Lecture 5
Carbohydrates and Glycobiology
Life Is Sweet
Outline
•Definitions
•Classification
•Structure & Function of
Carbohydrates
1. Introduction
• Carbohydrate, as the name implies,
consist of carbon, hydrogen, and
oxygen.
– Hydrate=(water) hydrogen and oxygen.
• The basic formula for carbohydrates
is CH2O, meaning that there is one
carbon atom, two hydrogen atoms,
and one oxygen atom as the ratio in
the structure of carbohydrates
– What would be the formula for a
carbohydrate that has 3 carbons?
(1) Where Do Carbohydrates Come
From?
• Plants take in
Carbon dioxide
(CO2), water (H2O)
and energy from
the sun and make
glucose–
photosynthesis.
6CO2+6H2O+energy (from sunlight)
C6H12O6+6O2
(2) Functions
• Provide Energy
• structural components
– cellulose in plants and chitin in arthropods.
• Transfer to other life molecules such as
lipids and protein
• Informational molecules for recognition
(3) Nomenclature
• Carbohydrates are
polyhydroxy
aldehydes or ketones,
or their derivatives.
(4) Classifying Carbohydrates
• Scientist use the word saccharides to
describe sugars.
• Monosaccharides (simple sugers):
– only one sugar molecule, cannot be hydrolyzed to
simpler sugars,
– eg：glucose, fructose and galactose
• Oligosaccharides:
– 'a few' ( from two to ten monosaccharide
units )covalently linked monosaccharides
– eg：sucrose, maltose, lactose
• polysaccharides:
– 'many' covalently linked monosaccharides
– eg：Starch, Glycogen, Cellulose (a form of fiber)
Glycoconjugates
•Also called complex saccharide
•Polysaccharide linked to proteins or
lipids including
•Proteoglycans
•Glycoproteins
•Glycolipids
2. Monosaccharide
(1) Monosacchrides can be divided into two
families: aldoses and ketoses.
H
H
C
aldehyde
group
H
O
C
OH
C
O
HO
C
H
H
C
OH
HO
C
H
H
C
OH
H
C
OH
H
C
OH
H
C
OH
CH 2OH
CH 2OH
D-Glucose,
an aldohexose
D-Fructose,
a ketohexose
keto
group
• Monosaccharides can be classified
according to the number of
carbon atoms they contain
–3
–4
–5
–6
–7
carbons
carbons
carbons
carbons
carbons
:
:
:
:
:
triose.
tetrose.
pentose.
hexose.
heptose.
3 carbons : Triose
• The simplest aldose is glyceraldehyde
• The simplest ketose is dihydroxyacetone
5 carbons : Pentose
6 carbons : Hexose
•The most abundant monosaccharide in nature
is the six-carbon sugar D-glucose.
(2) Configuration of
monosaccharide
•Monosaccharides
have
asymmetric centers: chiral carbon atom
except
dihydroxyacetone
•Opticity: D(Dextrorotatory) and L(Levorotatory)
•Glyceraldehyde is
conventionally used as
the standard for
defining D and L
configurations
enantiomers
H
1
CHO
2
C OH
CHO
HO
C
H
H
C
OH
H
3
C
4
C
OH
HO
C
H
H
5
C
OH
HO
C
H
HO
1
CH2OH
2
C O
H
CH2OH
6CH2OH
D-Glucose
L-Glucose
mirror plane
(an aldose)
D- and L-glucose
CH2OH
C
O
H
C
OH
H
3
C
4
C
OH
HO
C
H
H
5
C
OH
HO
C
H
HO
H
CH2OH
6CH2OH
D-Fructose
L-Fructose
mirror plane
(a ketose)
D- and
L-fructose
Enantiomers: mirror images of each other, D- and L-sugars.
• D-Sugars predominate in nature (in living
organisms) (e.g., D-ribose, D-glucose, Dgalactose, D-mannose, D-fructose)
• Each stereoisomer has a different
conventional name, ending with “-ose” suffix
• Ketoses are often named by inserting an “ul”
into the name of the corresponding aldoses
(e.g., aldopentose is named as ribose, the
ketopentose is named as ribulose
Stereoisomers of
D-family of aldose
Epimer: Two sugars
that differ only by
the configuration
around one carbon.
Stereoisomers of
D-family of ketose
epimer
(3) Cyclization of monosaccharide
• Less than 1% of
each of the
monosaccharides
exists in open-chain
(acyclic) form
(Fischer Projection).
• Rather, they are
predominantly found
in a ring form
(Haworth Projection).
• The aldehyde or
ketone group can
react with a
hydroxyl group to
form a covalent
bond.
• An aldehyde
reacts with a
hydroxyl group
creates a
hemiacetal.
• A ketone reacts
with a hydroxyl
group to form a
hemiketal.
Conversion of a linear form to a Haworth projection
anomeric carbon
anomer
• -OH group of anomeric carbon (C1) and –CH2OH group (C6) :
•On the contrary-α
•Same side - β
Pyranose form
(six-membered ring)
D-Glucose
Furanose form
(five-membered ring)
•D-Glucose can cyclize in two ways, forming either
furanose or pyranose structures.
