Chapter 12 Chapter 12 CarbohydratesCarbohydrates

CarbohydratesCarbohydrates

Carbohydrate:Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.

Monosaccharide:Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula

CCnnHH2n2nOOnn, where nn varies from 3 to 8.

• AldoseAldose:: A monosaccharide containing an aldehyde group.

• KetoseKetose:: A monosaccharide containing a ketone group.

MonosaccharidesMonosaccharidesThe suffix -ose-ose indicates that a molecule is a

carbohydrate.The prefixes tri-tri-, tetratetra, pentapenta, and so forth

indicate the number of carbon atoms in the chain.

Those containing an aldehyde group are classified as aldosesaldoses. .

Those containing a ketone group are classified as ketosesketoses..

There are only two trioses:

MonosaccharidesMonosaccharides There are only two trioses:

◦ Often aldo- and keto- are omitted and these compounds are referred to simply as trioses.

◦ Although “triose” does not tell the nature of the carbonyl group, it at least tells the number of carbons.

HC

HC

H2C OH

O

Glyceraldehydean aldotrioses

OH C

H2C

H2C OH

OH

O

Dihydroxyacetonea ketotrioses

MonosaccharideMonosaccharide Monosaccharides with

◦ three carbons: trioses◦ Five carbons: pentose◦ Six carbons: hexose ◦ And so on …

M M M M M M

Polysaccharide

hydrolysisn M

monosaccharide

MonosacharidesMonosacharides

Figure 12.1 Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.

EnantiomersEnantiomers Enantiomers: a molecule has a nonsuperimposable

mirror image◦ Chiral molecule – has four different groups

MonosaccharidesMonosaccharidesFischer projection:Fischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters.• Horizontal lines represent bonds projecting forward

from the stereocenter. • Vertical lines represent bonds projecting to the rear.• Only the stereocenter is in the plane.

(R)-Glyceraldehyde(3-D representation)

(R)-Glyceraldehyde(Fisher projection)

MonosacharidesMonosacharidesIn 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.

• D-monosaccharide:D-monosaccharide: the –OH is attached to the bottom-most assymetric center (the carbon that is second from the bottom) is on the right in a Fischer projection.

HC

HC

H2C OH

O

OH

achiral carbon

D-Glyceraldehyde

MonosacharidesMonosacharides

• L-monosaccharide:L-monosaccharide: the -OH is on the left in a Fischer projection.

CH

HC

H2C OH

O

HO

achiral carbon

L-Glyceraldehyde

Table 20-1 p532

Table 12.1

Table 20-2 p532

Table 12.2

ExamplesExamples Draw Fisher projections for all 2-ketopentoses.

Which are D-2-ketopentoses, which are L-2-ketopentoses? Prefer to table 12.2 (your textbook) to write their names

Amino SugarsAmino SugarsAmino sugars contain an -NH2 group in place of an -OH group. • Only three amino sugars are common in nature: D-

glucosamine, D-mannosamine, and D-galactosamine. N-acetyl-D-glucosamine is an acetylated derivative of D-glucosamine.

Cyclic StructureCyclic Structure• Aldehydes and ketones react with alcohols to form

hemiacetalshemiacetals • Cyclic hemiacetals form readily when the hydroxyl and

carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring.

EpimersEpimers Diastereomers that differ in configuration at only on

asymmetric center

HC O

OHH

OHH

OH

CH2OH

H

HC O

HHO

OHH

OH

CH2OH

H

D-ribose D-arabinose

1

2

3

4

5

1

2

3

4

5

C2-epimers*dif ferent conf iguration at C2

HC O

HHO

OHH

HHO

HC O

HHO

HHO

OHHO

CH2OH CH2OH

H OH OHH

C3-epimers*dif ferent conf iguration at C3

D-iodose D-talose

Haworth ProjectionsHaworth Projections• Figure 12.2 D-Glucose forms these two cyclic hemiacetals.

D-glucose

Β-D-Glucopyranoseβ-D-Glucose

-D-Glucopyranose-D-glucose

Same side

Haworth ProjectionsHaworth Projections• A five- or six-membered cyclic hemiacetal is represented

as a planar ring, lying roughly perpendicular to the plane of the paper.

• Groups bonded to the carbons of the ring then lie either above or below the plane of the ring.

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

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

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

Haworth ProjectionsHaworth Projections

In the terminology of carbohydrate chemistry,

◦ A six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose because these ring sizes correspond to the heterocyclic compounds furan and pyran.

