Carbohydrates IUG, Fall - PowerPoint Presentation

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Carbohydrates

IUG, Fall 2018Dr. Tarek Zaida

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CarbohydratesMost abundant class of biological molecules on Earth

Originally produced through CO2 fixation during photosynthesis2

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Roles of CarbohydratesEnergy storage (glycogen

, starch)Structural components (cellulose, chitin)Cellular recognition Carbohydrate derivatives include DNA, RNA, co-factors,

glycoproteins

,

glycolipids

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CarbohydratesMonosaccharides

(simple sugars) cannot be broken down into simpler sugars under mild conditions Oligosaccharides = "a few" - usually 2 to 10 Polysaccharides are polymers of the simple sugars4

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Monosaccharides

Polyhydroxy ketones (ketoses) and aldehydes (aldoses

)

Aldoses

and ketoses contain

aldehyde

and

ketone

functions, respectively

Ketose

named for “equivalent

aldose

” + “

ul” insertedTriose, tetrose, etc. denotes number of carbons Empirical formula = (CH2O)n

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Monosaccharides are

chiralAldoses with 3C or more and ketoses with 4C or more are chiral

The number of

chiral

carbons present in a

ketose

is always one less than the number found in the same length

aldose

Number of possible

stereoisomers

= 2

n

(n = the number of chiral carbons)

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Monosaccharides

Monosaccharides are classified according to the number of carbon atoms present (triose

,

tetrose

,

pentose

,

hexose

, and so on) and according to whether the carbonyl group is present as an

aldehyde

(

aldose

) or as a

ketone (ketose).There are only two trioses:

1. Glyceraldehyde and

2. DihydroxyacetoneEach has two hydroxyl groups, attached to different carbon atoms, and one carbonyl group.

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Glyceraldehyde is the simplest aldose

Dihydroxyacetone is the simplest ketose. Each is related to glycerol in that each has a carbonyl group in place of one of the hydroxyl groups.Other aldoses or ketoses can be derived from glyceraldehyde or dihydroxyacetone by adding carbon atoms, each with a hydroxyl group. In aldoses, the chain is numbered from the aldehyde

carbon. In most ketoses, the carbonyl group is located

at C-2.

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Name each of the following monosaccharides as analdose or ketose & according to its number of C atoms.

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Origin of Carbohydrates

1. Plants catch up CO2 from air & H2O from soil6CO2 + 6H

2

O

sun light

C

6

H12O6 + 6O2

The above reaction is called photosynthesis (formation of carbohydrates from CO

2

, H

2

O and sun light)

12enzymes, chlorophyllglucose

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2. The carbohydrates produced during photosynthesis are monosaccharides.Plants build disaccharides from monosaccharides:

C6H12O6 + C6H12O6 C12H22O11

+ H

2

O

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Disaccharide

glucose

glucose

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3. Plants as well as animals can combine many molecules of monosaccharides into large polysaccharide molecules: n

C6 H12O6 polymerization (C6H10O5)n + nH2

O

Polysaccharides in plants are:

- Cellulose in stalk and stem

- Starch in roots and seeds

- Mono & disaccharides are found in fruits

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Oxygen – Carbon dioxide cycle

1. Plants pick up CO2 from air, water from soil to build carbohydrates (monosaccharides).2. Animals can not do that, so they have to rely on plants to get their carbohydratesC6H12O6 + 6O2 6CO2

+ 6H

2

O + energy

The above reaction is the reverse of photo-synthesis.

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Krebs cycle

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During Photosynthesis: Energy from the sun needed for aerobic reactions.During catabolism of carbohydrates in animals same amounts of energy is liberated from oxidation of food.

So energy from metabolism in animals comes from the sun through plants that store solar energy in carbohydrates.16

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Stereoisomerism Stereoisomers

:Are compounds having the same molecular formula but a different structural formula.Stereoisomers have structures that are mirror image of one anotherIf you put your left hand on top of your right hand, you will see that they are not superimposable17

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Some molecules are superimposable with their mirror images and others are not: H-C-H H-C-H

Both methane and mirror image are said to be superimposable (achiral)18

H

H

H

H

methane

Mirror image

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In general any object with a plane of symmetry is achiralAny object or molecule does not have a plane of symmetry it is chiralIf there are 4 different atoms attached to a central carbon atom, it is said to be chiral

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Stereoisomers in Carbohydrates

Carbohydrates are chiral molecules since they have carbon atoms carrying four different groups.The simplest three carbon sugar is glyceraldehyde. This sugar exists as a pair of enantiomers. 20

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Stereochemistry

Enantiomers = mirror imagesPairs of isomers that have opposite configurations at o more than one chiral centers but are NOT mirror images are diastereomers

Epimers

= Two sugars that differ in configuration at only one

chiral

center

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Two forms of glyceraldehyde (D and L) have the same physical properties except they behave differently in the way they rotate polarized light and the way they are affected by catalysts.

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Remember: Compounds with n chiral carbon atoms has a maximum of 2n possible

stereoisomers and half that many pairs of enantiomers (mirror images).•This aldotetrosose, has 2 chiral carbon atoms and a total of 22 = 4 possible stereoisomers (2 pairs of enantiomers).

