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Chapter 22 Carbohydrates

Atlantic codfish, which can survive in frigid water. A glycoprotein functions as an antifreeze in the bloodstream and body fluids of Arctic fish.

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CarbohydratesCarbohydrates are:

A major source of energy from our diet. Composed of the elements C, H and O.Also called saccharides, which means “sugars.”Produced by photosynthesis in plants.

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Photosynthesis and Respiration

Photosynthesis in plants requires CO2, H2O, and energy from the sun and produces carbohydrates, such as glucose.In respiration, glucose is oxidized in living cells to produce CO2, H2O, and energy.

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Types of CarbohydratesMonosaccharides are the simplest carbohydrates. Disaccharides consist of two monosaccharides.Polysaccharides contain many monosaccharides.

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Classification of MonosaccharidesAldoses are monosaccharides with an aldehydegroup and many hydroxyl (-OH) groups.Ketoses are monosaccharides with a ketone group and many hydroxyl (-OH) groups.

Aldose Aldose Ketose

CHO

HO H

CH2OH

CHO

HO H

OHH

CH2OH

CH2OH

O

HHO

OHH

OH H

OH

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MonosaccharidesMonosaccharides are also classified according to the number of carbon atoms.A triose has three carbons; a tetrose has four carbons; a pentose has five carbons; and a hexose has six carbons.

triose tetrose pentoseCHO

HO H

CH2OH

CHO

HO H

OHH

CH2OH

CH2OH

O

HHO

OHH

OH H

OH

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Identify each as tetrose, pentose or hexose, and as aldose or ketose.

A B

CHO

HO H

HHO

HHO

CH2OH

Learning Check

CH2OH

O

HHO

OHH

OHH

CH2OH

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Solution

A BPentose, aldose hexose, ketose

CHO

HO H

HHO

HHO

CH2OH

CH2OH

O

HHO

OHH

OHH

CH2OH

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22.2 Handedness in CarbohydratesCarbohydrates are chiral molecules since they have carbon atoms carrying four different groups.The simplest three carbon naturally occurring carbohydrate glyceraldehyde lack a plane of symmetry and exist as a pair of enantiomers– a right handed D form or a left handed L form.

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Two forms of glyceraldehyde (D and L) have the same physical properties except they behave differently in the presence of a polarized light.Two forms of glyceraldehyde rotate plane of a polarized light in the opposite direction by the same amount.An instrument known as Polarimeter can be used to measure the degree of rotation of the plane of a polarized light.

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Fig 22.1 Principles of a polarimeter, used to determine optical activity. A solution of an optically active isomer rotates the plane of the polarized light by a characteristic amount.

Animation

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In general, compounds with n chiral carbon atoms has a maximum of 2n possible steroisomers and half that many pair of enantiomers.Glucose, an aldohexose, has four chiralcarbon atoms and a total of 24 = 16 possible stereoisomers (8 pairs of enantiomers).

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Fig 22.2 Two pairs of enentiomers. The four isomeric aldotetroses, 2,3,4-trihydroxybutanals.

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22.3D and L Families of Sugars

By convention, the letter L is assigned to the structure with the —OH on the left of Fisher projection.The letter D is assigned to the structure with —OH on the right.

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In a Fisher projection, the aldehyde or ketonecarbonyl group of a monosaccharide is always placed at the top.

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Fischer ProjectionsA Fischer projection

Is used to represent carbohydrates.Places the most oxidized group at the top.Uses horizontal lines for bonds that come forward.Uses vertical lines for bonds that go back.

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D and L MonosaccharidesThe —OH on the chiral atom farthest from the carbonyl group is used to assign the D or L configuration.

L-Galactose

C

HO H

OHH

OHH

HHO

CH2OH

O

OHC

H OH

HHO

OHH

OHH

CH2OH

O

D-Glucose

H

CHOH OH

OHHOHH

CH2OH

D-Ribose

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Learning CheckIndicate whether each is the D or L isomer.

Ribose Threose Fructose

CHO

HO H

HHO

HHO

CH2OH

CHO

H OH

HHO

CH2OH

CH2OH

O

HHO

OHH

OH H

CH2OH

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SolutionIndicate whether each is the D or L isomer.

L-Ribose L-Threose D-Fructose

CHO

HO H

HHO

HHO

CH2OH

CHO

H OH

HHO

CH2OH

CH2OH

O

HHO

OHH

OH H

CH2OH

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D-Glucose

D-Glucose is: Found in fruits, corn syrup, and honey (23-41%) An aldohexose with the formula C6H12O6.Known as blood sugar in the body.

