carbohydrates 2/9/2016dr seemal jelani introduction to biochemistry1
DESCRIPTION
They are produced by photosynthesis in plants. such as glucose are synthesized in plants from CO 2 from the air, H 2 O from the soil, and energy from the sun absorbed in chlorophyll to form carbohydrates and O2 Oxidized in living cells to produce CO 2, H 2 O, and energy. 2/9/2016 Dr Seemal Jelani Introduction to Biochemistry 3TRANSCRIPT
Carbohydrates
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Most abundant Their abundance in human
body is low but constitute about 75% by mass of dry plant materials
Green (chlorophyll-containing) plants produce carbohydrates via photosynthesis
A major source of energy from our diet composed of the elements C, H and O.
They also called saccharides, which means “sugars.”
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Carbohydrates
Carbohydrates They are produced by
photosynthesis in plants.
such as glucose are synthesized in plants from CO2 from the air, H2O from the soil, and energy from the sun absorbed in chlorophyll to form carbohydrates and O2
Oxidized in living cells to produce CO2, H2O, and energy.
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Cellulose (carbohydrates) serve as structural elements
Starch (carbohydrates) provide energy reserves for the plants
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Uses of carbohydrates in plants
Dietary intake of plant materials is the major carbohydrate source for humans and animals
Functions: Carbohydrate oxidation provides energy Carbohydrate storage in the form of
glycogen provides a short-term energy reserve
Carbohydrates supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, nucleic acids)
Carbohydrate form part of the structural framework of DNA and RNA
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Classification of Carbohydrates
The types of carbohydrates are • Monosaccharides, the simplest carbohydrates. • Disaccharides, which consist of two
monosaccharides.• Polysaccharides, which contain many
monosaccharides.
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Cn H2n On Carbohydrates are Polyhydroxy aldehydes,
Polyhydroxy ketones The carbohydrate glucose is Polyhydroxy
aldehyde and the carbohydrate fructose is Polyhydroxy ketone
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General Formula
CHIRALITY: Handedness in molecules
A chiral object is not superimposable on its mirror image they do not possess a plane of symmetry
Two forms of a chiral object are known as enantiomers
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Mirror image
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Mirror image
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Non superposable
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Chiral HANDS
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Chiral Shells
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The plane has the same thing on both sides for the flaskThere is no mirror plane for a hand
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If an object has a plane of symmetry it is necessarily the same as its mirror image
The lack of a plane of symmetry is called “handedness”, Chirality
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Hands, gloves are prime examples of chiral object
They have a “left” and a “right” version
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Why this chapter?
Handedness is important in organic and biochemistry
Molecular handedness makes possible specific interactions between enzymes and substrates
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Chirality in daily life
Most Biomolecules in nature are chiral (sugars, DNA, proteins, amino acids, steroids)
Human proteins are exclusively built from L-amino acids; this involves receptors which are chiral
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Aldehydes CHO AldoseKetone C=O Ketose
Functional groups in carbohydrates
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Carbohydrates may contain more than one chiral center
FP represents a method for giving molecularity specifications in two dimensions
FP is a two dimensional notation for showing the spatial arrangement of groups about chiral centers in molecule
Chirality in carbohydratesFischer projections
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Stereoisomers Enantiomers Diastereomers Handedness (Right and left) D and L Fischer Projection for 2,3,4-trihydroxybutanal Epimers
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Structural isomers have different properties Diastereoisomers have different properties Enantiomers have same properties expect
two: Their interaction with plane polarized light Their interaction with other chiral substances
Properties of Enantiomers
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Isomers whose atoms are connected in the same way but differ in their arrangement in space
The two non superimposable mirror-image forms of a chiral molecule are stereoisomers
Stereoisomers
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Two major causes: The presence of a chiral center in a molecule The presence of structural rigidity in a
molecule The structural rigidity is
caused by restricted rotation about chemical bonds
Causes of stereoisomerism
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PLANE-POLARIZED LIGHTPolarimeter:Polarimeter: a device for measuring
the extent of rotation of plane-polarized light
Plane-Polarized LightLight Light vibrating in all planes to direction of propagation
Plane-polarized light:Plane-polarized light: light vibrating only in parallel planes
optical activity
Plane-polarized lightPlane-polarized light the vector sum of the vector sum of left left and and right circularly polarized lightright circularly polarized light
Optically Activity Enantiomers (chiral) interact with Enantiomers (chiral) interact with circularly polarized light
rotating the plane one way with R center and opposite way with S
result: rotation of plane-polarized light clockwise (+)or counterclockwise (-)
Change in the polarized plane?
no change in the plane
achiral sample
Plane-Polarized Light (polarimeter)
rotates the plane
Plane-Polarized Light (polarimeter)Change in the polarized plane?
