Transcript week 4_carbohydrates

Structure and properties of
Carbohydrates
PTT 103 (Biochemistry)
Department of Chemical Engineering Technology,
UniMAP
[email protected]
LECTURE 3 –STRUCTURE AND
PROPERTIES OF CARBOHYDRATES
Structure, properties and function of carbohydrates and
their derivatives
• Classification of carbohydrates
Course outcome:
• Ability to differentiate basic structure, properties,
functions and classification of important biomolecules.
•
LECTURE 3 -CARBOHYDRATES
Section
Section
Section
Section
TOPICS:
1. Role & Significance of
Carbohydrates
2. Monosaccharide
3. Oligosaccharides
4. Polysaccharides
SIGNIFICANCE OF CARBOHYDRATES
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Carbohydrates are the most abundant biomolecules
in nature, having a direct link between solar energy
and the chemical bond energy in living organisms.
Source of rapid energy production
Structural building blocks of cells
Components of several metabolic pathways
Recognition of cellular phenomena, such as cell
recognition and binding (e.g., by other cells,
hormones, and viruses)
CARBOHYDRATES
Carbohydrate : compounds contains H, C & O with the comp :
(CH2O)n (Hydrate of carbon)
Carbohydrates : Consist of sugar (saccharum)
Sugars : compound that contains alcohol & carbonyl
functional group
Carbonyl func. group : >C=O
Adehyde  aldose
Ketone  ketose
Examples:
Classification
Carbohydrate
Mono
saccharide
Glucose, fructose
Ribose (aldopentose)
Deoxy ribose
Oligo
saccharide
disaccharides
Glycoproteins
(bacterial cell
walls
Poly
saccharide
cellulose, chitin,
starch, glycogen,
glucoaminoglycans
Glycoconjugates
glycoproteins
and
proteoglycans
Monosaccharides
Simple sugars, polyhydroxy aldehydes or
ketones
 Aldehyde functional group: aldose
 Ketone functional group: ketose
 Simplest aldose: glyceraldehyde
 Simplest ketose: dihydroxyacetone
 Classified according to no. of carbon.
 3 carbon (trioses), 4 carbon (tetroses, 5
carbon (pentose), 6 carbon (hexoses)
 Glucose: six carbon aldose: aldohexose
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Monosacharides : Exp. aldoses & ketoses
Aldotriose
Ketotriose
Aldotetrose
Ketotetrose
Aldopentoses
Ketopentose
Aldohexose
Ketohexose
2.2. MONOSACCHARIDES
STEREOISOMERS
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Isomers: same chemical formulas, different structures
Total no of possible isomers can be determined by Van Hoff’s rule:
compound with n chiral C atoms has a max of 2n possible
stereoisomers.
Chiral: asymmetric carbons, i.e carbon atom with four different
substituents
Eg: n = 4, there are 16 stereoisomers (8-L stereoisomers, 8-D
stereoisomers).
In optical isomers- the ref C is the asymmetric C that is most
remote from the C=O carbon.
In D-aldose family sugars, the OH group is to the right on the chiral
C atom farthest from the most oxidized C (aldehyde group) in the
molecule.
D- and L- enantiomers
Stereoisomers that are not enantiomers
(mirror-image) are called diastereoisomers.
 Eg: aldopentoses, D-ribose and L-ribose are
enantiomers.
 The D-ribose and D-arabinose are
diastereomers because they are isomers but
not mirror image.
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Diastereomers that differ in the
configuration at a single asymmetric C
atom are called epimers.
 Eg: D-glucose and D-galactose are
epimers because they differ only in the
configuration of the OH group at C-4.
 D-mannose and D-galactose are not
epimers- differ more than 1 C.
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MONOSACCHARIDES STEREOISOMERS
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The simplest aldose, glyceraldehyde, contains one chiral center (the middle
carbon atom) and has two different optical isomers, or enantiomers
the projection in which the carbohydrate backbone is
drawn vertically with the carbonyl shown on the top.
Cyclic structure of monosacharides
• in aqueous solution, monosaccharides with five or more carbon atoms in the
backbone occur predominantly as cyclic (ring) structures in which the carbonyl group
has formed a covalent bond with the oxygen of a hydroxyl group along the chain.
• Sir Norman Haworth showed that the linear form of glucose (and other aldohexose)
could undergo intramolecular reaction to form a cyclic hemiacetal.
• the analogous intramolecular reaction of ketose sugar yields a cyclic hemiketal.
The new chiral center in cyclic (c1) is called anomeric carbon
In aldose sugar…
α-sugar
β-sugar
• The OH of newly formed
hemiacetal occurs on C1 is at
‘down’ position
• In Fischer projection, α-anomeric
OH occurs on the right
• The OH of newly formed
hemiacetal occurs on C1 is at
‘up’ position
• In Fischer projection, β-anomeric
OH occurs on the left
Pyranoses& Furanoses
Pyranoses: six-membered ring compounds ( resemble pyran )
Furanoses : fivemembered rings, (resemble furan)
The structure systematic names glucose & fructose become
HAWORTH STRUCTURES
An English chemist W.N.