•In general, the
pyranose form is
favored over the
furanose ring for
aldohexose
sugars.
Glucose: linear form
Magic words: right, left, right, right; (top down)
Glucose: Ring form
Magic words: down, up, down (for carbons 2, 3, and 4)
Fructose
furanose ring
•Fructose also forms
pyranose rings.
•The pyranose form
predominates in fructose
free in solution, whereas
the furanose form is the
major one in most of its
derivatives.
Notice
• Terminology describing sugar structure
– Enantiomers
• D- and L-
– Epimers
• differ only by the configuration around one
carbon
– Anomers
• α and β configuration
(4) Reducing sugar
• Sugars that can reduce Fe3+ or Cu2+ ion are
called reducing sugars. The carbonyl group
is oxidized to carboxyl group.
• Monosaccharides are reducing sugars.
• In disaccharides or polysaccharides, the end
of a chain with a free anomeric carbon is
called the reducing end.
brick-red
precipitates
Fructose is a ketose that changes to
aldose in a basic solution.
H
C
C
R
ketose
OH
H
O
H
H
OH
C
C
R
enediol
O
C
OH
HC
R
aldose
• a ketone cannot be oxidized directly, a keto
sugar can be converted to an aldehyde to
migrate the carbonyl to the end of the chain.
OH
• Benedict's reagent and Fehling's reagent are used
to test for the presence of a reducing sugar.
• Fehling reagents: copper(II) sulfate (CuSO4)、sodium
hydroxide(NaOH)、potassium sodium tartrate
• Benedict reagents: CuSO4、sodium carbonate
(Na2CO3)、sodium citrate (Na3C6H5O7∙2H2O).
•The red precipitate
is copper(I) oxide,
Cu2O.
•These reagents can be used to measure the presence
of reducing sugars qualitatively and so can monitor the
concentration of blood glucose for diabetes.
(5) Glycosides
• Glycosides are molecules in which a sugar is
bound to a non-carbohydrate moiety,
usually a small organic molecule.
glycosidic bond
Glucose
glycone
aglycone
•Glycosides can be linked by an O-, N-, S- or Cglycosidic bond.
(6) The three important dietary
monosaccharides
– Glucose, which is produced in plants during
photosynthesis.
– Fructose, which is also produced in plants
during photosynthesis and found in fruit
juices and honey.
– Galactose, which is found in milk.
3.
Oligosaccharides
• Definition: 'a few' ( from two to
ten monosaccharide units )covalently
linked monosaccharides
• 3 disaccharides
– Sucrose = glucose-fructose
– Maltose = glucose-glucose
– Lactose = glucose-galactose
1). Sucrose
•Sucrose is known as
common table sugar.
•Composed of D-glucoses and D-fructose
linked by α-1-β-2 glycosidic bond
sugarcane
Glc(α1
β2)Fru
• The hydrolysis of sucrose, will
yield both glucose and fructose.
• This chemical reaction is
achieved by honeybees which
use invertase enzymes.
• honey - a
mixture of
glucose and
fructose
Sucrose is nonreducing sugar
H
The anomeric carbon
atom for glucose
is carbon 1
H
H
O
H
H
H
1
2
3
4
5
6
C
O
C
O
C
H
C
O
H
C
O
H
C
O
H
C
O
H
C
O
C
H
C
O
H
C
O
H
C
O
H
H
H
The anomeric carbon
atom for fructose
is carbon 2
H
H
1
2
• Since the anomeric carbon is
involved in a glycosidic bond,
sucrose is classified as a
nonreducing sugar.
H
O
H
H
H
3
4
5
6
H
2). Maltose
Maltose syrup
•Composed of two D-glucoses linked
by α-1, 4 glycosidic bond
• reducing sugar
Glc(α1
4)Glc
3).
Lactose
lactose also referred
to as milk sugar.
• Composed of D-galactose and Dglucose linked by β-1,4 glycosidic bond.
• reducing sugar
Digestion of lactose
•The intestinal villi
secrete an enzyme called
lactase (β-Dgalactosidase) to digest
lactose, and produce
glucose and galactose,
which can be absorbed.
• Lactose intolerance
– More than half of the world’s adults are lactose
intolerance.
– Lactose intolerance is the inability to metabolize
lactose, because the lactase is absent in the
intestinal system or its availability is lowered.
Lactose intolerance
• In the absence of lactase, lactose
remains uncleaved and passes intact into
the colon.
• The operons of enteric bacteria quickly
switch over to lactose metabolism, and
produces copious amounts of gas (a
mixture of hydrogen, carbon dioxide, and
methane).
• This, in turn, may cause a range of
abdominal symptoms, including stomach
cramps, bloating, and flatulence.
• Treatment for this disorder is simple to
remove lactose from diet.
