Carbohydrates

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Carbohydrates

Bioc. 201

ميحرلا نمحرلا هللا مسب

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Carbohydrates or Saccharides

• Carbohydrates are the most abundant organic molecules in nature.

• Essential components of all living organisms.

• The word carbohydrate means "hydrate of carbon“.

• The suffix –ose indicates that a molecule is a carbohydrate, and the prefixes tri-, tetr-, and so forth indicate the number of carbon atoms in the chain.

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Carbohydrates or Saccharides

• Carbohydrates are compounds containing C, H, and O.

• The general formula for a carbohydrate is (CH

₂

O)

_n

, where n≥3.

• All carbohydrates contain

C==O and ―OH

functional groups.

D- Glucose D- Fructose

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Functions of Carbohydrates

1. The major source of energy.

2. Acting as a storage form of energy in the body.

3. Serving as cell membrane component that mediate some forms of intercellular communication (e.g. cell adhesion in inflammation).

4. Serving as a structural component of many organisms, including cell wall of bacteria, exoskeleton of insects, and fibrous cellulose of plants.

5. Essential components of DNA and RNA (Ribose and

Deoxyribose)

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Classification of Carbohydrates

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Classification of Carbohydrates

 There are four classes of Carbohydrates, based on the number of sugar units:

1. Monosaccharide (simple sugars): cannot be broken down into simpler sugars under mild conditions.

2. Disaccharides: contain 2 monosaccharide units covalently linked.

3. Oligosaccharides: contain from 3 - 12 monosaccharide units covalently linked.

4. Polysaccharides: contain more than 12 monosaccharid units and can be hundreds of sugar units in length covalently linked.

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Monosaccharides or Simple Sugars

• Monosaccaharides are aldehyde or ketone derivatives of straight - chain polyhydroxy alcohols containing at least three carbon atoms.

• Such substances, for example, D-glucose and D- ribulose, cannot be hydrolyzed to form simpler saccharides.

D- Glucose D- Ribulose

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Classification of Monosaccharides

 Monosaccharides can be classified according to:

1. The number of carbon atoms they contain.

Examples of monosaccharides found in humans.

2. The location of the carbonyl (CO) functional group:

• Aldose: monosaccharide has a terminal carbonyl group (O==CH^-) called an aldehyde group.

• Ketose: monosaccharide has a carbonyl group (O==C) in the middle linked to two other carbon atoms, called a ketone group.

Examples of an aldose (A) and a ketose (B) sugar.

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Classification of Monosaccharides

• These terms may be combined so that, for example, glucose is an aldohexoses where ribulose is a ketopentose.

aldo trioses and keto trioses aldo tetroses and keto tetroses aldo pentoses and keto pentoses aldo hexoses and keto hexoses

• The simplest monosaccharide are the two 3- carbon trioses:

– Glyceraldehydes, an aldose – Dihydroxyacetone, a ketose.

D- Glucose D- Ribulose

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Classification of Monosaccharides

• The most abundant monosaccharides in nature are the hexoses, which include the aldohexose D-glucose and the ketohexose D-fructose.

D- Glucose D- Fructose

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Classification of Monosaccharides

The stereochemical relationships, shown in Fisher projection, among the D-aldoses with 3 to 6 carbon atoms. The configuration about C2 (red) distinguishes the members of each pair.

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Classification of Monosaccharides

The stereochemical relationships among the D-ketoses with 3 to 6 carbon atoms. The configuration about C3 (red) distinguishes the members of each pair.

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Structure of Carbohydrates

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Chirality

• Chiral objects:

cannot be superimposed on their mirror images

— e.g., hands, gloves, and shoes.

• Achiral objects can be

superimposed on the

mirror images — e.g.,

drinking glasses,

spheres, and cubes.

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Chirality

• Any carbon atom which is connected to four different groups will be chiral, and will have two nonsuperimposable mirror images; it is a chiral carbon or a center of chirality.

• If any of the two groups on the carbon are the same, the carbon atom cannot be chiral.