Haworth ProjectionsHaworth Projections◦ Aldopentoses also form cyclic hemiacetals.◦ The most prevalent forms of D-ribose and other pentoses

in the biological world are furanoses.

◦ The prefix “deoxydeoxy” means “without oxygen.” at C2

-D-Ribofuranose-D-Ribose

β-2-Deoxy-D-ribofuranose

Β-2-Deoxy-D-ribose

Haworth ProjectionsHaworth ProjectionsD-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal.

-D-Fructofuranose-D-Fructose

D-Fructose β-D-Fructofuranoseβ-D-Fructose

ExamplesExamples Give structure of the cyclic hemiacetal formed by

◦ 4-hydroxybutanal

◦ 5-hydroxypentanal

Chair ConformationsChair Conformations• For pyranoses, the six-membered ring is more accurately

represented as a strain-free chair conformationstrain-free chair conformation.

β-D-Glucopyranose

D-Glucose

-D-Glucopyranose

Chair ConformationsChair Conformations• In both Haworth projections and chair conformations, the

orientations of groups on carbons 1- 5 of -D-glucopyranose are up, down, up, down, and up.

O

CH2OH

OH

OH

OH

1

23

4

5

6

OH

HOH2C

HOHO

OH123

45

6

-D-glucose

OH

same = cis

Chair ConformationsChair Conformations

O

OH

CH2OH

OH

OH

OH

HOH2C

1

23

4

5

6

HOHO

OHOH

123

45

6

D-glucose

opposite = trans

ExamplesExamples Which OH groups are in the axial position in β-D-mannopyranose

β-D-idopyranose

MutarotationMutarotation Mutarotation: Mutarotation: The change in specific rotation that

accompanies the equilibration of - and -anomers in aqueous solution.◦ Example: When either -D-glucose or -D-glucose is

dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.

β-D-Glucopyranose

β-D-Glucopyranose D-Glucose

-D-Glucopyranose

Formation of GlycosidesFormation of Glycosides• Treatment of a monosaccharide, all of which exist almost

exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.

Glycosidic bond

Glycosidic bond

β-D-Glucopyranose

β-D-Glucose

Methyl β-D-glucopyranoside

Methyl β-D-glucoside

Methyl -D-glucopyranoside

Methyl -D-glucoside

Formation of GlycosidesFormation of Glycosides

• A cyclic acetal derived from a monosaccharide is called a glycosideglycoside.

• The bond from the anomeric carbon to the -OR group is called a glycosidic bondglycosidic bond.

• Mutarotation is not possible for a glycoside because an acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound.

Formation of GlycosidesFormation of Glycosides• Glycosides are stable in water and aqueous

base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide.

• Glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -ee is replaced by -ide-ide.

ExamplesExamples Draw a Haworth projection and a chair conformation

for methyl -D-mannopyranoside. Label the anomeric carbon and glycosidic bond

Reduction to AlditolsReduction to Alditols• The carbonyl group of a monosaccharide can be reduced

to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2 in the presence of a transition metal catalyst.• The reduction product is called an alditolalditol.• Alditols are named by changing the suffix -ose-ose to -itol-itol

AlditolsAlditols The product formed when the CHO group of

monosaccharide is reduced to CH2OH group

OHO

HO

HOH2C

OH

OH

CHO

OHH

HHO

OHH

OHH

CH2OH

CH2OH

OHH

HHO

OHH

OHH

CH2OH

D-Glucopyranose

D-Glucose D-GlucitolD-Sorbitol

NaBH4

•Sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae.•It is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics.

AlditolsAlditols These three alditols are also common in the

biological world. Note that only one of these is chiral.

Erythritol D-Mannitol Xylitol

Oxidation to Aldonic AcidsOxidation to Aldonic Acids

• The aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion.

• The oxidation product is called an aldonic acidaldonic acid.• A carbohydrate that reacts with an oxidizing agent to

form an aldonic acid is classified as a reducing sugarreducing sugar (it reduces the oxidizing agent).• Itself is being oxidized

H

C

C O

R

OHH

2-Ketose

C

C

HO H

R OH

C

C

O HC

C

O O

R

OHOHH

R

H

-OH -OH

enediol aldose aldonate

Oxidizingagent

• 2-Ketoses (e.g. D-fructose) are also reducing sugars.

Oxidation to Aldonic AcidsOxidation to Aldonic Acids

Oxidation to Aldonic AcidsOxidation to Aldonic Acids

β-D-Glucopyranose

D-GlucoseD-Gluconatean aldonic

acid

Oxidation to Aldonic AcidsOxidation to Aldonic Acids• The body uses glucuronic acid to detoxify foreign

alcohols and phenols.• These compounds are converted in the liver to

glycosides of glucuronic acid and then excreted in the urine.