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Fischer Projections of Sugar MoleculesExample

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Monosaccharides are divided into two families:D form and L form sugars.

D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right.L: the –OH group on the chiral C furthest

from the C=O comes out of the plane of paper and points to the

left

.

The D and L relate directly only to the position of –OH group on the bottom carbon in a Fischer projection.

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D: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the right.

L: the –OH group on the chiral C furthest from the C=O comes out of the plane of paper and points to the left.

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Chiral DrugsMany enzymes will interact with only one particular

enantiomerIn human body, enzymes will react with carbohydrates of D- form, but will react with proteins of L-form.Some drugs are enantiomers: 1. Ibuprofen used in pain relief 2. Indinavir used in Aids 3. Levofloxacin

used as antibiotic

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some important pentoses and hexoses, and their derivatives

1. D-glucose, also called dextrose or blood sugar , is the most important monosaccharide in human metabolism. 29

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2. D-fructose, or fruit sugar, is most common natural ketose. Honey contains about 38% fructose

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In deoxy sugars a hydrogen atom replaces one or more of the -OH groups in a monosaccharide.D-ribose and its derivative D-2-deoxyribose (deoxy = minus one oxygen atom) are found in various coenzymes and in DNA.

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The Cyclic Form of Monosaccharides1. Glucose

– A monosaccharide contains both an alcohol and an aldehyde group.– It can react with itself to form a cyclic hemiacetal.32

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Glucose is prepared by:

1. hydrolysis of sucrose 2. hydrolysis of starchGlucose is found in blood streamFound in tissue fluidsIf found in urine, it is an indicator of diabetes (glucosuria)Glucose may show up in urine during extreme excitement (

emotional

glucosuria

) or after ingestion of a large amount of sugar (

alimentary

glucosuria

)

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2. Galactose

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Glucose & Galactose are epimers.Epimers: are 2 sugars that differ only in about a single carbon atom. D-

galactose epimerase D-glucoseGalactosuria

Is a severe inherited disease results in the inability of infants to metabolize

galactose

because of a deficiency in either:

1. Galactose-1-phosphate-uridyl

transferase

(GALT)

2.

Galactokinase

(GALK) As a result the galactose concentration will increase in blood and urine (galactosuria)38

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3. Fructose

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Fructose is often called fruit sugar or levuloseCan be prepared by hydrolysis of sucroseIt also can be prepared by hydrolysis of

InulinFructose is 1.7 times sweeter than sucrose

Fructosemia

:

An inherited disease (prevents the conversion of fructose to glucose) due to a deficiency of the enzyme fructose-1-phosphate

aldolase

.

As a result fructose will increase in blood.

symptoms may include vomiting, hypoglycemia, severe malnutrition

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Reactions of hexosesHexoses which are either

aldoses or ketoses have reducing properties.This reducing property of hexoses is the basis of the test for sugar in urine or blood.42

Gluconic

acid

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Reduction of Monosaccharides

45Note:Accumulation of sorbitol in the eye is a major factor in the formation of cataracts due to diabetes.

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FermentationIn the presence of yeast (or enzymes of yeast) glucose forms ethanol and carbon dioxideFructose can ferment but

galactose can not.Pentoses do not ferment in the presence of yeast.46

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Derivatives of Monosaccharides

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Sugar Phosphates Phosphate Esters

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Amino SugarsAmino sugars (hexosamines) contain an amino group on place of an –OH group.

Examples of natural amino sugars:D- GlucosamineD-galactosamineD-mannosamineAmino sugars are components of some antibiotics such as Erythromycin

and

Carbomycin

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Sugar alcohols

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Protein-Sugar InteractionsWhen the glucose level in the blood is elevated over a period of time, normal hemoglobin binds to glucose covalently.

The amount of glucosylated hemoglobin HbA1c in blood is used as a measure of the effectiveness of blood glucose control in a diabetic patient.52

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The concentration of HbA1c directly reflects the elevation of blood glucose over the preceding several days.In diabetic patient HbA1c is about 7 – 11% of the total hemoglobin (HbA

)In nondiabetic person this value ranges 4 – 6%.53

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Disaccharides54

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A disaccharide consists of two sugars joined by an O-glycosidic bond.The three common Disaccharides are:MaltoseLactose

Sucrose55

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1. MaltoseMaltose comes from the hydrolysis of large polymeric oligosaccharides such as starch and glycogen and is in turn hydrolyzed to glucose by

maltase.Sucrase, lactase, and maltase are located on the outer surfaces of epithelial cells lining the small intestine56

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Cellobiose is an isomer of maltose.It differs from maltose only in having the β configuration at C-1 of the left glucose unit. Otherwise, all other structural features are identical, including a link from C-1 of the left unit to the hydroxyl group at C-4 in the right unit.

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2. LactoseLactose, the disaccharide of milk, consists of galactose

joined to glucose by β

-1,4-

glycosidic linkage.

