LECTERE 3LECTERE 3

Lecturer: Dmukhalska Yevheniya. B.

1. Carbohydrates.2. Biological role of carbohydrates in an

organism. 3. Classification of carbohydrates.4. Structure and stereoisomerism, chemical

properties of monosaccharides.

Plan Plan

Carbohydrates are polyhydroxy aldehydes such as D-glucose, polyhydroxy ketones such as D-fructose, and compounds such as sucrose that can be hydrolyzed to polyhydroxy aldehydes or polyhydroxy ketones.

Synthesis of carbohydrates

CO2+ H2O

photosynthesis

chlorophylllight

glucose

polymerizecellulosegiving structure to plants

polymerize

starch, plant seeds

eaten by animals

glucose glycogen (liver)CO2+H2O+energy

clothing fiberwood

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.

ClassificationClassification • 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.

Configurations of Aldoses

Configurations of Ketoses

• Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but which differ only in the three-dimensional orientation of their atoms in space.

• 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. Enantiomers are an optically active compound.

• The right- and left-handed enantiomers of a chiral compound rotate plane-polarized light in opposite ways.

• Monosaccharides are polyhydroxy aldehydes and polyhydroxy ketones, because Aldoses can react with themselves forming cyclic hemiacetals (a) and Ketoses can react with themselves forming cyclic hemiketals (b)

CHO

OH

H

OH

H

HO

H

H OH

CH2OH

HH OH

HHO

HOH()

OH

H

CH2OHO

C

H OH

HHO

HOH

H

CH2OHOH

O

H

OH()H OH

HHO

HH

OH

H

CH2OHO

D-Glucose

-D-Glucopyranose (-D-Glucose)

-D-Glucopyranose (-D-Glucose)

anomeric carbon

+

anomericcarbon

5

5

1

1

redraw to show the -OH on carbon-5 close to thealdehyde on carbon-1

•Five-membered rings are called furanoses

•six-membered rings are pyranoses

•Cyclic form of fructose is fructofuranose, while glucose in the pyranose form is glucopyranose

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

The - and -forms of monosaccharides are readily interconverted in aqueous environments

This spontaneous process, mutarotation, produces an equilibrium mixture of - and -forms in both furanose and pyranos ring structure

Open chain form can participate in redox reactions

Equilibrium Mixture of D-Glucose

MutarotatiMutarotationon

Isomerization—monosaccharides can undergo several types of isomerization

D-glucose incubated in an alkaline solution for several hours produces two isomers: D-mannose and D-fructose

Both involve an enediol intermediate

Isomerization

Oxidation can be done in several ways.

Tollens reagent (Ag+(NH3)2 or Benedict’s solution (Cu2+ tartrate complex). Not synthetically useful due to side reactions.

Bromine water oxidizes aldoses (not ketoses) to monocarboxylic acids (Aldonic Acids).

Nitric Acid oxidizes aldoses to dicarboxylic acids (Aldaric acids).

Enzyme catalyzed oxidation of terminal OH to carboxylic acid (Uronic Acid)

Oxidation of Monosaccharides

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Oxidation of Monosaccharides• Monosaccharides are reducing sugars if their carbonyl

groups oxidize to give carboxylic acids. • In the Benedict’s text, Fehling's text, Tollens’, and

Tromer’s solutions, D-glucose is oxidized to D-gluconic acid. Glucose is a reducing sugar.

C

C

C

C

C

CH2OH

HO

OHH

HO H

OHH

OHH

C

C

C

C

C

CH2OH

OHO

OHH

HO H

OHH

OHH

+ Cu2+

D-Glucose D-Gluconic acid

+ Cu2O(s)

Oxidation• Weak oxidizing agents oxidize the carbonyl

(aldehyde) group end of а monosaccharide to give a glyconic acidglyconic 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 acidsaldaric acids.. For glucose, such an oxidation produces glucaric 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).

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Reduction of Monosaccharides

• The reduction of the carbonyl group produces sugar alcohols, or alditols.

• D-Glucose is reduced to D-glucitol also called sorbitol.

Glycoside Formation.

Methyl--D-glucoside

Methyl--D-glucoside

H

H

H

H

OHOCH2

HO OH

NH2

O

N

N

anomericcarbon

a -N-glycosidicbond

Acylation and Alkylation of Monosaccharides

Phosphate ester formation

-D-Glucose-6-phosphate

 -D-Glucose-1-phosphate

Amino Sugar• Amino groups may be substituted for hydroxyl

group of sugars to give rise to amino sugars. Generally, the amino group is added to the second carbon of the hexoses. The most common aminosugars are Glucosamine and Galactosamine.

  -D-Glucosamine -D-Glalactosamine N-acety1-D-glucosanune

Oligosaccharides: 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

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 (Functioning as (Glycoside)

hemiacetal or an alcohol)

а hemiketal)

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 (1 - 4)-linkage

• Lactose is а reducing sugar

• 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 non-reducing 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 laevorotatoryThis 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 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.

А polysaccharide (glucans)

Starch is used for energy storage in plants

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.

Chitin, a polymer of N-acetylglucoasamine

• 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.

• 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.

• (1,4)-O--D-Glucopyranosyluronic acid-(1,3)-2-acetamindo-2-deoxy--D-glucopyranose.

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-6-O-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-sulfamindo-2-deoxy-6-O-sulfo--D-glucopyranose

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