• Many organic compounds (e.g. carbohydrates, amino acids) contain more than one chiral carbon.

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Stereoisomers

• The central carbons of a carbohydrate are asymmetric (chiral) – four different groups are attached to the carbon atoms. This allows for various spatial arrangements around each asymmetric carbon (also called stereogenic centers) forming molecules called stereoisomers.

2⁴=16 steroisomers Chiral Centers

Chiral Centers

2³=8 steroisomers

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Stereoisomers

• Stereoisomers have the same order and types of bonds but different spatial arrangements and different properties.

• For each asymmetric carbon, there are 2

ⁿ

possible isomers (e.g. an aldohexose contains four asymmetric carbons, there are 2

⁴

, or 16, possible isomers).

2⁴=16 steroisomers Chiral Centers

Chiral Centers

2³=8 steroisomers

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Stereoisomers

 What is the maximum number of possible

stereo-isomers of the following compounds?

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D- and L- Monosaccharides (Stereoisomers)

• D-monosaccharide:

a monosaccharide that, when written as a Fischer projection, has the -OH on its asymmetric carbon on the right.

• L-monosaccharide:

a monosaccharide that, when written as a Fischer projection, has the -OH on its asymmetric carbon on the left.

 According to the conventions proposed by Fischer:

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Isomers and Epimers

• Isomers are compounds that have the same chemical formula but have different structures.

• Fructose, glucose, mannose, and galactose are all

isomers of each other, having the same chemical

formula, C

₆

H

₁₂

O

₆

.

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Isomers and Epimers

• If two monosachharides differ in configuration

around only one specific carbon atom (with the

exception of the carbonyl carbon), they are defined

as epimers of each other (of course, they are also

isomers!).

_{C2 epimers C4 epimers}

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Isomers and Epimers

•

Glucose and galactose are C-4 epimers - their structures differ only in the position of the ―OH group at carbon 4.

[Note: the carbons in sugars are numbered beginning at the end that contain the carbonyl carbon - that is, the aldehyde or keto group].

C4 epimers C2 epimers

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Isomers and Epimers

•

Glucose and mannose are C-2 epimers - their structures differ only in the position of the ―OH group at carbon 2.

•

Galactose and mannose are

NOT epimers – they differ in

the position of ―OH groups at two carbons (2 and 4) and are, therefore, defined only as isomers.

C4 epimers C2 epimers

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Enantiomers and Diastereomers

•

A special type of isomers is found in the pairs of structures that are mirror images of each other. These mirror images are called enantiomers.

•

The two members of the pair are designated as a D- and L- sugar .

•

The vast majority of the

sugars in humans are

D- sugars.

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Enantiomers and Diastereomers

• Diastereomers:

stereoisomers that are not mirror images (e.g.

D-erythrose and D- threose).

• Enantiomers:

stereoisomers that are

mirror images (e.g. D-

erythrose and L-

erythrose).

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Representation of Carbohydrates

Structure

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Representation of Carbohydrates

 Several models are used to represent carbohydrates:

• Fisher Projection.

• Haworth Projection.

• Chair Conformation.

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Fischer Projections

• Fischer projections are a convenient way to represent mirror images in two dimensions.

• Place the carbonyl

group at or near the top

and the last achiral

CH

₂

OH at the bottom.

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Fischer Projections

 Naming Stereoisomers:

When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group:

• if the hydroxyl group points to right when the carbonyl is “up” it is the D-isomer.

• If the hydroxyl group points to the left, it is the L-isomer.

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Fischer Projections

 Identify the following compounds as D or L isomers, and draw their mirror images:

Xylose

Fructose Arabinose

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Configurations and Conformations

• Alcohols react with the

carbonyl groups of

aldehyde and ketones

to form hemiacetals

and hemiketals,

respectively.

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Configurations and Conformations

• A sugar with a 5-membred ring is known as a furanose in analogy with furan.

• A sugar with a 6-membred ring is known as a

pyranose in analogy with pyran.

