrates
End Chapter 19
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