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Blood Glucose Level

In the body, glucose has a normal concentration of 70-90 mg/dL.In a glucose tolerance test, blood glucose is measured for several hours after ingesting glucose.

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D-Fructose

D-Fructose: Is a ketohexose C6H12O6.Is the sweetest carbohydrate.Is found in fruit juices and honey (31-44%).Converts to glucose in the body.

D-Fructose

CH2OH

C

C

O

C

H

OHH

HO

C

CH2OH

OHH

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D-GalactoseD-Galactose:

Is not found free in nature.Is obtained from lactose, a disaccharide.Is important in cellular membranes.

CHO

C

C

OH

C

H

HHO

HO

C

CH2OH

OHH

H

D-Galactose

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Learning CheckDraw the structure of D-fructose.

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SolutionDraw the structure of D-fructose.

D-Fructose

CH2OH

C

C

O

C

H

OHH

HO

C

CH2OH

OHH

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22.4 Structure of Glucose and other Monosaccharides

Stable cyclic hemiacetals form when the C=O group and the -OH are part of the same molecule.For hexoses, the hydroxyl group on C-5 reacts with the aldehydegroup or ketone group. The cyclic structure of a D-isomer has the last CH2OH group located above the ring.

oCH2OH

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Formation of Cyclic GlucoseThe Haworth structure can be written from theFischer Projection.

The –OH group on the left (C3) is drawn up.The –OH groups on the right (C2, C4) are drawn down.

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α and β Anomers for D-Glucose

β

α

The new –OH on C1 is drawn down for the αanomer, and up for the β anomer.

α-D-Glucose β-D-Glucose

OCH2OH

OHOH

OH

OH

OCH2OH

OHOH

OH

OH

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MutarotationIn solution, α-D-glucose is in equilibrium with β-D-glucose.Mutarotation involves the conversion of the cyclic anomers into the open chain.At any time, there is only a small amount of open chain .

α-D-glucose D-glucose (open) β-D-glucose(36%) (trace) (64%)

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22.5 Some Important MonosaccharidesCyclic Structure of Fructose

As a ketohexose, fructose forms a cyclic structure when the —OH on C-5 reacts with the C=O on C-2.

CH2OH

C

C

C

C

CH2OH

O

HO H

OHH

OHH

D-Fructose

CH2OH

OH

OH

OH

CH2OHO

CH2OH

OH

OH

CH2OH

OHO

α-D-Fructose β-D-Fructose

5

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Write the cyclic form of α-D-galactose.

Learning Check

C

C

C

C

C

CH2OH

OH

OHH

HO H

HHO

OHH

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α-D-galactose

o

OH

OH

CH2OH

OH

OH

Solution

C

C

C

C

C

CH2OH

OH

OHH

HO H

HHO

OHH

Write the cyclic form of α-D-galactose.

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22.6 Reactions of MonosaccharidesMonosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids. In the Benedict’s text, 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)

Animation

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Reduction of MonosaccharidesThe reduction of the carbonyl group produces sugar alcohols, or alditols.D-Glucose is reduced to D-glucitol also called sorbitol.

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Glycosides and Glycosidic BondsWhen a cyclic monosaccharide reacts with an alcohol:

A glycoside is produced.The bond is a glycosidic bond.

β-D-Glucose Methanol Methyl-β-D-glucoside

glycosidic bond

O

OH

OH

CH2OH

O

OH

CH3

HOCH3+

O

OH

OH

CH2OH

OH

OH

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Learning CheckWrite the products of the oxidation and reduction of D-mannose.

C

HO H

HHO

OHH

OHH

CH2OH

O

H

D-Mannose

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SolutionWrite the products of the oxidation and reduction of D-mannose.

D-Mannitol

OxidationReduction

CH2OH

HO H

HHO

OHH

OHH

CH2OH

C

HO H

HHO

OHH

OHH

CH2OH

O

OHC

HO H

HHO

OHH

OHH

CH2OH

O

H

D-Mannose

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22.7 DisaccharidesA disaccharide consists of two monosaccharides.

Disaccharide MonosaccharidesMaltose + H2O Glucose + GlucoseLactose + H2O Glucose + GalactoseSucrose + H2O Glucose + Fructose

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MaltoseMaltose is:

A disaccharide in which two D-glucose molecules are joined by an α-1,4-glycosidic bond.Obtained from starch.Used in cereals, candies, and brewing.

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Lactose

Lactose is:Also called milk sugar.Composed of galactose and glucose linked by a β-1,4-glycosidic bond.

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SucroseSucrose:

Is the disaccharide known as table sugar.Is composed of glucose and fructose molecules joined by α,β-1,2-glycosidic bond.

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Sweetness of SweetenersEach sweetener is compared to sucrose (table sugar), which is assigned a value of 100.

