Sugars and
Polysaccharides
Importance of Carbohydrates
• Key intermediates of metabolism of food and
energy production (sugars)
• Structural components of plants, animals and
bacteria: (cellulose, peptidoglycan, cartilage)
• Central to materials of industrial products:
(paper, lumber)
• Key component of food sources: (sugars,
flour, fiber)
Outline of Carbohydrates
• Part 1: General structure, names, and
stereochemical properties of simple sugars
(chpt 11 sec 1)
• Names of simple sugars
• Fischer Projections of sugars
• Cyclic reactions: Hemiacetal formation
• Part 2: Disaccharides: structure, nomenclature
and biology (chpt 11 sec 2)
• Acetal formation
• Disaccharide nomenclature
• Part 3: Polysaccharides and Glycoproteins:
structures and biology
•
•
•
•
Starch, Glycogen structures
Cellulose, Chitin structures
Bacterial cell walls: peptidoglycan
Extracellular matrix: Hyaluronic acid
Classification of Carbohydrates
• Carbohydrates- are molecules, consisting only of
carbon (C), hydrogen (H), and oxygen (O), with the
empirical formula Cm(H2O)n (where m could be
different from n).
– Monosaccharide's (simple sugars) can't be
converted into smaller sugars by hydrolysis.
– Disaccharides- comes from two monosaccharides
(glucose linked to fructose; sucrose) linked
together by an acetal bond.
– Polysaccharides- made of three or more simple
sugars connected as acetals (aldehyde and
alcohol).
Biological Monosaccharides are
classified into two categories
Simple Sugars
Aldoses
Most oxidized
carbon
is an aldehyde
Ketoses
Most oxidized
carbon is a ketone
Three and four carbon
Aldoses: Aldotriose, Aldotetriose
3-Carbon
Most
Oxidized
carbon
4-Carbon
Chiral
center
D-Glyceraldehyde
D-Erythrose
D-Threose
Five Carbon Aldoses:
Aldopentoses
D-Ribose
D-Arabinose
D-Xylose
D-Lyxose
Six Carbon Aldoses: Aldohexoses
D-Allose
D-Gulose
D-Altrose
D-Idose
D-Glucose
D-Galactose
D-Mannose
D-Talose
Three and four carbon
Ketoses: Ketotriose, Ketotetriose
4-Carbon
3-Carbon
Dihydroxyacetone
D-Erythrulose
Five and six Carbon Ketoses
Ketohexoses
Ketopentoses
D-Ribulose
D-Xylulose
D-Psicose
D-Sorbose
D-Fructose
D-Tagatose
Abbreviations for some sugars that
are common components of
polysaccharides
Memorize the shaded abbreviations!!!
Structures you have to
memorize!!!
• Glyceraldehyde
• Dihydroxacetone phosphate
• Ribose & Deoxyribose
• Glucose
• Fructose
Write on board
Stereochemisry review
Chiral Centers
How do we deal with multiple
chiral centers
• Enantiomers- molecules that are not identical
to their mirror images. (This definition includes
multiple chiral centers).
• A general rule, a molecule with “N” chiral centers can
have 2N stereoisomers.
• N = 6; 26 =
64 possible stereoisomers!!!
• Diastereomers- stereoisomers that are not
mirror images.
Absolute configuration is
assigned using the R,S system
- The R,S system was developed long after many
biochemicals were discovered.
- Biochemists have been slow to adopt the R,S system.
Carbohydrate Stereochemistry:
Fischer Projections
• A chirality center C is projected
into the plane of the paper
• Groups forward from paper are
always in horizontal line.
• Vertical bonds represents groups
projecting into the plane of paper
Hermann Emil Fischer
From 1852-1919; Nobel Prize in 1902
Fischer convention for
carbohydrates (D, L)
• The hydroxyl group at the chiral center farthest
from the oxidized end of the sugar determines the
stereochemical reference (D, or L).
