Chapter 12 Carbohydrates Carbohydrates. Carbohydrates Carbohydrate: Carbohydrate: a...
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Transcript of Chapter 12 Carbohydrates Carbohydrates. Carbohydrates Carbohydrate: Carbohydrate: a...
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