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Cyclization of Monosaccharids

 The hydroxyl and either the aldehyde or the ketone functions of monosaccharides can likewise react intramolecularly to form cyclic hemiacetals and hemiketals.

• Cyclization using C1 to C5, results in hemiacetal formation.

• Cyclization using C2 to C5 results in hemiketal formation.

• In both cases, the carbonyl carbon is new chiral

center and becomes an anomeric carbon.

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Cyclization of Monosaccharids

•

Has the aldehyde or ketone at the top of the drawing.

•

The carbons are numbered starting at the aldehyde or ketone end.

•

The compound can be represented as a straight chain or might be linked to show a representation of the cyclic, hemiacetal

form.

^• Fisher projection of glucose. (Left) Open chain (linear form) Fisher projection.

(Right) Cyclic Fisher projection.

 Fischer Projection: D-Glucose

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Cyclization of Monosaccharids

• Fischer Projection: D- Fructose

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Cyclization of Monosaccharides

• Haworth Projection:

• Represents the compound in the cyclic form that is more representative of the actual structure.

• This structure is formed when the functional (carbonyl) group (ketone or aldehyde) reacts with an alcohol group on the same sugar to form a ring called either a hemiketal or hemiacetal ring, respectively.

The reaction of (a) D-glucose in its linear form to yield the cyclic hemiacetal α-D-glucopyranose, and (b) D-fructose in its linear form to yeild the hemiketal α-D-fructofuranose. The cyclic sugars are shown as both Hawoth projections and space-filling models.

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Haworth Projections

• A common way of representing the cyclic structure of monosaccharides

• Monosaccharides do not usually exist in solution in their “open-chain” forms: an alcohol group can add into the carbonyl group in the same molecule to form a pyranose ring containing a stable cyclic hemiacetal or hemiketal.

• All of the atoms on the right are pointed down in the Haworth structure.

• All of the atoms on the left are pointed up in the

Haworth structure.

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Cyclization of Monosaccharids

In the pyranose form of glucose, carbon-1 is chiral, and thus two stereoisomers are possible: one in which the OH group points down ( -hydroxy group) and one in which the OH group points up ( -hydroxy group). These forms are anomers of each other, and carbon-1 is called the anomeric carbon.

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Cyclization of Monosaccharids

• The new carbon stereocenter created in forming the cyclic structure is called an anomeric carbon.

• Stereoisomers that differ in configuration only at the anomeric carbon are called anomers.

• The anomeric carbon of an aldose is carbon 1; that of the most common ketoses is carbon 2.

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Cyclization of Monosaccharids

Anomeric carbon always above ring for D-saccharides

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Chair Conformation

• The use of Haworth formulas may lead to the erroneous impression that furanose and pyranose rings are planner.

• Chair Conformation: The most stable conformation of cyclohexane that resembles a chair.

• The chair structure consists of a six-membered ring where every C-C bond exists in a staggered conformation.

• Molecules will try to adopt the most stable conformation that minimizes strain.

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Chair Conformation

• When going from the Haworth projection to the chair conformation, the anomeric carbon’s substituent that points down in the Haworth projection is going to be axial, and the substituent that points up in the Haworth projection is going to be equatorial. An axial –OH on the anomeric carbon makes the sugar an α sugar, while an equatorial –OH on the anomeric carbon makes the monosaccharide a β sugar.

• Besides the substituents on the anomeric carbon, everything else is drawn relative to the Haworth projection. In other words, all the other substituents are drawn pointing up if they were pointing up in the Haworth projection, and pointing down if they were pointing down in the Haworth projection.

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Chair Conformation

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Physical and Chemical Properties of

Carbohydrates

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Physical Properties of Monosaccharides

• Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter than sucrose).

• They are solids at room temperature.

• They are extremely soluble in water:

– Despite their high molecular weights (MW), the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW.

– Glucose can dissolve in minute amounts of

water to make a syrup (1 g / 1 ml H2O).