Two kind disaccharides
(1)reducing disaccharide
(2) nonreducing disaccharide
• Key factor: free aldehyde or
ketone group ( a free reducing
group)
REDUCING SUGARS
•When Benedicts test is performed with the
disaccharides maltose and sucrose, the following result is
obtained:
Sucrose is a
non-reducing sugar
SUCROSE
RESULT
Maltose is a
reducing sugar
MALTOSE
RESULT
4.
Polysaccharides
• Homoglycans- homopolysaccharides
containing only one type of monosaccharide
• Heteroglycans - heteropolysaccharides
containing residues of more than one type
of monosaccharide
• Lengths and compositions of a
polysaccharide may vary within a population
of these molecules
•Polysaccharides may be composed of one,
two, or several different monosaccharides, in
straight or branched chains of varying length
(1). Starch
•Starch is a polysaccharide carbohydrate consisting of
a large number of glucose units joined together by
glycosidic bonds.
•Function: starch is produced by all green plants as an
energy store and is a major food source for humans
Amylose
Amylopectin
•Starch is a
mixture of
unbranched
amylose (α1-4
bonds) and
branched
amylopectin (α
1-4 and α1-6
branchpoints).
Starch structure
(2)
Glycogen
•The glucose storage device in animals
•D-Glucoses linked byα-1,4 and α-1,6 glycosidic
bonds
•Glycogen has the same
overall structure as
amylopectin but there is
significantly more branching
in this molecule
Glycogen
• Glycogen is
synthesized and
stored mainly in the
liver and the muscles.
(3).
Cellulose
•Cellulose is the structural component of the
primary cell wall of green plants.
•Cellulose is the most common organic compound
on Earth. About 33% of all plant matter is
cellulose.
•Cellulose is a polymer of β-glucose units.
Cellulose structure
• Cellulose is a straight
chain polymer.
• Intra- and interchain
H-bonding gives
strength.
hydrogen bonds between parallel chains of beta glucose
•
•
In cellulose, sugar
units are joined by
beta linkages.
The straight chains
formed by beta
linkages is optimal for
structural function.
•The multiple hydroxyl groups on the glucose residues
form hydrogen bonds, holding the chains firmly
together side-by-side and forming microfibrils.
Cellulose
• Cellulosa can not be digested by
mammals due to lack of the enzyme that
cleaves β-glycosidic bond.
• The functions of dietary fiber
– Decrease the absorption of glucose and
cholesterol from the intestine, increase the
bulk of feces, prevent constipation.
(4). Chitin
•Chitin is a polysaccharide forming the outer skeleton of
arthropods (such as insects, crabs, shrimps, and lobsters).
•It is a polymer of N-acetylglucosamine(NAG) in β-1 to 4
glycosidic linkage.
• Chitin is the main source of production of chitosan,
which is used in a number of applications, such as a
wound healing agent, surgical thread, flocculating agent
and a delivery vehicle for pharmaceuticals and genes.
(5). Testing for polysaccharides:
the Iodine Test
•When iodine solution is added
to a suspension of starch, the
iodine molecules pack inside
the amylose helix to give a
blue-black colour.
• All monosaccharides and all
disaccharides give negative
Iodine Tests.
5. Glycoconjugate (complex saccharide)
• Glycoconjugates: carbohydrates covalently linked
with other chemical species.
Glycoprotein
Sugar + protein
Proteoglycan
Glycoconjugate
Glycolipid
Sugar + lipid
Lipopolysaccharide (LPS)
•Glycoconjugates are very important compounds.
They are involved in cell-cell interactions, including
cell-cell recognition, and cell-matrix interactions.
(1). Glycoprotein
• Glycoproteins: proteins that contain
oligosaccharide chains (glycans)
covalently attached to their polypeptide
side-chains.
• O-Glycosidic and N-glycosidic linkages
• Oligosaccharide chains exhibit great
variability in sugar sequence and
composition.
O-Glycosidic and N-glycosidic linkages
N-acetylgalactosamine
(GalNAc)
N-acetylglucosamine
(GlcNAc)
• Glycoproteins play essential roles in the
body. For instance, in the immune system
almost all of the key molecules involved in
the immune response are glycoproteins.
•Glycoprotein in
cell membrane
(2). Proteoglycan
• A special type of
glycoprotein with
sugar weighing about
95%
•On cell surface or
Extracellular matrix
•Essential components
of tissue (particularly
connective tissue)
structure.
(3) Glycolipids
• Glycolipids are
carbohydrateattached lipids.
• Their role is to
provide energy
and also serve
as markers for
cellular
recognition.
Points
• Definition, function and classification of
carbohydrates
• Monosaccharides
–
–
–
–
Aldoses and ketoses
Fischer projections and Haworth structures
Reducing sugar
Terminology describing sugar structure
• Enantiomers,Epimers,Anomers
• Oligosaccharides
– Sucrose, maltose, lactose
• Polysaccharides
– Starch, Glycogen, Cellulose
– Iodine Test
• Glycoconjugate (complex saccharide)