HL Chemistry - Option B: Human Biochemistry

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HL Chemistry - Option B Human Biochemistry
The Discovery of Honey by Piero de Cosimo (1462)

• Carbohydrates

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Part 1

• Overview of Carbohydrates

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General Characteristics

• The term carbohydrate is derived from the French
  hydrate de carbone
• All carbohydrates are compounds composed of (at
  least) C, H, and O
• The general formula for a carbohydrate is
  (CH2O)n (e.g. when n 5 then the formula would
  be C5H10O5)
• Not all carbohydrates have this empirical formula
  (e.g. deoxysugars, aminosugars, etc.)
• Carbohydrates are the most abundant compounds
  found in nature (e.g. cellulose 100 billion tons
  annually)

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General Characteristics

• In nature, most carbohydrates are found bound to
  other compounds rather than as simple sugars
• Polysaccharides (starch, cellulose, inulin, gums)
• Glycoproteins and proteoglycans (hormones, blood
  group substances, antibodies)
• Glycolipids (cerebrosides, gangliosides)
• Glycosides
• Mucopolysaccharides (hyaluronic acid)
• Nucleic acid polymers

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Carbohydrate Functions

• Carbohydrates can be
• Sources of energy
• Intermediates in the biosynthesis of other basic
  biochemical entities (fats and proteins)
• Associated with other entities such as
  glycosides, vitamins and antibiotics)
• Structural tissues in plants and in
  microorganisms (cellulose, lignin, murein)
• Involved in biological transport, cell-cell
  recognition, activation of growth factors,
  modulation of the immune system

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Classification of Carbohydrates

• Carbohydrates can be classified by size
• Monosaccharides (monoses or glycoses)
• Trioses, tetroses, pentoses, hexoses
• Oligosaccharides
• Di, tri, tetra, penta up to 10
• (The disaccharides are the most important)
• Polysaccharides (or glycans)
• Homopolysaccharides (all the same type)
• Heteropolysaccharides (mixtures of momomer types)
• Complex carbohydrates (joined to non-carbohydrate
  molecules)

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Monosaccharides

• Monosaccharides are also known as simple sugars
• They are classified by (1) the number of carbons
  and (2) whether they are aldoses or ketoses (more
  to come on this!)
• Most (99) simple sugars are straight chain
  compounds
• D-glyceraldehyde is the simplest of the aldoses
  (aldotriose)
• All other sugars have the ending ose (glucose,
  galactose, ribose, lactose, etc)

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Monosaccharides

• Aldoses (e.g. glucose) have an aldo (aldehyde)
  group at one end

Ketoses (e.g. fructose) have a keto (ketone)
group (usually at C2)
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Aldose sugars
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Ketose sugars
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D- vs L- Designation

• D L designations are based on the configuration
  about the single asymmetric C in glyceraldehyde
• The lower diagrams are Fischer Projections.

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Sugar Nomenclature

• For sugars with more than one chiral center, D or
  L refers to the asymmetric C farthest from the
  aldehyde or keto group (in yellow)
• Most naturally occurring sugars are D isomers

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• D L sugars are mirror
• images of one another
• They have the same root
• name (but a different
• D/L designation),
• e.g. D-glucose
• L-glucose
• Other stereoisomers
• have unique names,
• (e.g. glucose, mannose,
• galactose, etc)
• The number of stereoisomers is 2n, where n is the
  number of asymmetric (chiral) centers
• The 6-C aldoses have 4 asymmetric centers. Thus
  there are 16 stereoisomers (8 D-sugars and 8
  L-sugars).

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Structure of a Simple Aldose and a Simple Ketose
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Enantiomers and Epimers
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Relationship Between D- L-Fructose
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Properties of Optical Isomers

• The differences in structures (configurations) of
  sugar optical isomers are responsible for
  variations in properties
• Physical Differences Between D- L- forms
• Crystalline structure solubility rotatory power
• Chemical Differences Between D- L- forms
• Reactions (oxidations, reductions, condensations)
• Physiological Differences Between D- L- forms
• Nutritive value (human, bacterial) sweetness
  absorption

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Structural Representation of Sugars

• Biomolecules (in this case sugars) can be
  represented in three main ways (visualized in the
  following slides)
• Fischer Projection straight chain representation
• Haworth Projection simple ring in perspective
• Conformational Representation chair and boat
  configurations

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• Pentoses and hexoses can cyclize as the ketone or
  aldehyde reacts with a distal OH. The top
  diagram is a Fischer Projection of D-Glucose
• Glucose forms an intra-molecular hemiacetal, as
  the C1 aldehyde C5 OH react, to form a
  6-member pyranose ring, named after pyran

The representations of the cyclic sugars (bottom)
are called Haworth Projections
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More Pyran Cyclization
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• Fructose forms either a
• 6-member pyranose ring reaction of the C2 keto
  group with the OH on C6, or
• 5-member furanose ring reaction of the C2 keto
  group with the OH on C5

