Biochemistry 462a - Carbohydrate Structure
Reading - Chapter 9
Practice problems - Chapter 9: 2,4a,4b,6,9,10,13,14,15,16a,17; Carbohydrate extra problems
Monosaccharides
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Carbohydrates (CH2O)n or saccharides are the
sugars.
Aldehydes or ketones with multiple hydroxyl
groups.
The simplest carbohydrates are the monosaccharides in which n=3-8. Monosaccharides have
either an aldehyde (aldoses) or ketone (ketoses)
functional group.
The simplest monosaccharides of biological interest
have n=3 (trioses).
Glyceraldehyde has a chiral carbon at C2.
There are two enantiomers (D and L) nonsuperimposable mirror images.
Most sugars are derived from the D isomer.
There are two D isomers for the tetroses.
There are eight D isomers
for the hexoses.
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D-glucose and L-glucose are enantiomers they are non-superimposable mirror images.
Diastereoisomers are isomers that are not
mirror images. D-glucose and D-galactose are
EPIMERS - different configuration ONLY at C4.
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Pentoses and hexoses can form five
(furanose) or six (pyranose)-membered
rings.
The formation of a ring creates a new
chiral center at C1, leading to two
anomers, α and β.
The rings can adopt either a chair or boat
conformation - the chair is more stable.
The α and β anomers freely
interconvert in a process called
mutarotation, which involves the open
chain form of the sugar.
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Derivatives of carbohydrates play important roles in
biochemistry.
Disaccharides
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A disaccharide is a glycoside formed between two monosaccharides.
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This is cellobiose, which has a β-(1→
→ 4) linkage
between the two glucose molecules.
Cellobiose is the building block of cellulose.
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This is maltose, which has a α-(1→
→ 4) linkage
between the two glucose molecules.
Maltose is the building block of glycogen and
starch
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Other common disaccharides
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Sucrose (glucose-α-1,2fructose)
Lactose (galactose-β-1,4glucose)
Trehalose (glucose-β-1,1glucose)
Polysaccharides
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Cellulose is a polymer of β-(1→
→ 4) linked glucose residues.
The conformation of the β-(1→
→ 4) linkage leads to flat, ribbon-like structure that is stabilized
by inter chain hydrogen bonds.
Amylose is a polymer of α-(1→
→ 4) linked glucose residues.
The conformation of the α-(1→
→ 4) linkage leads to a helical structure.
Chitin- the shells of insects- is composed of N-Acetylglucosaminyl-β-1,4 NAcetylglucosamine.
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Glycogen and starch are
storage polysaccharides that
have highly branched
structures.
In both the glucose residues are
joined by α-(1→
→ 4) linkages.
The branch points are created
by α-(1→
→ 6) linkages.
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Keeping track of blood glucose levels
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Non enzymatic glycosylation of proteins
takes place when the glucose
concentration is high.
This is the Amadori reaction and
proceeds through the formation of a
Schiff base.
In diabetes mellitus, blood sugar levels
are often quite high. Under these
conditions there is glycosylation of the Nterminal Val of hemoglobin.
The resulting protein, Hemoglobin A1C
can be readily measured.
The level of Hemoglobin A1C is another
measure of how blood glucose levels vary
in diabetic individuals.
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