Haworth gave a more accurate
picture of carbohydrate
structure.
Haworth Structures
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To convert from traditional Fischer formula of a D-pentose or
D-hexose to a Haworth formula, the following steps should
be followed:
Draw a 5 or 6-membered ring with the O placed as shown
below:
Starting with anomeric carbon to the right of the ring O, place
OH group either above or below the plane of the ring. Group
that pointing to the left in Fischer projection should go above
(β-) the plane of the ring, and those pointing right should go
below the ring (α-)
 In D-sugars, the last C position (eg: C-6 glucose) is always up
FISHER AND HAWORTH FORMS OF SUGAR
SUMMARY OF SUGAR STRUCTURES
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ISOMERS- compounds that have the same chemical formula e.g. fructose,
glucose, mannose, and galactose are isomers of each other having formula
C6H12O6.
EPIMERS- refer to sugars whose configuration differ around one specific
carbon atom e.g. glucose and galactose are C-4 epimers and glucose and
mannose are C-2 epimers.
ENANTIOMERS- a special type of isomerism found in pairs of structures
that are mirror images of each other. The mirror images are termed as
enantiomers and the two members are designated as D- and L- sugar. The
vast majority of sugars in humans are D-sugars.
CYCLIZATION OF SUGARS- most monosaccharides with 5 or more
carbon atoms are predominately found in a ring form, where the aldehyde
or ketone group has reacted with an alcoholic group on the same sugar
group to form a hemiacetal or hemiketal ring.
Pyranose ring- if the ring has 5 carbons and 1 oxygen.
Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen
Exercise 1
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Draw cyclic Haworth structure for:
D-fructose
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Name the anomers that result from the
cyclization of these molecules.
IMPORTANT REACTIONS IN MONOSACCHARIDES
Monosaccharides undergo the following reactions :
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6.
Mutarotation
Oxidation
Reduction
Isomerization
Esterification
Glycoside formation
IMPORTANT REACTIONS IN MONOSACCHARIDES
Details
1.
Mutarotation –
alfa and beta forms of sugars are readily interconverted when dissolved in
water. Mutarotation produces an equilibrium mixture of α and β- forms in
both furanose and pyranose ring structures.
2
Oxidation and reduction
in presence of oxidising agents, metal ions (Cu2+) and enzymes,
monosacchs undergo several oxidation reactions e.g. Oxidation of
aldehyde group (R-CHO) yields aldonic acid; of terminal CH2OH
(alcohol) yields uronic acid; and of both the aldehyde and CH2OH gives
aldaric acid. The carbonyl groups in both aldonic and uronic can react
with an OH group in the same molecule to form a cyclic ester known as
a lactone.
sugars that can be oxidized by weak oxidizing agent ie. Benedict’s reagent,
called reducing sugars. Because the reaction occurs only with sugars that
can revert to open chain form, all monosaccharides are reducing sugars.
REDUCING SUGARS
All monosacchs are reducing sugars.
 They can be oxidised by weak oxidising
agent such as Benedict’s reagent
 Benedict's reagent is a solution of copper
sulfate, sodium hydroxide, and tartaric
acid.
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Aqueous glucose is mixed with Benedict's reagent and heated.
The reaction reduces the blue copper (II) ion to form a brick
red precipitate of copper (I) oxide. Because of this, glucose is
classified as a reducing sugar.
3. REDUCTION
reduction of the aldehyde and ketone
groups of monosacchs yield sugar
alcohols (alditols) Sugar alcohols
e.g.sorbitol, are used commercially in
processing foods and pharmaceuticals.
sorbitol- improves the shelf-life of candyit helps prevent moisture loss.
IMPORTANT REACTIONS (Cont)
4.
ISOMERIZATION
Monosaccharides undergo several types of isomerization e.g. D-glucose in alkaline
solution for several hours contain D-mannose and D-fructose. Both isomerization
involves an intramolecular shift of a H atom and a relocation of double bond. The
conversion of glucose to mannose is termed s epimerization.
Important reactions… (cont)
5
ESTERIFICATION
Free OH groups of carbohydrates react with acids to
form esters. This reaction an change the physical and
chemical propteries of sugar.
6.
GLYCOSIDE FORMATIONHemiacetals and hemiketals reaction with alcohols to
form the corressponding aceta or ketal. On the
contrary when a cyclic hemiacetal or hemiketal form of
monosaccharide reacts with alcohol, the new linkage is
called glycosidic linkage and the compound glycoside.
IMPORTANT MONOSACCHARIDES
GLUCOSE
 FRUCTOSE
 GALACTOSE
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D-Glucose:
D-glucose (dextrose) is the primary fuel in living cells
especially in brain cells that have few or no mitochondria.
Cells such as eyeballs have limited oxygen supply and use
large amount of glucose to generate energy
Dietary sources include plant starch, and the disaccharides
lactose, maltose, and sucrose
Important monosaccharides. Cont
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FRUCTOSE
◦ D-fructose (levulose) is often referred as fruit sugar
and is found in some vegetables and honey
◦ This molecule is an important member of ketose
member of sugars
◦ It is twice as sweet as sucrose (per gram basis) and is
used as sweeting agent in processed food products
◦ It is present in large amounts in male reproductive
tract and is synthesised in the seminal vesicles.
