Download Biological role of carbohydrates

LECTERE 3
Lecturer: Dmukhalska Yevheniya. B.
Plan
1. Carbohydrates.
2. Biological role of carbohydrates in an
organism.
3. Classification of carbohydrates.
4. Structure and stereoisomerism, chemical
properties of monosaccharides.
Carbohydrates are polyhydroxy aldehydes such
as D-glucose, polyhydroxy ketones such as Dfructose, and compounds such as sucrose that can
be hydrolyzed to polyhydroxy aldehydes or
polyhydroxy ketones.
Synthesis of carbohydrates
polymerize
CO2
+
H2O
photosynthesis
glucose
clothing fiber
wood
cellulose
giving structure
to plants
polymerize
chlorophyll
light
starch, plant seeds
eaten by animals
CO2
+
H2O
+
energy
glucose
glycogen (liver)
Biological role of
carbohydrates
1. Carbohydrate oxidation provides energy.
2. Carbohydrate storage, in the form of glycogen,
provides а short- term energy reserve.
3. Carbohydrates supply carbon atoms for the
synthesis of other biochemical substances
(proteins, lipids, and nucleic acids).
4. Carbohydrates form part of the structural
framework of DNA and RNA molecules.
5. Carbohydrate "markers" on cell surfaces play
key roles in cell -cell recognition processes.
Classification
• Monosaccharides are carbohydrates that contain a single
polyhydroxy aldehyde or polyhydroxy ketone unit.
Glucose, fructose.
• Oligosaccharides are carbohydrates that contain from two
to ten monosaccharide units.
Lactose, sucrose.
• Polysaccharides are carbohydrates made up of many
monosaccharide units.
Cellulose, starch.
Classification of monosaccharides.
1. Monosaccharides are classified by the basis of type of
carbonyl group, which are present in molecule:
• Aldoses are monosaccharides that contain an aldehyde
group.
• Ketoses are monosaccharides that contain а ketone
group.
2. Monosaccharides are often classified by number of
carbon atoms.
• А six-carbon monosaccharide is an hexose.
• A five-carbon monosaccharide – pentose.
• A four-carbon monosaccharide – tertrose.
Configurations of Aldoses
Configurations of Ketoses
• Stereoisomers are isomeric molecules that have the
same molecular formula and sequence of bonded
atoms (constitution), but which differ only in the
three-dimensional orientation of their atoms in space.
• Any organic molecule containing а single carbon
atom with four different groups attached to it exhibits
chirality.
• А chiral center is an atom in а molecule that has
four different groups tetrahedrally bonded to it. It is
asymmetric atom.
• Enantiomers are stereoisomers whose
molecules are nonsuperimposable mirror
images of each other. Enantiomers are an
optically active compound.
• The right- and left-handed enantiomers of a
chiral compound rotate plane-polarized light
in opposite ways.
• Monosaccharides are polyhydroxy aldehydes
and polyhydroxy ketones, because Aldoses can
react with themselves forming cyclic
hemiacetals (a) and Ketoses can react with
themselves forming cyclic hemiketals (b)
1
CHO
OH
H
HO
H
H
OH
red raw to sh ow th e -OH
on carbon-5 close to the
aldeh yd e on carbon-1
CH2 OH
OH
H5
O
H
OH H C
1 H
HO
H 5 OH
CH2 OH
D -Glucose
H
anomeric
carbon
CH2 OH
O OH()
H
H
OH H
HO
H
H OH
-D -Glucopyranose
(-D -Glucose)
OH
anomeric
carb on
CH2 OH
O
H
H
H
+
OH H
HO
OH()
H OH
-D -Glucopyranose
( -D -Glucos e )
•Five-membered rings are called furanoses
•six-membered rings are pyranoses
•Cyclic form of fructose is fructofuranose, while glucose
in the pyranose form is glucopyranose
Haworth Projection Formulas
The D or L form of а monosaccharide is determined by the
position of the terminal СН2ОН group on the highest-numbered
ring carbon atom. In the D form, this group is positioned above the
ring. In the L form, which is not usually encountered in biological
systems, the terminal CH2OH group is positioned below the ring.
 or  configuration is determined by the position of the ОН group on carbon-1. In а  configuration OH group is
positioned above the ring; in an  configuration OH group
is positioned below the ring.