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions: REDUCTION

• The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, such as NaBH₄.

• Reduction of the C=O group of a monosaccharide gives sugar alcohols called alditols.

• The products named by replacing the -ose ending with -itol.

 Reduction of D-glucose:

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions: REDUCTION

D- Fructose D- Sorbitol

(sugar alcohol) D- Mannitol

 Reduction of D-fructose:

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions: OXIDATION

• Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids.

• Oxidation of an aldose yielding an aldonic acid (suffix –onic acid).

 Oxidation of D-glucose:

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions: OXIDATION

 Oxidation of D-glucose:

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions: OXIDATION

• In a basic solution, ketoses are converted into aldoses.

• Fructose, a ketohexose, is also a reducing sugar. In a basic solution such as Benedict's, the carbonyl group moves from carbon 2 to carbon 1, so it can be oxidized as glucose.

 Oxidation of D-fructose:

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions:

• Oxidation

– Loss of electrons – Gain of O

– Loss of H (H⁺ & e)

• Reduction

– Gain of electrons – Loss of O

– Gain of H (H⁺ & e)

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions:

The products of oxidation and reduction of D-Mannose are:

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Chemical Properties of Monosaccharides

 Oxidation-Reduction Reactions:

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Chemical Properties of Monosaccharides

 Amino Sugars:

•

Replace a hydroxyl group with an amino (-NH

₂

) group.

•

Only three amino sugars are common in nature: D-

glucosamine, D-mannosamine and galactosamine.

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Chemical Properties of Monosaccharides

 Formation of Phosphate Esters:

• Phosphate esters can form at the 6-carbon of aldohexoses and aldoketoses.

• Phosphate esters of monosaccharides are found in the sugar- phosphate backbone of DNA and RNA, in ATP, and as intermediates in the metabolism of carbohydrates in the body.

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Chemical Properties of Monosaccharides

 Glycosidic Bond Formation:

• Carbohydrates can form glycosidic bonds with other carbohydrates and with noncarbohydrates.

• Glycoside: a carbohydrate in which the -OH of the anomeric carbon is replaced by –OR (-H2O)

• Glycosidic bond: the bond from the anomeric carbon to the - OR group.

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Disaccharides

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Disaccharides

• Disaccharides are formed when two monosaccharide units are joint by a glycosidic linkage.

• On hydrolysis, disaccharides will split into two monosaccharides by disaccharide enzymes (e.g.

lactase).

• The most common disaccharides are:

-

Maltose (Glucose + Glucose).

-

Lactose (Glucose + Galactose).

-

Sucrose (Glucose + Fructose).

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Disaccharides

 Maltose:

•

Contains two D-glucose residues joined by a glycosidic linkage between carbon atom 1 (the anomeric carbon) of the first glucose residue and carbon atom 4 of the second glucose.

•

The

free anomeric carbon of its glucose residues makes

maltose a reducing sugar since it can be oxidized

α-D-glucopyranosyl-(1 4)-D-glucopyranose

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Disaccharides

 Lactose (milk sugar):

• Occurs naturally only in milk.

• Made up of D-galactose and one unit of D-glucose joined by a b -1,4-glycosidic bond.

• The free anomeric carbon of its glucose residues makes lactose a reducing sugar.

β-D-galactopyranosyl-(1 4)-D-glucopyranose

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Disaccharides

 Sucrose:

• The most abundant disaccharide, occur throughout the plant kingdom and is familiar to us as common table sugar.

• Sucrose is formed by one unit of D-glucose and one unit of D-fructose joined by an α-1,2-glycosidic bond.

α-D-glucopyranosyl-(1 2)-β-D-fructofuranoside

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Disaccharides

 Sucrose:

• Sucrose is a non-reducing sugar (C1 of glucose and C2 of fructose are the anomeric carbon atoms).

α-D-glucopyranosyl-(1 2)-β-D-fructofuranoside

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Oligosaccharides

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Oligosaccharides

• A linear or branched carbohydrate usually from 3 to 12 monosaccharide units joined by glycosidic bonds.