Important monosaccharides. Cont....
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GALACTOSE
◦ is necessary to synthesize a variety of biomolecules
(lactose-in mammalary glands, glycolipids, certain
phospholipids, proteoglycans, and glycoproteins)
◦ Galactose and glucose are epimers at carbon 4 and
interconversion is catalysed by enzyme epimerase.
◦ Medical problems – galactosemia (genetic disorder) where
enzyme to metabolize galactose is missing; accumulation of
galactose in the body can cause liver damage, cataracts, and
severe mental retardation
MONOSACCHARIDE DERVATIVES
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URONIC ACIDS – formed when terminal
CH2OH group of a mono sugar is oxidised
◦ Important acids in animals – D-glucuronic acid and
its epimer L-iduronic acid
◦ In liver cells glucuronic acid combines with
steroids, certain drugs, and bilirubin to improve
water solubility therby helping the removal of
waste products from the body
◦ These acids are abundant in the connective tissue
carbohydrate components.
Mono sugar derivatives
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AMINO SUGARS –
◦ Sugars in which a hydroxyl group (common on
carbon 2) is replaced by an amino group e.g. Dglucosamine and D-galactosamine
◦ common constituents of complex carbohydrate
molecule found attached to cellular proteins and
lipids
◦ Amino acids are often acetylated e.g. N-acetylglucosamine.
Mono sugar derivatives
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DEOXYSUGARS
◦ monosaccharides in which an - H has replaced an – OH
group
◦ Important sugars: L-fucose (formed from D-mannose by
reduction reactions) and 2-deoxy-D-ribose
◦ L-fucose – found among carbohydrate components of
glycoproteins, such as those of the ABO blood group
determinates on the surface of red blood cells
◦ 2-deoxyribose is the pentose sugar component of DNA.
GLYCOSIDIC BONDS
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Monosaccharides can be linked by glycosidic bonds (joining
of 2 hydroxyl groups of sugars by splitting out water
molecule) to create larger structures.
Disaccharides contain 2 monosaccharides e.g. lactose
(galactose+glucose); maltose (glucose+glucose);
sucrose (glucose+fructose)
Oligosaccharides – 3 to 12 monosaccharides units e.g.
glycoproteins
Polysaccharides – more than 12 monosaccharides units e.g.
glycogen (homopolysaccharide) having hundreds of sugar units;
glycosaminoglycans (heteropolysaccharides) containing a
number of different monosaccharides species.
Disaccharides and oligosaccharides
Monosaccharides are linked together by
glycosidic bonds to form a variety of
molecules that perform diverse biological
functions.
 Disaccharides are glycosides composed of
2 monosaccharides.
 While oligosaccharide is often used for
relatively small sugar polymers that
consist of 2-10 or more monosaccharide
units.
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If one monosaccharide molecule is linked
thru its anomeric carbon to the oh group
on carbon 4 of another monosaccharide,
the glycosidic linkage is designated as 1,4.
 Anomeric OH group may potentially be in
either α (down-position) or βconfiguration (up position).
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Important sugars of Disaccharides
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LACTOSE
(milk sugar) disaccharide found in milk; composed of one molecule of
galactose (OH group in C-1) and glucose (OH group at C-4) linked
through beta(1,4) glycosidic linkage (anomeric C of galactose is in βconfigurations); because of the hemiacetal group of the glucose
component, lactose is a reducing sugar
Lactose intolerance
Digestion of disaccharides and other carbohydrates is
mediated by enzymes synthesized by cells lining the
small intestines.
 Deficiency in any of these enzymes can cause an
unpleasant symptoms.
 Carbohydrates are absorbed as monosaccharides- any
undigested disaccharide molecules would pass into large
intestine, where osmotic pressure draws water from
surrounding tissue causing diarrhea.
 Bacteria in the colon digest disaccharides thru
fermentation, thus producing gas- cramp and bloating.
 Most common- lactose intolerance- deficiency in
enzyme lactase.
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• MALTOSE ( malt sugar)
An intermediate product of starch hydrolysis; it is a
disaccharide with an alfa(1,4) glycosidic linkage between
two D-glucose molecules; in solution the free anomeric
carbon undergoes mutarotation resulting in an equilibrium
mixture of alfa and beta – maltoses; it does not occur freely
in nature
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SUCROSE
common table sugar: cane sugar or beet sugar produced in
the leaves and stems of plants; it is a disaccharide containing
both alfa-glucose and beta-fructose residues linked by
alfa,beta(1,2)glycosidic bond.
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CELLOBIOSE
degradation product of cellulose
containing two molecules of glucose
linked by a beta (1,4) glycosidic bond; it
does not occur freely in nature
OLIGOSACCHARIDE SUGARS
Oligosaccharides are small polymers often
found attached to polypeptides in
glycoproteins and some glycolipids.
 They are attached to membrane and
secretory proteins found in endoplasmic
reticulum and Golgi complex of various cells
 Two classes: N-linked and O-linked
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