Monosaccharides consist of 3-6 carbon atoms typically. A carbonyl group (aldehyde or
ketone). Several hydroxyl groups. 2 types of monosaccharide structures:
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Biochemistry 30
Classification of Monosaccharides
Aldoses are monosaccharides
with an aldehyde group with many hydroxyl (-OH)
groups.triose (3C atoms)tetrose (4C atoms)pentose (5 C atoms)hexose (6 C atoms)
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Aldoses O ║
C─H aldose │ H─ C─OH │ H─ C─OH │
CH2OH
Erythose, an Aldotetrose
Ketoses are monosaccharides
with a ketone group with many hydroxyl (-OH)
groups.
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Ketoses CH2OH │ C=O ketose │ H─ C─OH │ H─ C─OH │
H─C─OH │ CH2OH
Fructose, a ketohexose
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Learning Check Identify each as aldo- or keto- and as tetrose, pentose, or hexose:
H
CH2OH
OHC
H
H
H
OH
OH
OH
C
C
C
HC
O
CH2OH
HHO
CH2OH
O
H OHC
C
C
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Structures of
Monosaccharides
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Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
A Fischer projection is used to represent carbohydrates. places the most oxidized group at the top. shows chiral carbons as the intersection of
vertical and horizontal lines.
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Fischer Projections
CH3CHOHC6H5
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Learning check
In a Fischer projection, the −OH group on the chiral carbon farthest from the carbonyl group
determines an L or D isomer. left is assigned the letter L for the L-form. right is assigned the letter D for the D-form.
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D and L Notations
LC of D and L Isomers of Monosaccharides
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glucose ribose galactose
O
CH2OH
H OHH OH
HO H
OHH
C H
CH2OH H OHH OH
OHHHC
OH
O
CH2OH
HO H
H OH
H OH
HHO
C
found in fruits, corn syrup, and honey.
an aldohexose with the formula C6H12O6 known as blood sugar in the body.
The monosaccharide in polymers of starch, cellulose, and glycogen.
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D-Glucose
D-Fructose is a ketohexose
C6H12O6. is the sweetest
carbohydrate. is found in fruit
juices and honey. converts to
glucose in the body.
H OH
CH2OH
C
HO
H OH
H
C
O
C
C
CH2OH
D-Fructose
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Cyclic structures are the widespread form of monosaccharides
with 5 or 6 carbon atoms.
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Cyclic Structures
O O
The cyclic forms of monosaccharides result from the ability of their carbonyl group to react intramolecularly with a –OH group
Cyclic structure is formed when the –OH group on C-5 reacts with the aldehyde group or ketone group
The result is a cyclic hemiacetal or cyclic hemiketal
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LE 5-4
Abbreviated ringstructure
Steps for cyclisationfor D-Glucose
All –OH groups to the right in the projection formula appear below the ring whereas –OH gps to the left in FP appear above the ring
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Drawing the Cyclic Structure for Glucose
Number the carbon chain and turn clockwise to form a linear open chain.
HHO
H
CH2OH
OHC
H
H
OH
OH
C
C
C
OHC
H
OHH
OH
C
H H
OH OH
C C CH
OCHOCH2
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1
2
3
4
5
6
6 5 4 3 2 1
Cyclic Structure for GlucoseSTEP 2 Fold into a hexagon. Bond the C5 –O– to C1. Place the C6 group above
the ring. Write the –OH groups on
C2 and C4 below the ring. Write the –OH group on
C3 above the ring. Write a new –OH on C1.