• Often found covalently

attached to proteins or

membrane lipids to

form glycoproteins and

glycolipids.

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Polysaccharides

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Polysaccharides

• Polysaccharides are formed by the linkage of many monosaccharide units by glycosidic bonds.

• On hydrolysis, polysaccharides will yield more than 10 monosaccharides.

• Polysaccharides are not reducing sugars

because the anomeric carbons are connected

through glycosidic linkages.

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Polysaccharides

 Polysaccharides are classified as:

• Homopolymers: consist of one type monosaccharide, e.g.:

– Glucans: are homopolymers of glucose.

– Galactans: are homopolymers of galactose.

• Hetropolymers: consist of more than one type monosaccharide.

 The most common polysaccharides are:

• Starch and glycogen (energy-storage polymers).

• Cellulose and chitin (structural polymers).

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Storage Polysaccharides

 Starch (plants)

• The storage form of carbohydrate in plants ingested by humans (Amylase enzyme hydrolyzes starch to disaccharides).

• Plants store starch mostly in the roots and seeds.

• When a plant seed is in an energy poor state, the starch is broken down and used for energy and/or precursors.

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Storage Polysaccharides

 There are two forms of starch:

1. α-amylose: a linear polymer of glucose units liked with α (1- 4) glycosidic bond.

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Storage Polysaccharides

 There are two forms of starch:

2. Amylopectin: a highly branched polymer of all α,D-glucose subunits. The linkages in the backbone are all α(1-4). Every 24 to 30 residues along a backbone there is a branch point. The linkages at the branch points are α(1-6).

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Storage Polysaccharides

 Glycogen (animals)

• The main storage

polysaccharide in animal cells.

• It is abundant in the liver and muscle.

• A branched homopolymer of glucose.

• On hydrolysis, it forms glucose, which maintains normal blood sugar level and provides energy. It is the energy-reserve carbohydrate for animals.

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Storage Polysaccharides

 Glycogen (animals)

• Similar to amylopectin:

– It has backbone of glucose residues linked in an α (1- 4) configuration and Branches that are linked α (1- 6).

• Different from

amylopectin:

– In that it is more highly branched.

– It contains α (1- 6) branch every 8-13 residues along a backbone.

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Structural Polysaccharides

 Cellulose (plants)

• The most abundant

polysaccharide in the world.

• The main structural component of plant cells.

• It is part of the cell wall and is a major component of wood.

• The rigid nature of this polymer makes it an excellent structural element.

• An unbranched linear polymer of D-glucose linked by β(1-4) glycosidic bonds.

• The only difference between cellulose and starch is the β- linkage.

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Structural Polysaccharides

 Cellulose (plants)

• The glucose β (1- 4) glucose bond are very stable and difficult to hydrolyzed by chemicals.

• A very few species of bacteria contain an enzyme that can hydrolyzed the β (1- 4) bond.

• Animals, such as cow that derive most of their nutrients from plant material, contain bacteria in their gut that can hydrolyze the glucose β (1- 4) glucose bond of cellulose.

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Structural Polysaccharides

 Chitin (invertebrates)

• The major structural component of the hard, shell- like exoskeleton of invertebrates, such as insects, lobsters, crabs, shrimp, and other shellfish; also occurs in cell walls of algae, fungi, and yeasts.

• composed of units of N-acetyl- β-D-glucosamine joined by β - 1,4-glycosidic bonds.

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References

• Biochemistry (Lippincott´s Illustrated Reviews series) by Champe P, Harvey R, and Ferrier D.

• Clinical Chemistry by Bishop M, Fody E and Schoeff L.

• Biochemistry, by Voet D and Voet J.

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http://www.wiley.com/college/voet/

047119350X/image_gallery/ch11/

index.html

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https://www.inkling.com/read/lippincotts- illustrated-biochemistry-ferrier-6th/chapter-7/

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