Bold
wedges
H
O
C
H
OH
CH2OH
D-Glyceraldehyde
Hash
wedges
Stereochemical Reference
• A compound is “D” if the hydroxyl group at the
chirality center farthest from the oxidized end of the
sugar is on the right or “L” if it is on the left.
“D” is when the hydroxyl group is on
the right and “L” is when it is on the left
“D” is when the hydroxyl group is on
the right and “L” is when it is on the left
Allowed Movements with Fisher
Projections
- Rotation of 90º is not allowed with a fisher projection since
this will change the chirality.
- Rotation of 180º is allowed with a fisher projection since
this will not change the chirality.
We can change the position of three
groups and leave one group the same
without changing the chirality
Specify the sugars as “D” or “L”
Most oxidized
carbon at or
closest to the top
Linear
chain
L
Least oxidized
carbon at or
closest to the top
Assign “D” or “L” to each
Monosaccharide
HO
H
HO
HO
OH
H
H
H
L
D
O
L
Specify the sugar as “D” or “L”
CH O
HO
H
CH2 OH
Hold
Steady
L
Epimers are two diastereomers that differ in
their configuration around a single carbon
Mechanism of hemiacetal,
hemiketal formation
Sugars of five or more carbons readily
adopt the cyclic conformations in solution
<1%
α-anomer
38%
β-anomer
62%
Haworth Projections
α- D-Glucose
β- D-Glucose
α- D-Fructose
β- D-Fructose
Haworth Projections
α- D-Glucose
β- D-Glucose
α- D-Fructose
β- D-Fructose
Sugars can undergo oxidationreduction reactions at the
anomeric carbon
• The exposed C1 (anomeric carbon) is referred to as the
reduced end (carbonyl can be reduced to a carboxyl).
Reduced
end
Reducing sugar test is the basis of blood sugar meters.
Carbohydrate Analytical Tests
Oxidizing
Agent
Benedict’s
solution
Fehling’s
Solution
Tollen’s
Reagent
Active
ingredient
CuSO4
CuSO4
Ag in NH3
Deep Blue
Deep Blue
Mirror
Oxidant
Cu+2 ----- Cu+
Cu+2 ----- Cu+
Ag+ ----- Ag(s)
Sugar
Product
Oxidized to
Carboxylate
Oxidized to
Carboxylate
Oxidized to
Carboxylate
Test
Result
Positive for
Aldoses
Positive for
Aldoses and
Ketoses
Positive for
Aldoses
Color
Carbohydrate Analytical Tests
Positive for Fehling’s
Positive for all three
tests
Negative for Tollen’s
Negative for all
three tests
Negative for Benedict’s
Fehling’s Tests- Positive for Aldoses and Ketoses
Benedict’s Test- Positive for Aldoses only
Tollen’s Test- Positive for Aldoses only
All tests are negative if the anomeric carbon is linked to another sugar!!
Mechanism of hemiacetal, acetal
formation
Disaccharide nomenclature
• Glycosidic bond- forms when the hydroxyl group of
one sugar reacts with the anomeric carbon of the
other.
Anomeric
carbon
Galactose
(β1 - 4)
Glucose
Example disaccharide: Maltose
Non
Reducing
End
Reducing
End
Naming Rules: nonreducing residue and configuration
to reducing residue
Glucose (α1 – β2) Fructose
Glucose (α1 – α1) Glucose
Types of Homopolysaccharides
Starch- polysaccharides found in plants
that contains glucose in two forms:
- Amylose (linear α1-4 linked glucose) (10-30%)
- Amylopectin (Linear + branched glucose)
Linear α1-4 linked glucose
Branched α1-6 linked glucose
- Branching occurs every 24-30 residues
Glycogen- polysaccharides found in animals.
Linear α1-4 linked glucose
Branched α1-6 linked glucose
- Branching occurs every 8-12 residues
Structure of Starch:
Amylose & Amylopectin
3-D Structure of Glycogen and Starch
Structure of Cellulose
Cellulose- is found in cell
walls of plants.