OH
OH
OHOH
CH2OH
O
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6 5
4 1
3 2
Cyclic Structure for Glucose (cont)
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OH
OH
OHOH
CH2OH
O
OH
OH
OHOH
CH2OH
O
-D-Glucose -D-Glucose
STEP 3 Write the new –OH on C1 • down for the form.• up for the form.
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Summary of the Formation of Cyclic Glucose
Alcohols react with the carbonyl groups of aldehydes and ketones to give hemiacetal
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-D-Glucose and β-D-Glucose in Solution
When placed in solution, cyclic structures open and close. -D-glucose converts to β-D-glucose and vice versa. at any time, only a small amount of open chain
forms.
-D-glucose D-glucose (open) β-D-glucose (36%) (trace) (64%)
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OH
CH2OH
OH
OC
H
OH
OHOH
OH
OHOH
CH2OH
OOH
OH
OHOH
CH2OH
O
Fructose and other ketoses with a long carbon chain also cyclizes to form hemiketal
D-fructose and D-ribose form a five-membered ring
Intramolecular Cyclic hemiketal Structure of
Fructose
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Cyclic Structure of FructoseFructose is a ketohexose. forms a cyclic structure. reacts the —OH on C-5 with the C=O on C-2.
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D-fructose -D-fructoseα-D-fructose
O CH2OH
OH
OH
OH
CH2OH
O OH
CH2OH
OH
OH
CH2OH
H OH
H OH
HHO
O
CH2OH
C
C
C
CCH2OH
This is two dimensional notation that specifies the 3-dimensional structure of cyclic form of carbohydrate
Haworth Projection
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The D and L form of a monosaccharide is determined by the position of the terminal CH2OH gp on the highest-numbered ring carbon atom
In D-form, this group is positioned above the ring
In L-form the terminal CH2OH gp is positioned below the ring (not encountered in biological systems)
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α and β configurations is determined by the position of the –OH gp on carbon no 1 relative to the CH2OH
In β-configuration both of these gps point in the same direction
In α-configuration the two gps point in opposites direction
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Oxidation Reduction Glycoside formation Phosphate ester formation Amino sugar formation
Reactions of Monosaccharides
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Yield three different types of oxidation products Oxidizing agent used to determined the productWeak oxidizing Agent:Tollen's ReagentBenedict SolutionReducing SugarsIs a carbohydrate that gives a positive test with
TR, FS and BS
Oxidation
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Strong oxidizing agent oxidizes both end of monosaccharide i.e
Terminal pri-alcohol and carbonyl group to give dicarboxylic acid
Such polyhydroxy dicarboxylic acids are known as –aric acids
The oxidation of glucose gives Glucaric acid
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Carbonyl group present in a monosaccharide ( aldose, ketose) can be reduced to a Hydroxyl group using Hydrogen as a reducing group
Product is called Sugar Alcohol D- Glucitol is also known as D-sorbitol These are used as moisturizing agents in
foods and cosmetics because of their affinity for water
Reduction
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Cyclic forms of monosaccharides are hemiacetals and hemiketals react with alcohols to form Acetals and Ketals
The general name for monosaccharide acetals and Ketals is Glycoside
Glycoside It is an Acetal or a Ketal formed from a cyclic
monosaccharide
Glycoside Formation
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Glycoside produced from glucose is Glycoside Glycoside produced from galactose is
Galactoside Exist in α and β forms Named as by listing alkyl or aryl group
attached to the oxygen followed by the name of a monosaccharide involved with the suffix-ide
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The –OH gp of a monosaccharide can react with Oxoacids to form Esters
Phosphate esters are formed from phosphoric acid and various monosaccharides commonly encountered in biological system
Example Specific enzymes in the body catalyze the
esterification of the carbonyl group (C1) and the primary alcohol (C6) of glucose to give
Glucose-1-Phosphate Glucose-6-Phosphate
Phosphate Ester formation
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These phosphate esters of glucose are stable in aqueous solution and play important roles in the metabolism of carbohydrates
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Amino sugars of glucose, mannose and galactose are common in nature
They are produced by replacing the –OH group on carbon 2 on the monosaccharide with an amino group
Amino sugars and their N-acetyl-derivatives are important building blocks of polysaccharides such as cartilage
The N-acetyl derivatives of D-glucosamine and D-galactosamine are present in the biochemical
markers on red blood cells, which distinguish the various blood type
Amino Sugar formation
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Disaccharides
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A disaccharide consists of two monosaccharides.
