Download LECTERE 11

LECTERE 3
THEME: Carbohydrates: classification, chemical and physical
properties. Structure of monosaccharides, olihosaccharides, homoand heteropolisaccharides.
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.
5. Olygosaccharides. Structure and biological
role.
6. Homopolysaccharides.
Structure
and
biological role.
7. Heteropolysacchrides.
Structure
and
biological role.
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.
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.
The smallest aldose, and the only one whose name does not
end in “ose,” is glyceraldehyde, an aldotriose.
• 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.
Properties of enantiomers
• Enantiomers are said to be optically active
because of the way they interact with planepolarized light. An optically active
compound is а compound that rotates the
plane of polarized light.
• An enantiomer that rotates plane-polarized light to the right
is said to be dextrorotatory (the Latin dexter means "right").
An enantiomer that rotates plane-polarized light to the left is
said to be levorotatory (the Latin laevus means "left").
• А plus or minus sign inside parentheses is used to denote
the direction of rotation of plane-polarized light by а chiral
compound. The notation (+) means rotation to the right
(clockwise), and (-) means rotation to the left
(counterclockwise). Thus the dextrorotstory enantiomer of
glucose is (+)-glucose.
• An equimolar mixture of two enantiomers is called а
racemic mixture, or а racemate. Since а racemic mixture
contains equal numbers of dextrorotating and levorotating
molecules, the net optical rotation is zero. А racemic
mixture is often specified by prefixing the name of the
compound with the symbol ( )
•Stereoisomerism results either from the presence of а
chrial center or from structural rigidity caused by
restricted rotation about chemical bonds.
• Enantiomers:
Stereoisomers
that
are
nonsuperimposable mirror images of each other.
Enantiomers rotate plane-polarized light in different
directions. (+) Enantiomers are dextrorotatory
(clockwise), and (-) enantiomers are levorotatory
(counterclockwise).
•Diastereomers - stereoisomers that are not mirror
images of each other.
•Epimers are called such diastereomers that differ in
configuration at only one asymmetric carbon.
Configurations of Aldoses
Configurations of Ketoses
Biologically important
monosaccharides.
D-Galactose D-Glucose
D-Fructose
D-Ribose 2-Deoxy-D-ribose
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.
• - and -forms of monosaccharides are
readily interconverted when dissolved in
water.
• This spontaneous process, called
mutarotation.
• A slow change in optical rotation to an
equilibrium value is known as
mutarotation.
Oxidation
• Weak oxidizing agents, such as Tollens, Fehling's,
and Benedict's solutions, 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.
• In biological systems enzymes can oxidize the
primary alcohol end of an aldose, to produce а
gylcuronic acid. For glucose, such an oxidation
produces D-glucuronic 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).
Osazone Formation
Glycoside Formation.
Methyl--D-glucoside
Methyl--D-glucoside
Acylation and Alkylation of
Monosaccharides
Phosphate ester formation
-D-Glucose-1-phosphate
-D-Glucose-6-phosphate
Amino Sugar
-D-Glucosamine
-D-Glalactosamine N-acety1-D-glucosanune
Isomerization
Oligosaccharides
• Upon hydrolysis, а disaccharide produces two
monosaccharides,
а
trisaccharide
three
monosaccharides,
а
hexasaccharide
six
monosaccharides, and so on.
• Disaccharides are carbohydrates composed of
two monosaccharide units covalently bonded to
each other.
• The common disaccharides have the general
formula C12H22O11.
• Disaccharides are sweet-testing crystalline,
water-soluble substances, easily hydrolysed by
enzymes and dilute mineral acids.
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 may be of two types, namely
non-reducing and reducing depending on
the fact that С1 of one hexose is linked to
the carbonyl carbon at in or any other
carbon atom of other hexose.
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)
Disaccharide
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 (glucans)
• 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.
• Cellulose. Structurally, cellulose is а linear (unbranched)
D-glucose polymer in which the glucose units are linked by
(1-4) glycosidic bonds.
• Starch is а polysaccharide containing only glucose units.
Two different polyglucose polysaccharides can be
isolated from most starches: amylose (15-20%) and
amylopectin (80-85%)
• 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.
• Amylose
Amylopectin
(1,4)-O--D-Glucopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-D-glucopyranose.
• 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.
• 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.
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
• Dermatan sulfate. Varying amounts of D-glucuronic
acid may be present. Concentration increases during
aging process.
(1,4)-O--L-Idopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy-4O-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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