3 major classes of carbohydrates

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Structure and Catabolism of Carbohydrates
3 major classes of carbohydrates
a monosaccharides b disaccharides c
polysaccharides
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(note technically the simplest compound with the
empirical formula of the class (CH2O)n when n
is 1 is H2C O (formaldehyde) and when n is
2 the compound is glycolic acid (an ?-hydroxy
acid used in skin creams)
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In metabolism the simplest monosaccharides are
Trioses
Glyceraldehyde and dihydroxyacetone have the same
atomic composition (C3H603)    They are
structural isomers of each other (differ in
location of their hydrogen and double
bonds) They can interconvert via an unstable
enediol intermediate but since the reaction is
very slow unless catalyzed, glyceraldehyde and
dihyroxyacetone each exist as very stable
compounds
There are two forms of a triose This will be a
common theme in carbohydrate metabolism carbohydr
ates are aldehyde or ketone compounds
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The second carbon of glyceraldehyde is chiral

• OH can be on the right or left of the chiral
  carbon
• Right D
• Left L

These isomers are stereoisomers
stereoisomers chiral molecules with different
configurations about at least one of their
asymmetric centers, but other wise identical
There are two kinds of stereoisomers
enantiomers are stereoisomers with two
nonsuperimposable mirror images of one another
(the same composition and order of atomic
connections but different molecular arrangements
in space) Enantiomers are also called optical
isomers
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Is there a stereoisomer of dihydroxyacetone?
Q Where do the designations D and L come from
in describing enantiomers? A Originally, D
(dextro ie right) and L (laevo ie left)
indicated the direction of rotation of the plane
of polarization of polarized light note in
carbohydrates the D-monosaccharides predominate
(why?) (but some L-monosaccharides are found)
Q What happens when monosaccharides have more
than three carbons? A We get another structural
complication. There may be more than one chiral
carbon This results in having two types of
stereoisomers one type is an enantiomer the
other type is a diastereoisomers   diastereoisomer
s are stereoisomers that are not mirror images of
each other and have different properties (melting
point, boiling point, solubility etc.)
Diastereoisomers are first encountered in the
tetroses
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Tetroses adding another carbon to make a tetrose
Monosaccharides are built up by the successive
additions of one CHOH group just below the
carbonyl carbon Other groups move down When
this happens, what used to be the number 2 carbon
in D-glyceraldehyde is now the number 3 carbon in
both aldotetroses and there is a new chiral
carbon at position 2
The CHOH group that was added (and is now the
number 2 carbon) has two different arrangements
in space These two different arrangements are
designated erythrose and threose The designation
D or L refers to the chiral carbon furthest from
the carbonyl group (which is carbon 3 in the
aldotetroses shown) so erythrose and threose
both have the D designation
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Tetroses (CH2O)4 have two chiral carbons in the
aldose form
note for tetroses, the ketose and aldose forms
are also interconvertible via tautomerization (as
they were for trioses)

• aldotetroses have two chiral carbons
• (2 and 3) and therefore have four steroisomers
• two diastereoisomers (D-threose and D-erythrose
  which are not mirror images of each other)
• D-threose does have a mirror image (L-threose)
  and are enantiomers of each other
• D-erythrose also has a mirror image (L-erythrose)
  and are enantiomers of each other