-D-Monosaccharide
-D-Monosaccharide
Haworth projection and Fischer
projection
-form
-form
Mutarotation
 The - and -forms of
monosaccharides are
readily interconverted in
aqueous environments
 This spontaneous
process, mutarotation,
produces an equilibrium
mixture of - and -forms in
both furanose and pyranos
ring structure
 Open chain form can
participate in redox reactions
Equilibrium Mixture of D-Glucose
 Isomerization—
monosaccharides
can undergo several
types of
isomerization
 D-glucose incubated
in an alkaline
solution for several
hours produces two
isomers: D-mannose
and D-fructose
 Both involve an
enediol intermediate
Isomerization
Oxidation of Monosaccharides
Oxidation can be done in several ways.
Tollens reagent (Ag+(NH3)2 or Benedict’s solution (Cu2+
tartrate complex). Not synthetically useful due to side
reactions.
Bromine water oxidizes aldoses (not ketoses) to
monocarboxylic acids (Aldonic Acids).
Nitric Acid oxidizes aldoses to dicarboxylic acids (Aldaric
acids).
Enzyme catalyzed oxidation of terminal OH to carboxylic
acid (Uronic Acid)
Oxidation of Monosaccharides
• Monosaccharides are reducing sugars if their
carbonyl groups oxidize to give carboxylic acids.
• In the Benedict’s text, Fehling's text, Tollens’, and
Tromer’s solutions, D-glucose is oxidized to
D-gluconic acid. Glucose is a reducing sugar.
O
O
H
C
C
H C OH
HO C H
H C OH
OH
H C OH
+ Cu2+
HO C H
H C OH
H C OH
H C OH
CH2OH
CH2OH
D-Glucose
+ Cu2O(s)
D-Gluconic acid
37
Oxidation
• Weak oxidizing agents oxidize the carbonyl
(aldehyde) group end of а monosaccharide to give a
glyconic acid. Oxidation of glucose produces
gluconic acid.
• Strong oxidizing agents can oxidize both
ends of а monosaccharide at the same time to
produce а dicarboxylic acid - aldaric acids.
For glucose, such an oxidation produces
glucaric acid.
Reduction.
• Aldoses and ketoses, the product of the reduction is
the corresponding polyhydroxy alcohol (sugar
alcohol). The reduction D-glucose gives D-glucitol
(D-sorbitol).
Reduction of Monosaccharides
• The reduction of
the carbonyl group
produces sugar
alcohols, or
alditols.
• D-Glucose is
reduced to Dglucitol also called
sorbitol.
41
Glycoside Formation.
Methyl--D-glucoside
Methyl--D-glucoside
NH2
N
O
HOCH2
O
H
H
N
a -N -glycosid ic
bond
H
H
HO
OH
anomeric
carbon
Acylation and Alkylation of
Monosaccharides
Phosphate ester formation
-D-Glucose-1-phosphate
-D-Glucose-6-phosphate
Amino Sugar
• Amino groups may be substituted for hydroxyl
group of sugars to give rise to amino sugars.
Generally, the amino group is added to the second
carbon of the hexoses. The most common
aminosugars are Glucosamine and
Galactosamine.
-D-Glucosamine
-D-Glalactosamine N-acety1-D-glucosanune
Oligosaccharides: Biological role
• Within the human body, oligosaccharides are
often found associated with proteins and lipids in
complexes that have both structural and regulatory
functions.
• Free oligosaccharides, other than disaccharides,
are seldom encountered in biological systems.
• Complete hydrolysis of an oligosaccharide
produces monosaccharides.
Disaccharides
Nоn-reducing disaccharides. In these disaccharides the
two hexose units are linked together through their
reducing groups which is С, in aldoses and С, in
ketoses. Important example of non-reducing
disaccharides is sucrose.
Reducing disaccharides. In these disaccharides, one
hexose unit is linked through its reducing carbon to the
non-reducing carbon (C4 or С6). Maltose and lactose –
reducing disaccharides.
Disaccharides formation
Monosaccharide + monosaccharide = disaccharide + Н2O
(Functioning as а
hemiacetal or
а hemiketal)
(Functioning as
an alcohol)
(Glycoside)
Maltose
• Malt sugar, is produced whenever the polysaccharide starch breaks
down, as happens in plants when seeds germinate and in human
beings during starch digestion.
• Structurally, maltose is made up of two D-glucose units, one of which
must be -D-glucose.
• -D-Glucose
-D-Glucose
-(1-4)-linkage
• The glycosidic linkage between the two glucose units is called an (1 - 4)
linkage. Maltose is а reducing sugar.
