chapter 16 carbohydrates denniston topping caret 6 th edition copyright the mcgraw-hill companies,...
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Chapter 16
Carbohydrates
Denniston Topping Caret
6th Edition
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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16.1 Types of Carbohydrates
• Carbohydrates are synthesized by photosynthesis in plants– Grains, cereals, bread, sugar cane
• Glucose is major energy source– A gram of digested carbohydrate gives about 4 kcal of
energy
– Complex carbohydrates are best for diet
– USDA recommends about 58% daily calories from carbohydrates
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Basic Carbohydrate Types• Monosaccharides
– e.g., glucose, fructose– One sugar (saccharide) molecule
• Disaccharides– e.g., sucrose, lactose– Two monosaccharides linked together– Linkage is called a glycosidic bond
• Oligosaccharides– Three to ten monosaccharides linked by glycosidic
bonds
• Polysaccharides– e.g., starch, glycogen, cellulose– Chains of linked monosaccharide units16
.1 T
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16.2 Monosaccharides
• Monosaccharides are composed of:– Carbon– Hydrogen– Oxygen– Basic Formula = (CH2O)n n = any integer 3 – 7
• Many monosaccharides also contain chemical modifications– Amino groups– Phosphate groups
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Naming Monosaccharides
Named on the basis of • Functional groups
– Ketone carbonyl = ketose– Aldehyde carbonyl = aldose
• Number of carbon atoms in the main skeleton– 3 carbons = triose– 4 carbons = tetrose– 5 carbons = pentose– 6 carbons = hexose
• Combining both systems gives even more information16.2
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16.3 Stereoisomers and Stereochemistry
• Prefixes D- and L- in a monosaccharide name identify one of two isomeric forms– These isomers differ in the spatial arrangement of
atoms and are stereoisomers
• Stereochemistry is the study of different spatial arrangements of atoms
• The stereoisomers D- and L- glyceraldehyde are nonsuperimposable mirror image molecules and are called enantiomers (a subset of stereoisomers)
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Enantiomers16
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Chirality• A carbon atom that has four different
groups bonded to it is called a chiral carbon atom
• Any molecule containing a chiral carbon can exist as a pair of enantiomers
• Chirality in glyceraldehyde (the simplest
carbohydrate) is conveyed by a chiral carbon
• Larger biological molecules often have more than one chiral carbon16
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• Glyceraldehyde has a chiral carbon and thus, has two enantiomers – The D isomer has the -OH on the stereocenter to the
right– The L isomer has the -OH on the stereocenter to the left
C
C
O H
CH2OH
OHH
C
C
O H
CH2OH
HOH
the D isomer the L isomer
Mirrorplane
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Chiral Carbon:connected to
four different atoms
or groups
Chirality of Glyceraldehyde
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Structural Formulas of D- and L- Glyceraldehyde
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Optical Activity
• Enantiomers are also called optical isomers
• Enantiomers interact with plain polarized light to rotate the plane of the light in opposite directions – This interaction with polarized light is called
optical activity – Optical activity distinguishes the isomers – It is measured in a device called a polarimeter
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Polarized Light
• Normal light vibrates in an infinite number of directions perpendicular to the direction of travel– When the light passes through a polarizing
filter (Polaroid sunglasses) only light vibrating in one plane reaches the other side of the filter
– A polarimeter allows the determination of the specific rotation of a compound
• Measures its ability to rotate plane-polarized light16
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Schematic Drawing of a Polarimeter
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The Relationship Between Molecular Structure and Optical Activity