- Cellulose uses the
β configuration of glucose
- Mammals lack the enzyme
required to hydrolyze the
β configuration of glucose
Structural Polysaccharides
Composition similar to storage
polysaccharides, but small structural
differences greatly influence properties
• Cellulose is the most abundant natural
polymer on earth
• Cellulose is the principal strength and
support of trees and plants
• Cellulose can also be soft and fuzzy - in
cotton
Amino Acids
By
Doba Jackson, Ph.D.
Outline of Amino Acids, Peptides
& Proteins
• Amino Acid Structure (Chpt 4-text)
• Backbone
• Side Chains
• Acid-Base Properties of A.A’s (Chpt 4-text)
• pKa’s of -COOH, -NH3, side chains
• Levels of Protein Structure (Chpt 7-text; Introduction,
p163-164)
– We will skip Chpt 4- section 2 (Optical Activity) and Chpt 4section 3 (non-standard amino acids)
Amino Acids
Building Blocks of Proteins
Alpha
Hydrogen
Alpha
Carbon
Amino
Group
Side Chain
Chiral
Center
Carboxyl
Group
Classification of Amino Acids
based on the R-group
• Non-polar, Aliphatic (6)
• Non-polar, Aromatic (3)
• Polar, Uncharged (7)
• Polar, Acidic (2)
• Polar, Basic (3)
You should know names, structures, pKa
values, 3-letter and 1-letter codes!!!!!
Non-polar, Aliphatic (R-group) Amino Acids
Non-polar, Aromatic (R-group) Amino Acids
*
Tyrosine can also be considered polar, uncharged
because of its polar hydroxyl group
Polar, Uncharged (R-group) Amino Acids
Polar, Acidic (R-group) Amino Acids
Polar, Basic (R-group) Amino Acids
Histidine could be considered aromatic but its absorption is
very weak compared to other aromatic amino acids, it is also
not aromatic under high pH conditions
At the isoelectric point, the neutral form of the amino acid
is the predominant species.
Acid-Base Properties of Amino Acids
-Main species at
Low pH (<2)
-Main species at
Neutral pH (7.0)
-Both functional
groups contain
the maximum #
of protons
-Amino group has a
proton carboxyl
group loses a proton
-Amino group
loses proton
-Net charge is +1
-Net charge is zero
-Net charge is -1
Main species at
High pH (>12)
What Is the Fundamental Structural Pattern
in Proteins?
“Peptides”
• Short polymers of amino acids
• 2, 3 residues – dipeptide, tripeptide
• 12-20 residues - oligopeptide
What is this peptide sequence?
SGYAL
Levels of Protein Structure
• Primary structure- A description of the covalent bonds
linking amino acids in a peptide chain
• Secondary Structure- An arrangement of amino acids
giving rise to structural patterns
• Tertiary Structure- Describes all aspects of three
dimensional folding of a polypeptide
• Quarternary Structure- The arrangement in space of
polypeptide units
Lipids and Biological
Membranes
Definition of a Lipid
• A lipids are defined as compounds that have low
solubility in water and high solubility in non-polar
solvents.
–Hydrophobic (nonpolar only)
–Amphipathic (both polar and nonpolar groups)
Relevant Biology
• Biological membranes
• Energy storage
• Biological recognition on cell membrane
• Cellular signalling: ie. Steroids
• Free radicle scavengers: Vitamin E
• Insulation
• Many unknown functions
Classes of Lipids
• 1- Fatty acids
• 2- Triacylglycerols
• 3- Glycerophospholipids
• 4- Sphingolipids
• 5- Waxes
• 6- Isoprene-based lipids (including steroids)
Fatty acids
Know the common names and structures for
fatty acids up to 20 carbons long
• Saturated
–
–
–
–
–
Lauric acid (12 C)
Myristic acid (14 C)
Palmitic acid (16 C)
Stearic acid (18 C)
Arachidic acid (20 C)
• Nomenclature: fatty acids are denoted with the chain
length and number of double bonds separated by a
colon.