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Tetroses (CH2O)4 have one chiral carbons in the
ketose form
ketotetroses have only one chiral carbon (3) and
therefore only one form of stereoisomer two
enantiomers stereoisomers with two
nonsuperimposable mirror images of one another
The new carbon is added below the carbonyl
What was the number 3 carbon in dihydroxyacetone
is now the number 4 carbon in D-erythrulose And
there is now a chiral carbon (number 3 the
carbon that was added)
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Pentoses 5 carbons - aldopentoses have three
chiral centers
D-ribose and D-arabinose are stereoisomers what
kind? (they are not mirror images so they are
diasterioisomers) Is there an L-arabinose?
D-arabinose and L-arabinose are stereoisomers
what kind? (they are mirror images so they are
enantiomers)
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Pentoses 5 carbons ketopentoses have two chiral
centers
ketopentoses have two chiral carbons, therefore
four stereoisomers D-ribulose and D-xylulose
and L-ribulose and L-xylulose D-D and L-L are
diasterioisomers (they are not mirror
images) D-L and D-L are enantiomers (they are
mirror images)
In general a molecule with n chiral centers will
have 2n stereoisomers (ie two possibilities at
each chiral center) example 2 chiral centers
you would expect 22 or four stereoisomers example
3 chiral centers you would expect 23 or eight
stereoisomers
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Hexoses 6 carbons
Q How many chiral centers does an aldohexose
have? A four (carbons 2,3,4, 5) and 24
sixteen stereoisomers
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Epimers
Two sugars that differ only in the configuration
around one carbon atom are called epimers
D-glucose and D-mannose are diasterioisomers
(they are not mirror images) but they differ in
stereochemistry only around C-2 so they are a
subset of diastereoisomers
D-glucose and D-galactose differ in
stereochemistry only around C-4
Are D-mannose and D-galactose epimers of each
other?
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Ring structures of pentoses and hexoses
In aqueous solutions, aldotetroses and all
monosaccharides with five or more carbons atoms
in the backbone occur predominantly as cyclic
(ring) structures The carbonyl group forms a
covalent bond with the oxygen of a hydroxyl group
along the chain
A new chiral center has been created at the
carbonyl carbon in the hemiacetal and hemiketal
Substitution of a second alcohol molecule
produces an acetal or ketal
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The new chiral carbon at C-1 results in two
stereoisomeric forms of D-glucose
Reaction between the aldehyde group at C-1 and
the hydroxyl group at C-5 forms a hemiacetal
linkage producing either of two stereoisomers
These stereoisomers are designated as a or b
anomers which differ only in the stereochemistry
around the hemiacetal carbon
Mutarotation the interconversion of a and b
anomers
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Formation of ring structure by a pentose
C-1 oxygen and C-4 hydroxyl
C-1 oxygen and C-5 hydroxyl
Anomeric forms are possible for both the furanose
and pyranose rings
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Pyranoses and furanoses
Pyranoses six membered ring compounds Fura
noses five membered ring compounds
The four most common hexoses
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Monosaccaarides with more than six carbons
heptoses (seven carbons) octoses (eight
carbons) do exist in nature and one heptose
(sedopeptulose) plays a major role in fixation of
CO2 in photosynthesis
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Derivatives of the monosaccharides
monosaccharides carry a number of hydroxyl
groups (OH) where - things can be attached or
- the hydroxyl groups can be replaced by other
functional groups
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Phosphate esters
phosphate esters of the monosaccharides are major
participants in many metabolic pathways Condensa
tion of phosphoric acid with one of the hydroxyl
groups of a sugar forms a phosphate ester   Note
the standard free energy of hydrolysis for these
phosphate esters of monosaccarides are all less
negative than the free energy of hydrolysis of
ATP, therefore, ATP can act as a phosphate donor
to monosaccharides   but, since hydrolysis of the
phosphate esters of sugars is thermodynamically
favorable, these derivitives act as activated
compounds in many metabolic reactions
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Oxidation products of monosaccharides
example mild oxidation of the carbonyl
(anomeric) carbon of glucose with alkaline CU(II)
produces gluconate ( gluconic acid) which is
an an aldonic acid ( a sugar acid)
the production of a red precipitate of Cu2O is a
classic sugar test and was used in the past to
test for excess sugar in the urine of persons
thought to have diabetes (a more sensitive test
now uses glucose oxidase)
Glucose and other carbohydrates that are capable
of reducing ferric or cupric ions are called
REDUCING SUGARS
Oxidation of the carbon at the other end of the
carbon chain (C-6) forms the corresponding
uronic acid (glucuronic acid) Both aldonic and
uronic acids form stable intramolecular esters
called lactones
These are found in some naturally occurring
polysaccharides
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ALDITOLS (sugar alcohols)
a result of the mild reduction of the carbonyl
group on a sugar alditols are linear sugars that
cannot cyclize
example D-Glucitol (sorbitol) - in diabetics,
sorbitol accumulates in the lens of the eye and
leads to the formation of cataracts
Alditols are sweet tasting and sorbitol, mannitol
and xylitol are widely used to sweeten sugarless
gum
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AMINO SUGARS
Two amino derivatives of monosaccarides are
widely distributed in many naturally occurring
polysaccacharides
Further modifications of these amino sugars are
common For example the following compounds are
derived from ?-glucosamine
Two other common amino sugar derivatives
Modified sugars (especially the amino sugars) are
most often found as monomer residues in complex
oligosaccharides and polysaccharides.
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Modified sugars (especially the amino sugars) are
most often found as monomer residues in complex
oligosaccharides and polysaccharides.
Abbreviations have been developed for simple
sugars
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When the anomeric carbon of a sugar reacts with a
hydroxyl group and water is eliminated its
called an O- glycosidic bond
Glycosidic bonds
Remember that glucose is a reducing sugar When
its anomeric carbon participates in a glycosidic
bond it can no longer mutarotate and the bond is
in a fixed position What about the anomeric
carbon in the second molecule of glucose can it
still mutarotate? Yes and this is the reducing
end of the molecule The end with the anomeric
carbon involved in this glycosidic bond is the
non-reducing end
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Some examples of glycosides found in plant and
animal tissues
In O-glycosidic bonds, the hydroxyl that the
anomeric carbon bonds with does not have to be on
another monosaccharide
Amygdalin upon hydrolysis yeilds hydrogen
cyanide (HCN). Found in almond seeds   Some are
very toxic substances and often act as inhibitors
of enzymes involved in ATP utilization
Ouabain inhibits enzymes in NaK membrane
pump, therefore disrupting electrolyte balance.
Originates in an African shrub. Now used in some
cardiac treatments
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Glycosidic bonds can also be formed between a
sugar and a amide nitrogen
Glycosidic bond bonds between a sugar and
another molecule (through an intervening oxygen
or nitrogen atom)
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