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• Cyclization of glucose produces a new asymmetric
  center at C1. The 2 stereoisomers are called
  anomers, a b
• Haworth projections represent the cyclic sugars
  as having essentially planar rings, with the OH
  at the anomeric C1
• a (OH below the ring)
• b (OH above the ring)

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• Because of the tetrahedral nature of carbon
  bonds, pyranose sugars actually assume a "chair"
  or "boat" configuration, depending on the sugar
• The representation above reflects the chair
  configuration of the glucopyranose ring more
  accurately than the Haworth projection

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Chair (top) and Boat (bottom) forms of the
Pyranose Ring
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Optical Isomerism and Polarimetry

• Recall that optical isomerism is a property
  exhibited by any compound whose mirror images are
  non-superimposable
• Also, compounds with asymmetric carbons rotate
  plane polarized light
• Measurement of optical activity in chiral or
  asymmetric molecules uses plane polarized light
• Molecules may be chiral because of certain
  atoms or because of chiral axes or chiral
  planes
• Measurement uses an instrument called a
  polarimeter (Lippich type)
• Rotation is either () dextrorotatory or (-)
  levorotatory

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Polarimeter
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Polarimetry

• Magnitude of rotation depends upon
• 1. The nature of the compound
• 2. The length of the tube (cell or sample
  container) usually expressed in decimeters (dm)
• 3. The wavelength of the light source employed
  usually either sodium D line at 589.3 nm or
  mercury vapor lamp at 546.1 nm
• 4. Temperature of sample
• 5. Concentration of carbohydrate in grams per 100
  ml

Selected Rotations D-glucose 52.7 D-fructose -9
2.4 D-galactose 80.2 L-arabinose 104.5 D-mann
ose 14.2 D-arabinose -105.0 D-xylose
18.8 Lactose 55.4 Sucrose 66.5 Maltose 130.4
Invert sugar -19.8 Dextrin 195
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Part 2

• Oligosaccharides
• and selected derivatives

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Oligosaccharides

• The most common oligosaccarides are the
  disaccharides
• Sucrose, lactose, and maltose
• Maltose hydrolyzes to 2 molecules of D-glucose
• Lactose hydrolyzes to a molecule of glucose and a
  molecule of galactose
• Sucrose hydrolyzes to a molecule of glucose and a
  molecule of fructose

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Glycosidic Bonds

• The anomeric hydroxyl and a hydroxyl of another
  sugar or some other compound can join together,
  splitting out water to form a glycosidic bond
• R-OH HO-R' ? R-O-R' H2O
• e.g. methanol reacts with the anomeric OH on
  glucose to form methyl glucoside
  (methyl-glucopyranose).

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Disaccharides Maltose, a cleavage product of
starch (i.e. amylose), is a disaccharide with an
a(1 4) glycosidic link between the C1 - C4 OHs
of 2 glucoses. It is the a anomer (C1 O points
down)

• Cellobiose, a product of cellulose breakdown, is
  the otherwise equivalent b anomer (O on C1 points
  up).
• The b(1 4) glycosidic linkage is represented as
  a zig-zag, but one glucose is actually flipped
  over relative to the other

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• Other disaccharides include
• Sucrose, common table sugar, has a glycosidic
  bond linking the anomeric hydroxyls of glucose
  fructose.
• Because the configuration at the anomeric C of
  glucose is a (O points down from ring), the
  linkage is a(1?2)
• The full name of sucrose is
  a-D-glucopyranosyl-(1?2)-b-D-fructopyranose.)
• Lactose, milk sugar, is composed of galactose
  glucose, with b(1?4) linkage from the anomeric OH
  of galactose. Its full name is b-D-galactopyranosy
  l-(1? 4)-a-D-glucopyranose

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Sucrose

• Probably the most famous sugar, and everyones
  favorite, is sucrose
• a-D-glucopyranosido-b-D-fructofuranoside
• b-D-fructofuranosido-a-D-glucopyranoside
• Also known as table sugar
• Commercially obtained from sugar cane or sugar
  beet
• Hydrolysis yield glucose and fructose (invert
  sugar) ( sucrose 66.5o glucose 52.5o
  fructose 92o)
• Used pharmaceutically to make syrups

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Lactose

• Lactose is another famous disaccharide, resulting
  from b-D-galactose joining to a-D-glucose via a
  b-(1,4) linkage
• Milk contains the a and b-anomers in a 23 ratio
• b-lactose is sweeter and more soluble than
  ordinary a- lactose
• Used in infant formulations, medium for
  penicillin production and as a diluent in
  pharmaceuticals

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Starch

• Starch is the most common storage polysaccharide
  in plants
• It is composed of 10 30 a-amylose and 70-90
  amylopectin (depending on the source)
• The chains are of varying length, having
  molecular weights from several thousands to half
  a million

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Polysaccharides

• Plants store glucose as amylose or amylopectin.
  Glucose polymers collectively are called starch.
  Glucose storage in polymeric form minimizes
  osmotic effects.
• Amylose is a glucose polymer with a(1?4)
  linkages. It adopts a helical conformation (see
  above)
• The end of the polysaccharide with an anomeric
  C1 not involved in a glycosidic bond is called
  the reducing end

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• Amylopectin is a glucose polymer with mainly
  a(1?4) linkages, but it also has branches formed
  by a(1?6) linkages (see above). Branches are
  generally longer than shown above.
• The branches produce a compact structure
  provide multiple chain ends at which enzymatic
  cleavage can occur.

