Chapter 12Chapter 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• aldose:aldose: a monosaccharide containing an aldehyde

group• ketose:ketose: a monosaccharide containing a ketone group

MonosaccharidesMonosaccharides• Monosaccharides are classified by their number

of carbon atoms

Hexose

Heptose

Octose

TrioseTetrose

Pentose

FormulaName

C3H6 O3C4H8 O4

C5H1 0O5

C6H1 2O6

C7H1 4O7C8H1 6O8

MonosaccharidesMonosaccharides• There are only two trioses

• often aldo- and keto- are omitted and these compounds are referred to simply as trioses

• although this designation does not tell the nature of the carbonyl group, it at least tells the number of carbons

Dihydroxyacetone (a ketotriose)

Glyceraldehyde (an aldotriose)

CHO

CHOH

CH2OH

CH2OH

C=O

CH2OH

MonosaccharidesMonosaccharides• Glyceraldehyde, the simplest aldose, contains a

stereocenter and exists as a pair of enantiomers

CHO

CH OH

CH2OH

CHO

C

CH2OH

HHO

MonosaccharidesMonosaccharides• Fischer projection:Fischer projection: a two dimensional

representation for showing the configuration of tetrahedral stereocenters• horizontal lines represent bonds projecting forward • vertical lines represent bonds projecting backward

CHO

CH OH

CH2OH

H OHCHO

CH2OH

convert to a Fischerprojection

D,L MonosaccharidesD,L Monosaccharides• In 1891, Emil Fischer made the arbitrary

assignments of D- and L- to the enantiomers of glyceraldehyde

• D-monosaccharide:D-monosaccharide: the -OH on its penultimate carbon is on the right

• L-monosaccharide:L-monosaccharide: the -OH on its penultimate carbon is on the left

L-GlyceraldehydeD-Glyceraldehyde

CHOCHO

H OH

CH2OH CH2OH

HHO

[]25 = +13.5°D

[]25 = -13.5°D

Table 19.1, p.468

Table 19.2, p.469

D,L MonosaccharidesD,L Monosaccharides• the most common D-tetroses and D-pentoses

• the three common D-hexoses

CH2OH

CHO

OHOHH

H

CH2OH

CHO

OHHHO

HH

CH2OH

CHO

OHOHH

OHH

CH2OH

CHO

OHHH

HOHH

D-Erythrose D-Threose D-Ribose 2-Deoxy-D-ribose

CHO

HOHH

HOOHH

CH2OHOHH

CHO

HOHH

HOHHO

CH2OHOHH

CH2OH

HHOC=O

OHH

CH2OHOHH

D-FructoseD-Glucose D-Galactose

Amino SugarsAmino Sugars• Amino 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-galactosamineCHO

OHOHHNH2

HH

HOH

CH2OH

CHO

OHOHHH

HH

HOH2N

CH2OH

CHO

OHOHHNHCCH3

HH

HOH

CH2OH

OCHO

OHHHNH2

HHOHO

H

CH2OH

4

2

D-Mannosamine(C-2 stereoisomer of D-glucosamine

D-Glucosamine D-Galactosamine(C-4 stereoisomer of D-glucosamine)

N-Acetyl-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

O-HH

O

CO O

H

H

O O-H

H4-Hydroxypentanal

A cyclic hemiacetal

14

14

redraw to show -OH and -CHO

close to each other

Haworth ProjectionsHaworth Projections• D-Glucose forms these cyclic hemiacetals

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

OHH OH

HHO

HH

OH

H

CH2OHO

D-Glucose

-D-Glucopyranose (-D-Glucose)

()

()

-D-Glucopyranose (-D-Glucose)

+

anomericcarbon

5

5

1

1

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

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 an 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 ketoses is C-2

Haworth ProjectionsHaworth Projections• In the terminology of carbohydrate chemistry,

• means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH

• means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH

• a six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose

PyranFuranOO

Haworth ProjectionsHaworth Projections• aldopentoses also form cyclic hemiacetals• the most prevalent forms of D-ribose and other

pentoses in the biological world are furanoses

OH ()

H

HOH OH

H HOHOCH2

H

OH ()

HOH H

H HOHOCH2

-D-Ribofuranose(-D-Ribose)

-2-Deoxy-D-ribofuranose(-2-Deoxy-D-ribose)

Haworth ProjectionsHaworth Projections• D-fructose also forms a five-membered cyclic

hemiacetal

HO

HOCH2 OH

HHO

CH2OH

OHH

H

C=O

CH2OH

HOH

CH2OH

OHH

HO HOH

HOHOCH2

HO HCH2OH

OH

D-Fructose

1

2

5

5

5

1

2

2

()

-D-Fructofuranose(-D-Fructose)

-D-Fructofuranose(-D-Fructose)

()

1

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

accurately represented as a chair conformationchair conformation

OCH2OH

HOHO

OHOH()

CHOH

HO

CH2OHOHHO

OHO

OH()HO

HO

CH2OHO

(-D-Glucose)

(-D-Glucose)

-D-Glucopyranose

-D-Glucopyranose

D-Glucose

anomericcarbon

Chair ConformationsChair Conformations• in both a Haworth projection and a chair conformation,

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

OCH2OH

HOHO

OHOH()H

H OH

HHO

HOH()

OH

H

CH2OHO

-D-Glucopyranose(chair conformation)

-D-Glucopyranose(Haworth projection)

123

4

5

6

1

23

4

5

6

Physical PropertiesPhysical Properties• Monosaccharides are colorless crystalline solids,

very soluble in water, but only slightly soluble in ethanol• sweetness relative to sucrose:

Carbohydrate

fructose

glucose

galactose

sucrose (table sugar)

lactose (milk sugar)

honey

SweetnessRelative to Sucrose

1.741.000.970.74

0.320.16

Artificial Sweetener

SweetnessRelative to Sucrose

maltose 0.33

saccharin 450acesulfame-K 200aspartame 180

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

almost exclusively in a cyclic hemiacetal form, with an alcohol gives an acetal

HH OH

HHO

HOH

OH

H

CH2OHO

CH3OHH+

-H2O

OCH2OH

H

OH

OCH3H

HOH

OHH

H

OCH2OH

H

OH

HH

HOH

OHH

OCH3

(-D-Glucose)-D-Glucopyranose

Methyl -D-glucopyranoside(Methyl -D-glucoside)

anomeric carbon

+

+

Methyl -D-glucopyranoside(Methyl -D-glucoside)

glycosidicbond

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

• 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

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

OHOH

HOHO

CH2OHO

CHOOHHHHOOHH

CH2OHOHH

NaBH4

CH2OHOHHHHOOHH

CH2OHOHH

D-Glucitol(D-Sorbitol)

D-Glucose-D-Glucopyranose

Reduction to AlditolsReduction to Alditols• 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

• these three alditols are also common in the biological world

CH2OH

CH2OH

OHHOHH

CH2OH

CH2OH

OHHHHOOHH

CH2OHHHOHHOOHH

CH2OHOHH

D-Mannitol XylitolErythritol

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• any carbohydrate that reacts with an oxidizing agent to

form an aldonic acid is classified as a reducing sugarreducing sugar (it reduces the oxidizing agent)

OCH2OH

HOHO

OHOH

COHHHHOOHH

CH2OHOHH

O HC

OHHHHOOHH

CH2OHOHH

O O-

oxidizingagent

D-GluconateD-Glucose-D-Glucopyranose(-D-Glucose)

basicsolution

Oxidation to Uronic AcidsOxidation to Uronic Acids• Enzyme-catalyzed oxidation of the primary

alcohol at C-6 of a hexose yields a uronic aciduronic acid• enzyme-catalyzed oxidation of D-glucose, for example,

yields D-glucuronic acid

CHO

CH2OH

OHHHHOOHHOHH

CHO

COOH

OHHHHOOHHOHH OH

OH

COOHO

HOHO

D-Glucose

enzyme-catalyzedoxidation

D-Glucuronic acid(a uronic acid)

D-Glucuronic AcidD-Glucuronic Acid• D-glucuronic acid is widely distributed in the plant and

animal world• in humans, it is an important component of the acidic

polysaccharides of connective tissues• it is used by the body to detoxify foreign phenols and

alcohols; in the liver, these compounds are converted to glycosides of glucuronic acid and excreted in the urine

OHOHO

OHO

COO-

HO

Propofol A urine-soluble glucuronide

Phosphate EstersPhosphate Esters• Mono- and diphosphoric esters are intermediates

in the metabolism of monosaccharides• for example, the first step in glycolysis is conversion of

D-glucose to -D-glucose 6-phosphate• note that at the pH of cellular and intercellular fluids,

both acidic protons of a phosphoric ester are ionized, giving it a charge of -2

CHO

CH2O-P-O-

OHHHHOOHHOHH

O-

O

CH2

OP O--O

OHO

HO

OHHO

O

D-Glucose 6-phosphate

DisaccharidesDisaccharides• Sucrose (table sugar)

• sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets

• sucrose is a nonreducing sugar

HOOH

OH

CH2OH

O

OH

HOO

CH2OH

HOCH2

OHO

HO

O

OH

CH2OH

OH

HOO

CH2OH

HOCH2

1

1

2

1

2

1

a unit of -D-glucopyranose

a unit of -D-fructofuranose

DisaccharidesDisaccharides• Lactose

• lactose is the principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk

• it consists of D-galactopyranose bonded by a -1,4-glycosidic bond to carbon 4 of D-glucopyranose

• lactose is a reducing sugar

O

OH

HOOH

O

CH2OH

O

HOOH

OH

CH2OHOOH O

OH

OH

CH2OH

O OH

OH

OH

CH2OH

1

1

4

4

-1,4-glycosidic bond

DisaccharidesDisaccharides• Maltose

• present in malt, the juice from sprouted barley and other cereal grains

• maltose consists of two units of D-glucopyranose joined by an -1,4-glycosidic bond

• maltose is a reducing sugar

OHO

HOOH

OOHO OH

OH

CH2OH

CH2OHO

OH

O

OHHO

O OH

HO

OH

CH2OH

HOCH2 1

4

-1,4-glycosidicbond

1 4

DisaccharidesDisaccharides• Maltose

• present in malt, the juice from sprouted barley and other cereal grains

• maltose consists of two units of D-glucopyranose joined by an -1,4-glycosidic bond

• maltose is a reducing sugar

OHO

HOOH

OOHO OH

OH

CH2OH

CH2OHO

OH

O

OHHO

O OH

HO

OH

CH2OH

HOCH2 1

4

-1,4-glycosidicbond

1 4

PolysaccharidesPolysaccharides• Polysaccharide:Polysaccharide: a carbohydrate consisting of

large numbers of monosaccharide units joined by glycosidic bonds

• 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

Fig 19.3, p.485

amylopectin

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

PolysaccharidesPolysaccharides• CelluloseCellulose 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• Cellulose (cont’d)

• humans and other animals cannot use cellulose as food because our digestive systems do not contain -glucosidases, enzymes that catalyze hydrolysis of -glucosidic bonds

• instead, we have only -glucosidases; hence, the polysaccharides we use as sources of glucose are starch and glycogen

• many bacteria and microorganisms have -glucosidases and can digest cellulose

• 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

Fig 19.4, p.486

cellulose

End End Chapter 12Chapter 12

CarbohydratesCarbohydrates