• Lactose is made up of а -D-galactose unit
and а D-glucose unit joined by -(1 - 4)
glycosidic linkage.
• -D-galactose
-D-Glucose
• Lactose is а reducing sugar
(1 - 4)-linkage
• The two monosaccharide units present in -D-sucrose
molecule are -D-glucose and -D-fructose. It is instead
an ,(1-2) glycosidic linkage. Sucrose is а nonreducing sugar
Sucrose is dextrorotatcry. Sucrose hydrolysis (digestion)
produces an equimolar mixture of glucose and fructose.
Now since fructose is more strongly laevorotatory than the
dextrorotatory property of glucose, the mixture (product)
after hydrolysis will be laevorotatory.
dextrorotatcry
laevorotatory
This reaction is also as inversion of sugar because the
dextrorotatory case sugar is converted into laevorotatory
product due to hydrolysis. The mixture of glucose and
fructose is called invert sugar..
• А polysaccharide contains many monosaccharide units
bonded to each other by glycosidic linkages.
• Polysaccharides may be divided into two classes:
homopolysaccharides, which are composed of one type
of monosaccharide units, and heteropolysaccharides,
which contain two or more different types of
monosaccharide units.
• Homoglycans (glucans or glucosans): starch, glycogen
and cellulose. They are made of only glucose.
• Heteropolysaccharides
(Mucopolysaccharides):
hyaluronic acid and chondroitin sulphates. They are
made up of different monosaccharide units.
А polysaccharide (glucans)
• Cellulose. Structurally, cellulose is а linear (unbranched) Dglucose polymer in which the glucose units are linked by (1-4)
glycosidic bonds.
Starch is used for energy storage in plants
Starch is а polysaccharide containing only glucose units.
Two different polyglucose polysaccharides can be isolated
from most starches: amylose (15-20%) and amylopectin (8085%)
• Amylose: Up to 1000 glucose units; no branching;
molecular mass is 50,000 amu or more
• Amylopectin: Up to 100,000 glucose units; branch
points every 24-30 glucose units; molecular mass is
300,000 or more for
• Glycogen: Glycogen is about three times more
highly branched than amylopectin, and it is
much larger, with а molar mass of up to
3,000,000 amu. Up to 1,000,000 glucose units;
branch points every 8-12 glucose units.
• Liver cells and muscle cells are the storage sites
for glycogen in humans.
Chitin, a polymer of N-acetylglucoasamine
• Hyaluronic acid is а principal component of the ground
substance of connective tissue. Among other places it is
found in skin, synovial fluid, vitreous hemour of the eye,
and umbilical cord. Synovial fluid which contains about
0.02 – 0.05% of hyaluronate.
• Hyaluronic acid: It consists N- acetylglucosamine (NAG)
and glucuronic acid linked according to the principle
discussed above. Note also, in this structure, the alternating
pattern of glycosidic bond types, (1-3) and (1-4). It is а
highly viscous substance and has а molecular weight in
several 100 millions.
• (1,4)-O--D-Glucopyranosyluronic acid-(1,3)-2acetamindo-2-deoxy--D-glucopyranose.
Chondroitin sulphate. It has similar structure as hyaluronic
acid with the difference that the N-acetyl glucosamine unit of
the latter is replaced by N-acetyl galactosamine 6 sulphate
unit. The two other chondriotin sulphates are А and В; the
type А nas sulphate group in position 4 while the type В has
L-iduronate (а stereoisomer оf D-glucuronic acid) in place of
D-glucuronic acid. Chondroitin sulphates are found in
cartilage, bone, heart valves, tendons and cornea.
(1,4)-O--D-Glucopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-6O-sulfo--D-galactopyranose
Heparin. It is naturally occurring anticoagulant found
mainly in the liver, and also in lung, spleen, kidney and
iatestinal mucosa. It prevents blood clotting by inhibiting the
prothrombin-thrombin conversion and thus eliminating the
thrombin effect on fibrinogen. This polysaccharide is
composed of glucosamiae-N-sulphate aad sulphate ester of
glucuronic acid linked via 1  4 - 1 4 linkages
(difference from hyaluronic acid and chondroitin sulphates).
(1,4)-O--D-Glucopyranosyluronic acid-2-sulfo-(1,4)-2-sulfamindo2-deoxy-6-O-sulfo--D-glucopyranose
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