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Another view of amylose and amylopectin, the two
forms of starch. Amylopectin is a highly branched
structure, with branches occurring every 12 to 30
residues
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Glycogen

• Glycogen is also known as animal starch (not
  really an accurate description!)
• It is stored in muscle and liver tissue
• Also present in cells as granules (high MW)
• It contains both a-(1,4) links and a-(1,6)
  branches at every 8 to 12 glucose unit
• Complete hydrolysis yields glucose
• Glycogen and iodine gives a red-violet color
• Hydrolyzed by both a and b-amylases and by
  glycogen phosphorylase these are enymes

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• Glycogen, the glucose storage polymer in
  animals, is similar in structure to amylopectin,
  but glycogen has more a(1?6) branches
• The highly branched structure permits rapid
  release of glucose from glycogen stores, i.e. in
  muscle during exercise. The ability to rapidly
  mobilize glucose is more essential to animals
  than to plants

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Cellulose

• Cellulose is a polymer of b-D-glucose attached by
  b-(1,4) linkages
• It yields glucose upon complete hydrolysis
• Partial hydrolysis yields cellulobiose
• Cellulose is the most abundant of all
  carbohydrates
• Cotton flax 97-99 cellulose
• Wood 50 cellulose
• Cellulose gives no color with iodine
• Held together with lignin in woody plant
  tissues

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• Cellulose, a major constituent of plant cell
  walls, consists of long linear chains of glucose
  with b(14) linkages.
• Every other glucose is flipped over, due to the
  b linkages. This promotes intra-chain and
  inter-chain H-bonds and van der Waals
  interactions. This cause cellulose chains to be
  straight rigid, and pack with a crystalline
  arrangement in thick bundles called microfibrils

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The Linear Structures of Cellulose and Chitin
(chitin is found in the exoskeleton of insects,
crayfish, etc (these are the two most abundant
polysaccharides in nature)
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The Molecular Structure of Cellulose
(Notice the presence of sheets that can be
pealed away. Think about a piece of celery and
how you can strip off the fibers)
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Suspensions of amylose in water adopt a
helical conformation Iodine (I2) can insert
in the middle of the amylose helix to give a blue
color that is characteristic and diagnostic for
starch
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(a) The structure of starch shows a linkages
(b) The structure of cellulose shows b linkages
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Oligosaccharides that are covalently attached to
proteins or to membrane lipids may be linear or
branched chains

• O-linked oligosaccharide chains of glycoproteins
  vary in complexity.
• They link to a protein via a glycosidic bond
  between a sugar residue and a serine or threonine
  OH
• O-linked oligosaccharides have roles in
  recognition, interaction, and enzyme regulation

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The Structures of Serine or Threonine O-linked
Saccharides
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O-linked glycoproteins are found in the blood of
Arctic and Antarctic fish, enabling them to live
at sub-zero water temperatures
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• N-acetylglucosamine (GlcNAc) is a common
  O-linked glycosylation product of serine or
  threonine residues
• Many cellular proteins, including enzymes
  transcription factors, are regulated by
  reversible GlcNAc attachment
• Often attachment of GlcNAc to a protein OH
  alternates with phosphorylation, with these 2
  modifications having opposite regulatory effects
  (stimulation or inhibition)

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• N-linked oligosaccharides of glycoproteins tend
  to be complex and branched. First
  N-acetylglucosamine is linked to a protein via
  the side-chain N of an asparagine residue in a
  particular 3-amino acid sequence.

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The Structure of Aspargine N-linked Glycoproteins
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More Examples of N-Linked Glycoproteins
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Selected Facts About Oligosaccharide Derivatives

• Many proteins secreted by cells have attached
  N-linked oligosaccharide chains
• Genetic diseases have been attributed to
  deficiencies of particular enzymes involved in
  synthesizing or modifying these glycoprotein
  oligosaccharide chains
• Such genetic diseases, and gene knockout studies
  in mice, have been used to define pathways of
  modification of oligosaccharide chains in
  glycoproteins and glycolipids.
• Carbohydrate chains of plasma membrane
  glycoproteins and glycolipids usually face the
  outside of the cell
• Plasma membrane glycoproteins and glycolipids
  have roles in cell-cell interaction and
  signaling, as well as forming a protective layer
  on the surface of some cells

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Special Monosaccharides Deoxy Sugars

• Some monosaccharides lack one or more hydroxyl
  groups on the molecule. These are deoxy sugars
• One ubiquitous deoxy sugar is 2-deoxy ribose
  which is the sugar found in DNA
• 6-deoxy-L-mannose (L-rhamnose) is used as a
  fermentative reagent in bacteriology

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A Few Examples of Deoxysugar Structures