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Title: Carbohydrates made simple
Description: It is important for any student of Biology or Biochemistry to understand carbohydrates. In these notes, I have made it very easy for one to grasp the basics of carbohydrates.
Description: It is important for any student of Biology or Biochemistry to understand carbohydrates. In these notes, I have made it very easy for one to grasp the basics of carbohydrates.
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Ms C
...
Monosaccharides
2
...
Oligosaccharides
Yield 3-6 monosaccharide units on
hydrolysis
Oligosaccharides in most cells do not
occur as free entities but are joined to
nonsugar molecules (lipids and proteins)
to form glycoconjugates
Monosaccharides ,disaccharides and
oligosaccharides are water soluble because of
high hydrogen binding potential
4
...
Molecules tend to interact
more strongly with each other than with
water
Aldoses
◦ Three carbon aldoses: Glyceraldehyde
◦ Four carbon aldoses: D-Erythrose, D-Threose
◦ Five carbon aldoses: D-Ribose, D- Arabinose, D-Xylose, DLyxose
◦ Six Carbon aldoses: D- Glucose, D-Mannose, D-Galactose etc
Ketoses
◦
◦
◦
◦
Three carbon ketoses: Dihydroxyacetone
Four carbon aldoses: D-Erythrulose
Five carbon aldoses: D-Ribulose, D-Xylulose
Six Carbon aldoses: D- Fructose etc
D and L isomerism
Epimers
Aldose –ketose isomerism
Anomers
In general a molecule with n asymmetric centres
has 2n stereoisomeric forms
For aldotrioses, n=1, - there are two stereo
isomers, D- and L–glyceraldehyde
Called enantiomers (mirror images) of each
other
Addition of an HCOH group gives four aldotetroses
because n=2
Two of them D sugars and the other two
enantiomeric L sugars
D-erythrose and D-threose have the same
configuration at C-3 but opposite configurations at
C-2 -called diastereoisomers
The 5 carbon aldoses have three asymmetric
centers with 8 (23) stereoisomers
The 6 carbon aldoses have 4 asymmetric centers
with 16 (24) stereoisomers
Sugars differing in configuration at one
asymmetrical center are epimers
D-glucose and D-mannose are epimers at C-2
D-glucose and D-galactose are epimers at C-4
Most of the monosaccharides occurring in
mammalian metabolism are of the D-configuration
1
CH O
D-Glyceraldehyde
2 H C OH
3
CHO
CHO
3 HCOH
HO C H
D-Erythrose
H COH
CH2OH
4 CH 2OH
2 H C OH
3 H C OH
4 H COH
5
1
C HO
2 H C OH
C HO
HO C H
H C OH
H C OH
HCOH
H COH
6
CH OH
2
H C OH
HC OH
CH OH
2
H C OH
5
OH C H
CH OH
2
3 H C OH
4
C HO
CH OH
2
C HO
H C OH
H OC H
HC OH
H COH
CH OH
2
CHO
D-Arabinose
C HO
D-Ribose
1
D-Threose
HCOH
HO C H
HCOH
CHO
H OCH
HOC H
HCOH
CH2 OH
C HO
CHO
H OCH
HCOH
H OC H
HC OH
H C OH
H COH
CH OH
2
HOC H
HCOH
CH2OH
CHO
H OCH
HC OH
HOCH
H COH
CH 2OH
CH2OH
CHO
H COH
D-Lyxose
2H C OH
D-Xylose
1
CH OH
2
CHO
H OCH
HOCH
HOCH
HOCH
HOC H
HCOH
CH2OH
HCOH
CH2OH
11
1 CH2 OH
2 C
1
1
Dihydroxyacetone
O
3 CH2 OH
1
CH2 OH
2
C
3
H COH
CH2OH
1 CH2 OH
2 C
D-Erythrulose
O
CH2 OH
C
O
3 HCOH
H OCH
D-Ribulose
CH2OH
5 CH2OH
2 C
O
3 HCOH
4 HC OH
5 HC OH
6 CH2 OH
D-Psicose
D-Xylulose
HCOH
4 HCOH
1 CH2 OH
O
CH2 OH
C
O
HOCH
H C OH
H C OH
CH2 OH
D-Fructose
CH2 OH
C
O
HC OH
HOCH
HCOH
CH2OH
D-Sorbose
CH2 OH
C
O
HOC H
HOCH
HCOH
CH2OH
D-Tagatose
12
Most of the monosaccharides occurring in
mammalian metabolism are of the Dconfiguration
D- Fructose is the most abundant ketose
Pentoses and hexoses cyclize to form
furanose (5-membered )and pyranose (6membered) rings
D-Glucose is the most common aldose
Hexoses of physiological importance
Sugar
Importance
Reactions
D-glucose
Sugar carried in the blood and
the principal one used by the
tissues
Reducing sugar
...
D-galactose
Can be changed to glucose in
the liver
...
D-mannose
A constituent of prosthetic
polysaccharides of albumins,
globulins, mucoproteins
...