• When an enantiomer in a solution is placed in the polarimeter, the plane of rotation of the polarized light is rotated– One enantiomer always rotates light in a clockwise
(+) direction • This is the dextrorotatory isomer
– The other isomer rotates the light in a counterclockwise (-) direction
• It is the levorotatory isomer
• Under identical conditions, the enantiomers always rotate light to exactly the same degree, but in opposite directions16
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Fischer Projection Formulas
• A Fischer projection uses lines crossing through a chiral carbon to represent bonds– Projecting out of the page (horizontal lines)
– Projecting into the page (vertical lines)
• Compare the wedge to the Fischer diagrams
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C
C
O H
C
OHH
CH2OH
H OH
CH2OH
C
C
O
OH
C OHH
C
H
H OH
CH2OHD-erythrosean aldotetrose
D-fructosea ketohexose
12
3
4
1
23
4
5
6
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Monosaccharides
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The D- and L-System
• Monosaccharides are drawn in Fischer projections – With the most oxidized carbon closest to the top– The carbons are numbered from the top– If the chiral carbon with the highest number has the OH to the
right, the sugar is D – If the OH is to the left, the sugar is L
• Most common sugars are in the D form
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CHO
C
CH2OH
OHH
CHO
C
CH2OH
HOH
the D isomer the L isomer
CHO
CH2OH
OHH
CHO
CH2OH
HOH
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16.4 Biological Monosaccharides
• Glucose is the most important sugar in the human body– Found in many foods– Several common names include: dextrose and blood
sugar– Its concentration in the blood is regulated by insulin
and glucagon• Under physiological conditions, glucose exists in a
cyclic hemiacetal form where the C-5 hydroxyl reacts with the C-1 aldehyde group– Two isomers are formed which differ in the location of
the -OH on the acetal carbon, C-1• An aldohexose with molecular formula C6H12O6
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Cyclic Form of Glucose• The cyclic form of glucose is shown as a
Haworth projection
OCH2OH
HH
OHH
OH
OH
HOH
H
OCH2OH
HH
OHH
OH
OH
H O
HH
OCH2OH
HH
OHH
OH
OH
HH
OH
form(alpha)
form(beta)
arrows showelectron movement
Pyranose ring form
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Cyclization of Glucose 16
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Fructose• Fructose is also called:
– Levulose– Fruit sugar
• Found in large amounts in:– Honey – Corn syrup– Fruits
• The sweetest of all sugars
• KetohexoseD-fructose
CH2OHCC
OOH
C OHHC
H
OHCH2OH
H
1
2
3
4
5
6
O CH2OHCH2OH
HOHH
OH
O
CH2OH
CH2OH
HOHH
OH
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Galactose Galactose is the principal sugar found in
mammalian milk Aldohexose very similar to glucose -D-galactosamine is a component of the
blood group antigens
-D-galactose
CHOCC
OHOH
COH HC
H
OHCH2OH
H
H
1
2
3
4
5
6
OCH2OH
H
H
OH
H
OH
OH
HOH
H
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Glucose and galactose differ only in the orientation of one hydroxyl group
Galactose Orientation
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Ribose and Deoxyribose, Five-Carbon Sugars
• Components of many biologically important molecules
• Exists mainly in the cyclic form• Aldopentose
OHCCH2OH
H
CHO
CC
H OHH OH
CH2OH
HH
OH
H
OH
O H
OH
-D-ribose
Deoxyribose has an -H here replacing the -OH
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Reducing Sugars
+ Cu2O (red-orange)
• The aldehyde groups of aldoses are oxidized by Benedict’s reagent, an alkaline copper(II) solution
• The blue color of the reagent fades as reaction occurs reducing Cu2+ to Cu+ with a red-orange precipitate forming as Cu2O results
• Test can measure glucose in urine
CC
O H
CH2OHOHH
CC
O O
CH2OHOHH+2 Cu2+16
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Reducing Sugars• All monosaccharides and the disaccharides
except sucrose are reducing sugars• Ketoses can isomerize to aldoses and react also
D-glucose
CHCC
OHOH
C OHHC
H
H OHCH2OH
HO
D-fructose
CH2OHCC
OOH
C OHHC
H
H OHCH2OH
CHCC
OHOH
C OHHC
H
H OHCH2OH
OH
enediol