• The intravenous anesthetic propofol is converted to the following water-soluble glucuronide and excreted.

Formation of Phosphoric Formation of Phosphoric estersesters

CHO

OHH

HHO

OHH

OHH

CH2OH

D-Glucose

Enzyme-catalyzed phosphorylation

CHO

OHH

HHO

OHH

OHH

CH2O

D-Glucose 6-phosphate

P

O

O

O

OHO

HO

H2C

OHOH

O

P

O

OO

-D-Glucose 6-phosphate

What are Disaccharides and What are Disaccharides and Oligosaccharides?Oligosaccharides? Disaccharide: A carbohydrate containing two

monosaccharide units joined by a glycosidic bond

Oligosaccharide: A carbohydrate containing from six to ten monosaccharide units, each joined to the next by glycosidic bond

Polysaccharide:Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.

SucroseSucrose• Table sugar, obtained from the juice of sugar cane and

sugar beet.

-1,2-Glycosidic

bond

Sucrose

LactoseLactose The principle sugar present in milk.

◦ About 5 - 8% in human milk, 4 - 5% in cow’s milk.◦ Has no sweetness

β-1,4-Glycosidic bond

β-1,4-Glycosidic bond

Lactose

MaltoseMaltose• From malt, the juice of sprouted barley and other

cereal grains.-1,4-

Glycosidic bond

Maltose

PolysaccharidesPolysaccharides

Starch:Starch: A polymer of D-glucose.• Starch can be separated into amylose and

amylopectin.• Amylose is composed of unbranched chains of

up to 4000 D-glucose units joined by -1,4-glycosidic bonds.

• Amylopectin contains chains up to 10,000 D-glucose units also joined by -1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by -1,6-glycosidic bonds.

PolysaccharidesPolysaccharides• Figure 12.3 Amylopectin is a branched polymer of D-

glucose units joined by -1,4-glycosidic bonds. Branches consist of D-glucose units that start with an -1,6-glycosidic bond.

-1,6-Glycosidic bond

-1,4-Glycosidic bonds

PolysaccharidesPolysaccharides

• GlycogenGlycogen is the energy-reserve carbohydrate for animals.• Glycogen is a branched polysaccharide of approximately

106 glucose units joined by -1,4- and -1,6-glycosidic bonds.

• The total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle.

PolysaccharidesPolysaccharidesCelluloseCellulose is a linear polysaccharide of D-glucose units joined by -1,4-glycosidic bonds.• It has an average molecular weight of 400,000

g/mol, corresponding to approximately 2200 glucose units per molecule.

• Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds.

• This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength.

• It is also the reason why cellulose is insoluble in water.

PolysaccharidesPolysaccharides• Figure 12.4 Cellulose is a linear polysaccharide of D-

glucose units joined by -1,4-glycosidic bonds.

β-1,4-Glycosidic bonds

PolysaccharidesPolysaccharides

Cellulose (cont’d)◦ Humans and other animals can not digest cellulose

because their digestive systems do not contain -glycosidases, enzymes that catalyze the hydrolysis of -glycosidic bonds.

◦ Termites have such bacteria in their intestines and can use wood as their principal food.

◦ Ruminants (cud-chewing animals) and horses can also digest grasses and hay.

◦ Humans have only -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen.

◦ Many bacteria and microorganisms have -glucosidases.

ExampleExample Draw a chair conformation for a disaccharide in

which two units of D-glucopyranose are joined by a β -1,3-glycosidic bond

Acidic PolysaccharidesAcidic PolysaccharidesAcidic polysaccharides:Acidic polysaccharides: a group of polysaccharides that contain carboxyl groups and/or sulfuric ester groups, and play important roles in the structure and function of connective tissues.• There is no single general type of connective tissue.• Rather, there are a large number of highly

specialized forms, such as cartilage, bone, synovial fluid, skin, tendons, blood vessels, intervertebral disks, and cornea.

• Most connective tissues are made up of collagen, a structural protein, in combination with a variety of acidic polysaccharides.

Acidic PolysaccharidesAcidic PolysaccharidesHeparin (cont’d)◦ Heparin is synthesized and stored in mast cells of

various tissues, particularly the liver, lungs, and gut.

◦ The best known and understood of its biological functions is its anticoagulant activity.

◦ It binds strongly to antithrombin III, a plasma protein involved in terminating the clotting process.

HeparinHeparin• Figure 12.5 The repeating pentasaccharide unit of

heparin.

p546