Note that most natural fatty acids contain an even number of
carbon atoms.
Fatty acids
• Know the common names and structures for
unsaturated fatty acids up to 20 carbons long
• Unsaturated fatty acids
– Palmitoleic acid (16:1 (Δ9))
– Oleic acid (18:1 (Δ9))
– Linoleic acid (18:2 (Δ9,12))
– -Linolenic acid (18:3 (Δ9,12,15))
– Arachidonic acid (20:4 (Δ5,8.11,14))
• Nomenclature: position of double bonds are denoted by the
Δ symbol next to the first carbon of the double bond.
Structure of unsaturated fatty
acids
• Double bonds are never conjugated and
always separated by one methylene
group.
• Double bonds are always cis in naturally
occuring fatty acids.
• Double bonds increase solubility in water
because of the decreased ability to pack
together.
• Double bonds lower the melting point of
the fatty acid.
• The most favorable conformation
of a fatty acid is the fully extended
form.
• There is not rotation allowed
across a double bond.
• Cis double bonds adds a bend to
the fatty acid.
• It takes less energy to disorder
poorly ordered arrays of
unsaturated
fatty acids.
Triacylglycerols
Also called triglycerides
• A major energy source for many organisms
• Why?
– Most reduced form of carbon in nature
– No solvation needed
– Efficient packing
Triacylglycerols
• When glycerol has two
different fatty acids at C1 and
C3 then C2 becomes a chiral
center.
• Simple triacylglycerols with
the same fatty acid are names
tripalmitin, tristearin, etc.
Speciallized cells (adipocytes) store large amounts
of triacylglycerols that nearly fill the cell.
Adipocytes contain lipases, enzymes that cleave the ester
bond and release fatty acids for use as fuel.
Other advantages accrue to users
of triacylglycerols
Insulation
Metabolic water
Structure of Lipids in
membranes
• Membrane lipids are amphipathic molecules
that form bilayers in solution.
– Five types of membrane lipids
•
•
•
•
•
Glycerolphospholipids
Glycolipids: Galactolipids & Sulfolipids
Etherlipids (archeabateria)
Spingolipids
Sterols
Glycerolphospholipids
*Glycerolphospholipids have a
glycerol backbone esterified to
2 fatty acids a phosphate and
a head group.
*
*
*
*
*
*Charges contribute to the surface
charges of the membrane
Sphingolipids are derivatives
of Sphingosine
Features of sphingosines
A hydrocarbon backbone
An amide linkage of the fatty acid
A free alcohol at C3
Sphingolipids
• Sphingomyelins: contain phosphocreatine or phosphocholine.
Resembles phosphatidylcholine. Present in significant quantities in the
myelin sheath that surrounds axons.
• Cerebrosides: have a sugar linked to ceramide. Commonly found in
plasma membranes.
• Globosides: Neutral lipids with a few linear sugars attached.
• Gangliosides: have sugars attached as heads which terminates with
N-acetyl-Neuraminic acid. Commonly found in plasma membranes and
are points of biological recognition.
Some important Gangliosides
Diphytanyl tetraether lipids are found
in archeabacteria under extreme
conditions
Glycerol dialky glycerol tetraethers
Able to withstand low pH, high ionic strengths and
high temperature
Ether Lipids: found in many
tissues (heart) and unicellular organisms
The ether group is resistant to cleavage by most lipases
Phospholipases breakdown lipids
in the lysosome
When one fatty acid has been removed from the lipid, the second fatty
acid is removed by lysophospholipase
Sterols: cholesterols, steroids
The steroid nucleus is a
planar rigid ring with no
C-C bond rotation among
the nucleus.
Analysis of lipids in membranes
Relative proportion of components in plasma membranes differ for
each species and tissue
Typical erythrocyte plasma membrane
Low temperature: Thermal
motion is constrained
Tc= when 50% of each phase is
present in solution
High temperature: Thermal
motion is rapid.
Theory: Cells seek to balance these two phases providing
enough disorder for lateral movement but less freedom for acyl chains