14
Pentoses and hexoses cyclize to form furanose and
pyranose rings
The predominant forms of glucose and fructose in solution are not
open chains – the open chain forms cyclize to form rings
In general an aldehyde can react with an alcohol to form a
hemiacetal
H
O
R
C
H
Aldehyde
+
HOR
R
C
O R’
OH
Alcohol
Hemiacetal
15
The C-1 aldehyde in the open-chain form of glucose reacts with the C5 hydroxyl group to form an intramolecular hemiacetal
The resulting 6-membered ring is called a pyranose
6CH2OH
5
H
H
O
1
H
C
OH
3
HO
C
H
4
5
6
4
C
2
H
H
C
C
OH
OH
CH2OH
D-Glucose
(Open chain)
H
OH
HO
CH2OH
H
OH
H
C
C
H
OH
H
C
C
H
OH
HO
3
H
O H
H
1
2OH
OH
α-D-Glucopyranose
C
O
H
HO
CH2OH
O OH
H
OH H
H
OH
H
β-D-Glucopyranose
16
An additional asymmetric center is created when
glucose cyclizes
C-1, the carbonyl carbon becomes the
asymmetric center in the ring form of the sugar
Two ring structures can be formed
α-D-glucopyranose and β-D-glucopyranose
The C-1 carbon is called the anomeric
carbon atom; the α and β forms are called
anomers
α-D-glucopyranose
The hydroxyl group attached to C-1 is below the
plane of the ring (or the OH group and the CH2OH
group of the two carbons atoms linked by the oxygen
are trans to each other)
β-D-glucopyranose
The hydroxyl group attached to C-1 is above the
plane of the ring (or the OH group and the CH2OH
group of the two carbons atoms linked by the oxygen
are cis to each other)
Similarly a ketone can react with an alcohol to form a hemiketal
R
C
R
O
R’
Ketone
+
HOR’’
Alcohol
OR’’
C
R’
OH
Hemiketal
• The C-2 group in the open-chain form of fructose can react with the
C-5 hydroxyl group to form an intramolecular hemiketal
• This five-membered ring is called furanose
19
•
In D-fructofuranose
α-D-fructofuranose
The hydroxyl group attached to C-2 is below
the plane of the
ring
β-D-fructofuranose
The hydroxyl group attached to C-2 is above
the plane of the
ring
• The C-2 carbon is called the anomeric carbon atom;
the α and β forms are called anomers
• Fructose also forms pyranose rings
Disaccharides
(such as maltose,
lactose, and sucrose) consist of two
monosaccharides joined covalently
by an O-glycosidic bond
...
Maltose
6 CH 2OH
H H5
4
OH
HO
3
H
Anomeric
carbons
6CH 2OH
O H
H
2
1
OH
H
O
4
5
H
OH
3
H
O H
H
2
1
OH
OH
α-D-Glucopyranosyl-(1→ 4)-α-D-
glucopyranose
Maltose comes from digestion by amylase or hydrolysis
of starch
Can be hydrolyzed to glucose by maltase
Because of the free anomeric carbon (C-1 of glucose on
the right), maltose can be oxidized; hence its reducing
disaccharide
23
Sucrose (table sugar)
HOCH2
H
HO
O
O H
H OCH2
H
OH
H
H
OH
1 2
O
H
OH
H
HO
H
CH2OH
α-D-Glucopyranosyl-(1→2)-α-Dfructofuranoside
The anomeric carbon atoms of a glucose unit and a fructose unit are joined
in this disaccharide; the configuration of this glycosidic bond is α for
glucose and α for fructose
Sucrose contains no free anomeric carbon atom; its therefore a nonreducing
sugar
Nonreducing disaccharides are known as glycosides; the positions joined are
the anomeric carbons
Hydrolyzed by sucrase
...
Branches occur about once in
ten glucose units
This branching increases the solubility of glycogen
and makes its sugar units accessible
Abundant in the liver (10% of liver mass)
...
Because each branch in glycogen ends with a nonreducing
sugar unit, a glycogen molecule has as many nonreducing
ends as the branches, but only one reducing end
Degradative enzymes that act only at the nonreducing ends
can work simultaneously on the many branches speeding up
the process
Glucose can not be stored in its monomeric form because
The concentration of glucose in the cells that would be equivalent
to stored glycogen is too high, and this can lead to osmotic
destruction of the cells
The free-energy change for entry of glucose into the cells against
the concentration gradient would be prohibitively large
The nutritional reservoir in plants
Two types
Amylose (10-30%)
-Unbranched
-Glucose residues linked by α1→4 bonds
-Reacts with iodine to give a dark blue complex
Amylopectin (70-90%)
-The branched form
-Has about one α1→6 linkage per 30 α1→4 linkages (like
glycogen except for the lower degree of branching)
-Gives a red-violet colur with iodine
Both amylose and amylopectin are hydrolyzed by α –
amylase (found in human saliva)
α –amylase hydrolyzes internal α1→4 linkages to yield
maltose (2 glucose residues in α1→4 linkage),
maltotriose (3 glucose residues in α1→4 linkages), and
α-dextrin (several glucose units joined by an α1→6
linkage in addition to α1→4 linkages)
Maltose and maltotrioses are hydrolyzed to glucose
by maltase
α-dextrin is hydrolyzed to glucose by α-dextrinase
β-amylase (in plants) is an exoglucosidase (act on
terminal α1→4 linkages)
Storage polysaccharide in yeasts and
bacteria
Consists only of glucose residues
Nearly all linkages are α1→6
Occasional branches are formed by
α1→2, α1→3 α1→4 depending on species
Dextrans formed by bacteria are
components of dental plaque
Title: Carbohydrates made simple
Description: It is important for any student of Biology or Biochemistry to understand carbohydrates. In these notes, I have made it very easy for one to grasp the basics of carbohydrates.
Description: It is important for any student of Biology or Biochemistry to understand carbohydrates. In these notes, I have made it very easy for one to grasp the basics of carbohydrates.