Download Chapter 25 Biomolecules: Carbohydrates

Chapter 25
Biomolecules: Carbohydrates
The Importance of Carbohydrates
• Carbohydrates are…
– widely distributed in nature.
– key intermediates in metabolism (sugar).
– structural components of plants (cellulose).
– key components of industrial products (wood,
fibers).
– key components of food sources (sugar,
flour).
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Chemical Formula
• Carbohydrates are highly oxidized.
– They have approximately as many oxygen atoms as
carbon atoms.
• Carbons of carbohydrates are usually bond to
an alcohol and hydrogen atom; therefore, the
empirical formula is roughly (C(H2O))n.
H OH
HO
HO
HO
H
H OH
OH
H
D+ Glucose
(C6H12O6)
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Sources of Carbohydrates
• Glucose is produced in plants from CO2
and H2O via photosynthesis.
• Plants convert glucose into other small
sugars and polymers (cellulose, starch).
• Dietary carbohydrates provide the major
source of energy required by organisms.
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Classifications of Carbohydrates
• Monosaccharide: simple sugars that can not be
converted into smaller sugars by hydrolysis
• Carbohydrate (Oligosaccharide,
Polysaccharide): two or more simple sugars
connected as acetals
• Sucrose: disaccharide of two monosaccharides
(glucose linked to fructose)
• Cellulose: polysaccharide of several thousand
glucose units connected by acetal linkages
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Aldose and Ketose
• The prefixes aldo- and keto- identify the
nature of the carbonyl group.
– Aldo: carbonyl is located at the end of the
chain
– Keto: carbonyl is located within the chain
• The suffix -ose denotes a carbohydrate.
• The number of carbons is indicated by the
root.
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Aldose and Ketose
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Fischer Projections
• Carbohydrates have multiple chiral
centers.
• A chiral center carbon is projected into the
plane of the paper and other groups are
drawn as horizontal and vertical lines.
• The oxidized end of the molecule is
always “up” on the paper.
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Fischer Projections
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Minimal Fischer Projections
• In order to work with the structure of an aldose
more easily, only the essential components are
shown.
• An alcohol is designated by a “-” and a carbonyl
is designated by an “↑”.
• The terminal OH in the CH2OH is not shown.
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Stereochemical References
• The reference compounds for
stereochemistry are the two enantiomers
of glyceraldehyde (C3H6O3).
• The stereochemistry depends on the
hydroxyl group attached to the chiral
center farthest from the oxidized end of
the sugar.
– D: hydroxyl group is on the right
– L: hydroxyl group is on the left
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Stereochemical References
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The “D” Sugar Family
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D and L Sugars
• The two enantiomers of glyceraldehyde were
first identified by their opposite rotation of plane
polarized light.
• Naturally occurring glyceraldehyde rotates light
in a clockwise rotation and is denoted as “+”.
• The enantiomer rotates light counterclockwise
and is denoted as “-”.
• The direction of the rotation of light does not
correlate to structural features.
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Configurations of Aldoses
• Because R and S designations are difficult
to work with when multiple chiral centers
are present, the D,L designations are used
with aldoses.
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Aldotetrose
• Aldotetroses have two chiral centers; therefore,
there are two pairs of enantiomers.
• There and four sterioisomeric aldotertroses.
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Aldopentose
• Aldopentoses have three chiral centers, four
enantiomers and eight stereoisomer.
• Only D enantiomers are shown.
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Aldohexose
• Aldohexose has eight pairs of
enantiomers: allose, altrose, glucose,
mannose, gulose, idose, galactose, talose.
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Hemiacetal Formation
• Alcohols add reversibly to aldehydes and
ketones to form hemiacetals.
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Hemiacetals in Sugar
• Intramolecular nuclephillic addition creates a cyclic
hemiacetal in sugars.
• Five- and six-membered rings are stable.
• The formation of a cyclic hemiactal creates an additional
chiral center creating two diasteromeric forms called
anomer, which are designated α and β.
– α: the OH at the anomer center is on the same side as the
hydroxyl that determines D,L naming in the Fischer projection
– β: the OH at the anomer center is on the opposite side of the
hydroxyl that determines D,L naming in the Fischer projection
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Fischer Projections of Anomers
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Williamson Ether Synthesis
• Treatment with a alkyl halide in the
presence of a base
• Silver oxide is used as a catalyst for basesensitive compounds.
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Glycosides
• Carbohydrate acetals are named by
sighting the alkyl group and replacing the
-ose ending of the sugar with -oside.
• Glycosides are stable in water; therefore,
they require an acid catalyst for hydrolysis.
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Glycoside Formation
• Treatment of a monosaccharide
hemiacetal with an alcohol and an acid
catalyst yields an acetal in which the
anomeric -OH has been replace with an
-OR group.
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Reduction of Monosaccharides
• Treatment of an aldose or ketose with
NaBH4 reduces it to a polyalcohol (alditol).
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Oxidation of Monosaccharides
• Br2 in water is an effective oxidizing
reagent for converting an aldose to an
aldonic acid (carboxylic acid).
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Maltose and Cellobiose
• Maltose: two D-glycopyranose units with a 1,4’-αglycoside bond
– Formed from the hydrolysis of starch
• Cellobiose: two D-glycopyranose units with a 1,4’-βglycoside bond
– Formed from the hydrolysis of cellulose
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Lactose
• Lactose: 1,4-D-galactopyranosyl-Dglucopyranoside
• Lactose is a disaccharide that occurs naturally in
milk.
• Lactose is cleaved during digestion to form
glucose and galactose.
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Sucrose
• A disaccharide that hydrolyzes to glucose
and fructose.
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Cellulose
• Cellulose: thousands of D-glucopyranosyl
1-4’-β-glucopyranosides
• Cellulose molecules form a large
aggregate structure held together by
hydrogen bonds.
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Starch
• Starch: 1,4--glupyranosylglucopyranoside polymer
• Starch is digested into glucose
• Starch is made of two components
– Amylose
• insoluble in water – 20% of starch
– Amylopectin
• soluble in water – 80% of starch
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Glycogen
• Glycogen is a polysaccharide that serves
the same energy storage function in
animals that starch does in plants.
• Glycogen is highly branched and contain
up to 100,000 glucose units.
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