Lactose is hydrolyzed to these

monosaccharides

by

lactase

in human beings and by

β

-

galactosidase

in bacteria.

Lactose is formed in the urine of pregnant women, and it gives a positive test with Cu2+ containing reagents since it is a reducing sugar

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3. SucroseSucrose is the common table sugarIs obtained commercially from sugar cane or sugar beets.

The anomeric carbon atoms of a glucose unit and a fructose unit are joined in this disaccharide; The configuration of this glycosidic linkage is α

for glucose and

β

for fructose.

Sucrose can be cleaved into its component

monosaccharides

by the enzyme

sucrase

.

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Reducing Properties of DisaccharidesDisaccharides with 1,4 linkage are reducing because they have a free aldehyde group in one of the two

monosaccharides.Lactose & maltose are reducing sugars.Sucrose with 1,2 linkage, no aldehyde or ketone group is free, therefore sucrose is a nonreducing sugar60

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FermentationSucrose & maltose will ferment in the presence of yeast, because yeast does contain

sucrase & maltse

.

Lacotse

on the other hand will not ferment when yeast is added, because it does not contain

lactase

.

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Sweetness and Sweeteners (Sugar Substitutes)Sweetness is literally a matter of taste. Although individuals vary greatly in their sensory perceptions, it is possible to make some quantitative comparisons of sweetness.

For example, we can take some standard sugar solution (say 10% sucrose in water) and compare its sweetness with that of solutions containing other sugars or sweetening agents. 62

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If a 1% solution of some compound tastes as sweet as the 10% sucrose solution, we can say that the compound is 10 times sweeter than sucrose.D-Fructose is the sweetest of the simple sugars—almost twice as sweet as sucrose. D-Glucose is almost as sweet as sucrose. On the other hand, sugars like lactose and galactose

have less than 1% of the sweetness of sucrose.63

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Many synthetic sweeteners are knownThe most common sugar substitutes are:Saccharin is made commercially from toluene.

Cyclamates: they are not used anymore because they might cause cancer.64

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3. Aspartame became the first new sweetener to be approved by the U.S. Food and Drug Administration (FDA) 25 years ago. It is about 160 times sweeter than sucrose.Structurally, aspartame is the methyl ester of a dipeptide of two amino acids that occur naturally in proteins—

aspartic acid and

phenylalanine

—and is sold under the trade name

NutraSweet.

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Polysaccharides

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Polysaccharides

Contain many linked monosaccharides and vary in chain length and molecular weigh.Most polysaccharides give a single mono-saccharide on complete hydrolysis.

The monosaccharide units may be linked linearly, or the chains may be branched.

Polysaccharides formed from

pentoses

are called

pentosans

, while the ones made from

hexoses

are called

hexosans

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Comparison of Polysaccharides to Mono- & Disaccharides

Property

Di- &

Monosaccharides

Polysaccharides

Taste

sweet

tasteless

Solubility in water

soluble

insoluble

Molecular Weight

low

Very highSize of particlespass through membranesDo not pass through membranesTest with Cu2+positive except for sucrosenegative

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Examples of Polysaccharides

1. Starch

I

s the energy-storing carbohydrate of plants.

It is the form in which glucose is stored by plants for later use.

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s made up of glucose units joined mainly by 1,4-

α

-

glycosidic

bonds, although the chains may have a number of branches attached through 1,6-

α

–

glycosidic bonds. Partial hydrolysis of starch gives maltose, and complete hydrolysis gives only d-glucose.69

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Amylose and

Amylopectin70

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Starch can be separated into two fractions:

amylose and

amylopectin

.

In

amylose

, which constitutes about 20% of starch, the glucose units (50 to 300) are in a continuous chain, with 1,4 linkages

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Amylopectin

is highly branched.May contain 300 to 5000 glucose units,Chains with consecutive 1,4 links average only 25 to 30 units in length.These chains are connected at branch points by 1,6 linkages.Because of this highly branched structure, starch granules swell and eventually form colloidal systems in water.72

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2. Glycogen

Is the energy-storing carbohydrate of animals. Like starch, it is made of 1,4- and 1,6-linked glucose units.Glycogen has a higher molecular weight than starch (perhaps 100,000 glucose units), and its structure is even more branched than that of amylopectin

, with a branch every 8 to 12 glucose units.

Glycogen is produced from glucose, transported to the liver, muscles, where it is stored for later use.

Glycogen helps maintain the glucose balance in the body by removing and storing excess glucose from ingested food and later supplying it to the blood when various cells need it for energy.

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glycogen

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3. Cellulose

Cellulose is an unbranched polymer of glucose joined by 1,4-β-glycosidic bonds.

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Other

Polysacchrides4. Dextrin: Produced during the hydrolysis of starch.

5. Heparin:

Made up of repeated units of

glucuronic

acid and glucosamine, used as a blood anti-coagulant. Is the strongest organic acid in the body.

6. Dextran

: produced by bacteria when grow on sucrose.

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Dextrans

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7.

Pectin: Obtained from fruits and berries, used in making jellies.They are linear polymers of d-galacturonic acid, linked with 1,4-α-glycosidic bonds.

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