Carbohydrates 2/9/2016Dr Seemal Jelani Introduction to Biochemistry1

Carbohydrates

05/04/23Dr Seemal Jelani Introduction to Biochemistry 1

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.”

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.

Cellulose (carbohydrates) serve as structural elements

Starch (carbohydrates) provide energy reserves for the plants

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

Classification of Carbohydrates

The types of carbohydrates are • Monosaccharides, the simplest carbohydrates. • Disaccharides, which consist of two

monosaccharides.• Polysaccharides, which contain many

monosaccharides.

Cn H2n On Carbohydrates are Polyhydroxy aldehydes,

Polyhydroxy ketones The carbohydrate glucose is Polyhydroxy

aldehyde and the carbohydrate fructose is Polyhydroxy ketone

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

05/04/23dr seemal jelani 8

Mirror image

Non superposable

Chiral HANDS

Chiral Shells

The plane has the same thing on both sides for the flaskThere is no mirror plane for a hand

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

Hands, gloves are prime examples of chiral object

They have a “left” and a “right” version

Why this chapter?

Handedness is important in organic and biochemistry

Molecular handedness makes possible specific interactions between enzymes and substrates

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

Aldehydes CHO AldoseKetone C=O Ketose

Functional groups in carbohydrates

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

Stereoisomers Enantiomers Diastereomers Handedness (Right and left) D and L Fischer Projection for 2,3,4-trihydroxybutanal Epimers

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

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

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

05/04/23Dr Seemal Jelani Chem-241 25

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:

Aldoses and ketoses05/04/23Dr Seemal Jelani Introduction to

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)

Aldoses O ║

C─H aldose │ H─ C─OH │ H─ C─OH │

Erythose, an Aldotetrose

Ketoses are monosaccharides

with a ketone group with many hydroxyl (-OH)

groups.

Ketoses CH2OH │ C=O ketose │ H─ C─OH │ H─ C─OH │

H─C─OH │ CH2OH

Fructose, a ketohexose

Learning Check Identify each as aldo- or keto- and as tetrose, pentose, or hexose:

Structures of

Monosaccharides

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.

Fischer Projections

CH3CHOHC6H5

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.

D and L Notations

LC of D and L Isomers of Monosaccharides

glucose ribose galactose

H OHH OH

CH2OH H OHH OH

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.

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.

D-Fructose

Cyclic structures are the widespread form of monosaccharides

with 5 or 6 carbon atoms.

Cyclic Structures

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

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

Drawing the Cyclic Structure for Glucose

Number the carbon chain and turn clockwise to form a linear open chain.

C C CH

OCHOCH2

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.

Cyclic Structure for Glucose (cont)

-D-Glucose -D-Glucose

STEP 3 Write the new –OH on C1 • down for the form.• up for the form.

Summary of the Formation of Cyclic Glucose

Alcohols react with the carbonyl groups of aldehydes and ketones to give hemiacetal

-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%)

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

Cyclic Structure of FructoseFructose is a ketohexose. forms a cyclic structure. reacts the —OH on C-5 with the C=O on C-2.

D-fructose -D-fructoseα-D-fructose

O CH2OH

CCH2OH

This is two dimensional notation that specifies the 3-dimensional structure of cyclic form of carbohydrate

Haworth Projection

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)

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

Oxidation Reduction Glycoside formation Phosphate ester formation Amino sugar formation

Reactions of Monosaccharides

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

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

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

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

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

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

These phosphate esters of glucose are stable in aqueous solution and play important roles in the metabolism of carbohydrates

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

Disaccharides

A disaccharide consists of two monosaccharides.

Monosaccharides Disaccharide glucose + glucose maltose + H2Oglucose + galactose lactose + H2Oglucose + fructose sucrose + H2O

Important Disaccharides

Monosaccharide + Monosaccharide(Functioning as (Functioning as A hemiacetal or an alcohol)hemiketal)

Disaccharide + H2O

glycoside

Disaccharides

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

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

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

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

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

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.

α-form

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.

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

Learning Check

Polysaccharides

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.

α-D-Glucose

Structures of Amylose and Amylopectin

AmyloseAmylose is a polymer of α-D-

glucose molecules. linked by -1,4

glycosidic bonds. a continuous

(unbranched) chain.

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.

Starches like amylose and amylopectin hydrolyze to dextrins (smaller polysaccharides)

Contain 3-8 glucose units

Dextrins

Glycogen

Glycogen is the polysaccharide

that stores α-D-glucose in muscle.

is similar to amylopectin, but is more highly branched.

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.

Carbohydrates 2/9/2016Dr Seemal Jelani Introduction to Biochemistry1

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