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A Reduced Sugar• The most important reduced sugar is deoxyribose
CC
O H
CHH
CH OH
OHHCH2OH
D-deoxyribose
-D-2-deoxyribose
O OH
H
CH2OH
HH
OH
H
H
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16.5 Biologically Important Disaccharides
• The anomeric -OH can react with another -OH on an alcohol or sugar• Process is forming a glycosidic bond• Water is lost to form an acetal
O
CH2OH
HH
OHH
OH
OH
H
H
OH
+ CH3 OH
O
CH2OH
HH
OHH
OH
OH
H
H
O CH3
+ H2O
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Maltose• Maltose is formed by linking two -D-
glucose molecules to give a 1,4 glycosidic linkage
• Maltose is malt sugar• Formed as an intermediate in starch hydrolysis• Reducing sugar due to the hemiacetal hydroxyl
OH
OCH2OH
HH
H
OH
OH
H
H
O
OCH2OH
HH
OHH
OH
OH
HH
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Formation of Maltose16
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Lactose• Lactose is formed by joining D-galactose to
-D-glucose to give a 1,4-glycoside• Lactose is milk sugar
– For use as an energy source, must be hydrolyzed to glucose and galactose
– Lactose intolerance results from lack of lactase to hydrolyze the glycosidic link of lactose
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Lactose Glycosidic Bond
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Galactosemia• In order for lactose to be used as an energy
source, galactose must be converted to a phosphorylated glucose molecule
• When enzymes necessary for this conversion are absent, the genetic disease galactosemia results
• People who lack the enzyme lactase (~20%) are unable to digest lactose and have the condition lactose intolerance
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Sucrose• Sucrose is formed by linking D-glucose with D-fructose to give a 1,2 glycosidic linkage
– Nonreducing – negative reaction in Benedict test – The glycosidic O is part of an acetal and a ketal
• Important plant carbohydrate– Water soluble– Easily transported in plant circulatory system
• Cannot by synthesized by animals• Sucrose called:
– Table sugar– Cane sugar– Beet sugar– Linked to dental caries
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Glycosidic Bond Formed in Sucrose
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16.6 PolysaccharidesStarch• Starches are storage forms of glucose found in
plants• They are polymers of linked glucose• If the links are:
– Only 1,4 links, the polymer is linear = amylose• Amylose usually assumes a helical configuration
with six glucose units per turn• Comprises about 80% of plant starch
– Both 1,4 and 1,6 links then, the polymer structure is branched = amylopectin
• Highly branched with branches of approximately 20-25 glucose units
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Structure of Amylose16
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Comparison of Amylose to Amylopectin
amylopectin(-1,6 link)
()
O
OCH2
HH
OH
OH
OH
HH
OHO
CH2
HH
OH
OH
OH
HH
OH
OCH2
HH
OH
OH
OH
HO
HOH
OCH2
HH
OH
OH
OH
HHO
CH2
HH
OH
OH
OH
HH
OHO
CH2
HH
OH
OH
OH
HH
OH
(
amylose(-1,4 links)
()
OHO
CH2
HH
OH
OH
OH
HO
HOH
OCH2
HH
OH
OH
OH
HHO
CH2
HH
OH
OH
OH
HH
OHO
CH2
HH
OH
OH
OH
HH
OH
16.6
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Glycogen
• The major glucose storage carbohydrate in animals is glycogen
• A highly branched chain polymer like amylopectin – More frequent branching – 10 monomers
• Glycogen is stored in: – Liver – Muscle cells16
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Cellulose• Cellulose is the major structural polymer in plants• It is a liner homopolymer composed of -D-
glucose units linked -1,4 • The repeating disaccharide of cellulose is -
cellobiose • Animals lack the enzymes necessary to hydrolyze
cellulose • The bacteria in ruminants (e.g., cows) can digest
cellulose so that they can eat grass, etc.
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Structure of Cellulose
OO
CH2OH
HH
OH
OH
OH
H HOCH2OH
HH
H
OH
OH
H HOO
CH2OH
HH
OH
OH
OH
H H
-(1->4) glycosidic bond
16.6
Pol
ysac
char
ides
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Glycogen and Amylopectin Structures
Glycogen andAmylopectin are(1-4) chains withwith (1-6)branches
Amylopectin Glycogen
16.6
Pol
ysac
char
ides