Monosaccharides Disaccharide glucose + glucose maltose + H2Oglucose + galactose lactose + H2Oglucose + fructose sucrose + H2O
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Important Disaccharides
Monosaccharide + Monosaccharide(Functioning as (Functioning as A hemiacetal or an alcohol)hemiketal)
Disaccharide + H2O
glycoside
Disaccharides
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Maltose is
A disaccharide also known as malt sugar. Composed of two D-glucose molecules. Obtained from the hydrolysis of starch. Used in cereals, candies, and brewing. Found in both the - and β - forms. The Glycosidic linkage between the two glucose
units is called (1-4) linkage
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Maltose
Cyclic forms of monosaccharides are hemiacetals and hemiketals, they react with alcohols to form acetals and Ketals
The bond that links two monosaccharides of a disaccharide together is called a Glycosidic linkage
A Glycosidic linkage is the carbon-Oxygen-carbon bond that joins the two components of Glycoside together
Glycosidic Formation
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Formation of Maltose
Free α-OH
Produced as an intermediate in the hydrolysis of the polysaccharide cellulose
Contains two D-glucose monosaccharide units Differ from maltose –must have a β-
configuration β- (1-4)Glycosidic linkage Reducing sugar having three isomeric forms in
Aq. Solu and on hydrolysis produces two D-glucose molecules
Cellobiose
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Different in biological behaviors Differences are related to stereochemistry of
their glycosidic linkages Maltase enzyme which breaks glucose-
glucose α (1-4) linkage present in maltose is found in human body and in yeast
That’s why maltose is easily digested by human body and readily fermented by yeast
Difference in maltose and Cellobiose
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Both the human body and yeast lacks enzyme cellobiase which is needed to break the glucose-glucose β (1-4) glycosidic linkage of Cellobiose
Cellobiose cannot be digested by humans or fermented by yeast
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LactoseLactose is a disaccharide of
β-D-galactose and α- D-glucose.
contains a β -1,4-glycosidic bond.
is found in milk nearly 4-9% and milk products.
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α-form
α-form
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SucroseSucrose or table sugar• is obtained from sugar cane and sugar beets.• consists of α-D-glucose and β-D-fructose..• has an α,β-1,2-glycosidic bond.
α-D-glucose
β -D-fructose
Sugars and artificial
sweeteners
differ in sweetness.
are compared to sucrose (table sugar), which is assigned a value of 100.
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Sweetness of Sweeteners
60 000
Identify the monosaccharides in each of the following: A. lactose
(1) α-D-glucose (2) β-D-fructose (3) β-D-galactose
B. maltose(1) α-D-glucose (2) β-D-fructose (3) β-D-galactose
C. sucrose(1) α-D-glucose (2) β-D-fructose (3) β-D-galactose
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Learning Check
Polysaccharides
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Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
PolysaccharidesPolysaccharides Are polymers of D-glucose. Include Amylose and
Amylopectin, starches made of α-D-glucose.
Include glycogen (animal starch in muscle), which is made of α-D-glucose.
Include cellulose (plants and wood), which is made of β-D-glucose.
O
CH2OH
OHOH
OH
OH
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α-D-Glucose
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Structures of Amylose and Amylopectin
AmyloseAmylose is a polymer of α-D-
glucose molecules. linked by -1,4
glycosidic bonds. a continuous
(unbranched) chain.
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AmylopectinAmylopectin is a polymer of α-D-
glucose molecules. is a branched-chain
polysaccharide. has α-1,4-glycosidic
bonds between the glucose units.
has α-1,6 bonds to branches.
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Starches like amylose and amylopectin hydrolyze to dextrins (smaller polysaccharides)
Contain 3-8 glucose units
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Dextrins
Glycogen
Glycogen is the polysaccharide
that stores α-D-glucose in muscle.
is similar to amylopectin, but is more highly branched.
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CelluloseCellulose is a polysaccharide
of glucose units in unbranched chains.
has β-1,4-glycosidic bonds.
cannot be digested by humans because humans cannot break down β-1,4-glycosidic bonds.
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