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Identify the monosaccharides in each as 1) glucose, 2) fructose 3) galactose.A. Lactose

B. Maltose

C. Sucrose

Learning Check

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Identify the monosaccharides in each as 1) glucose, 2) fructose 3) galactose.A. Lactose 1) glucose, 3) galactose

B. Maltose 1) glucose, 1) glucose

C. Sucrose 1) glucose, 2) fructose

Solution

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22.8 Variations on the Carbohydrate Theme

Several important variations on the carbohydrate theme incorporate the modified glucose molecules shown below. Their distinctive functional groups are highlighted in yellow.

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Hyaluronate molecules contain up to 25,000 disaccharide units and form a very viscous mixture that is the synovial fluid that lubricates joints. Chondroitin 6-sulfate (also the 4-sulfate) is present in tendons and cartilage, where it is linked to proteins. It has been used in artificial skin.

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Heparin is valuable medically as an anticoagulant.Heparin is made of a variety of monosaccharides, many of them containing sulfate groups.Heparin binds strongly to a blood-clotting factor and in this way prevents clot formation.

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Proteins that contain short carbohydrate chains are known as glycoproteins. The carbohydrate is connected to the protein by a glycosidic bond between an anomericcarbon and a side chain of the protein. The bond is either a C-N glycosidic bond or a C-O glycosidic bond.

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The shells of lobsters, beetles, and spiders are made of chitin, the second most abundant polysaccharide in the natural world. Cellulose is the most abundant.Chitin is a hard, structural polymer. It is composed of N-acetyl-D-glucosamine rather than glucose but is otherwise identical to cellulose.

► Glycoproteins have important functions at the surfaces of cells, they function as receptors for molecular messengers, other cells, pathogens, or drugs. They are also responsible for the familiar A, B, O system of typing blood.

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PolysaccharidesPolysaccharides are polymers of D-glucose Important polysaccharides are:Starch (Amylose and

Amylopectin)GlycogenCellulose

O

OH

CH2OH

OH

OH

OH

D-Glucose

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Amylose, Amylopectin, and Glycogen

Amylose is a continuous chain of glucose molecules linked by α-1,4 glycosidic bonds.

Amylopectin is a branched chain of glucose molecules linked by α-1,4- and α-1,6-glycosidic bonds.

Glycogen is similar to amylopectin, but more highly branched.

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Structures of Amylose and Amylopectin

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CelluloseCellulose is a polymer of glucose molecules linked by β-1,4 glycosidic bonds.Enzymes in saliva can hydrolyze α-1,4 glycosidicbonds in starch, but not β-1,4 glycosidic bonds in cellulose.

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Identify the types of glycosidic bonds in:1) Amylose

2) Glycogen

3) Cellulose

Learning Check

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Identify the types of glycosidic bonds in1) Amylose α-1,4 glycosidic bonds

2) Glycogen α-1,4- and α-1,6-glycosidic bonds

3) Cellulose β-1,4 glycosidic bonds

Solution

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Chapter SummaryMonosaccharides are compounds with 3 to 7 carbon atoms, an aldehyde group on carbpn 1 or a ketone on carbon 2, and hydroxyl group on all other carbon atom. Monosaccharides are chiral molecules. A monosaccharide with n chiral carbon atom may have 2n stereoisomers, half that number of pair of enantiomers.Fisher projection formula represent open-chain

structures of monosaccharides.

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In solution, open-chain monosaccharides with five or six carbons establish equilibria with cyclic forms that are hemeacetals.The hemiacetal carbon is refereed to as the anomericcarbon.Oxidation of a monosaccharide can results in a carboxyl group on the carbon 1.Reaction with an alcohol converts the –OH group on the anomeric carbon to a –OR group through a bond known as gycosidic bond.

Chapter Summary Cond.

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Disaccharides result from glycosidic bond formation between two monosaccharides. Polysaccharides result from glycosidic bond formation between many monosaccharides. Chitin is a hard structural polysaccharide found in the shells of lobster and insects.Heparin is a polysaccharide that plays role in blood clotting.Glycoproteins have short carbohydrate chains bonded to proteins; the carbohydrate segments function as receptors at cell surface.

Chapter Summary Cond.

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Cellulose is a fibrous substance that provides structure in plants. They consist entirely of several thousand β-units joined together in a long straight chain by β-1,4-links.Starch, like cellulose, is a polymer of glucose. Starch is fully digestible and is an essential part of human diet. In starch, glucose units are joined by α-1,4-links.Glycogen, also called animal starch, serves as the energy storage role as starch serves in plants.

Chapter Summary Cond.

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End of Chapter 22