Transcript Carbohydrates

Why Carbohydrates?
• Play a number of important roles in
biochemistry:
 Major energy sources
 Play a key role of processes that take place on the
surfaces of cells. eg. cell-cell interactions
 Essential structural components of several classes
organisms.
Eg. Cellulose:
components in grass and
trees
Carbohydrates
• Consist of 3 sub-classes
- monosaccharide
- oligosaccharide
- polysaccharide
* saccharide = 1 unit sugar, normally
refers to glucose
Carbohydrate
• General formula – Cn(H2O) n
• Only the simple sugars (monosaccharides) fit this
formula.
• Oligosaccharides and polysaccharides are based on
the monosaccharides unit and have slightly different
formula.
• Oligosaccharides?
• Polysaccharides?
• Compounds containing C, H and O
• Produced in plants by photosynthesis
6CO2 + 6H2O  C6H12O6 + 6O2
• All have C=O and –OH functional group.
• The C=O may be as HC=O or by itself.
• As HC=O( aldehyde) so the sugars are
also known as aldoses . Just as C=O, the
sugars are also known as ketoses.
• Suffix – ‘ose’ indicates that molecule is a
carbohydrate.
• Major roles in energy metabolism ( energy
storage and energy transport ) and structural
component.
Monosaccharides
- monosaccharides is a molecule of single sugar unit.
e.g glucose, galactose, mannose, fructose, xylose
- Are the simplest sugars
- Can be used for fuel
- Can be combined into polymers
- Glucose, galactose, and mannose are 6C molecules or
aldohexoses.
- Xylose is a 5C molecule/aldopentose .
D-Glucose
• This is a Fischer
Formula
• 6C monomeric
molecule
• Is an aldohexose
• D to denote the –OH
on C5 is on the right
side of the C chain
• Emil Fischer ( 1852 1919 ) was a Germanborn scientist who won
the noble prize in
chemistry.
Stereochemistry of monosacharides
• Anomers :- Sugars that differ only in the
configuration around the anomeric carbon
(carbonyl becomes a new chiral; C1)
• Epimers :- Sugars that differ only by the
configuration at one C atom ( excluding the
anomeric carbon )
• Enantiomers :- Molecules that are
superimposible mirror images of one another.
• Isomers : Isomers are molecules with the
same molecular formula, but different
arrangements of atoms.
• Stereoisomers : molecules that differ from
each other only in their configuration ( three
dimensional shapes )also called optical
isomers. Glucose n Galactose.
• Diastereomers :non superimposable,non
mirror image stereoisomers. ( L- Threose with
both D n L erythrose. )
The figures show the two
isomeric forms that are
mirror images of each other
→ stereoisomers.
• The two forms differ in the
position of –OH group
bonded to the central C.
• the mirror-image
stereoisomers are also
called enantiomers,
D-Glyceraldehyde and LGlyceraldehyde are
enantiomers.
Fig. 16-1b, p.435
Epimers
Mirror image
diastereoisomers
The aldotetroses have two chiral C
; C-2 and C-3.
Therefore, the stereoisomers of
aldotetroses are 22, or four possible
stereoisomers.
Diastereoisomers that differ from
each other at only one chiral C
are known as epimers.
D-erythrose and D-threose are
epimers.
Fig. 16-3, p.437
Haworth
projections
formula
The most abundance monosaccharide in nature.
Glucose Anomers
Oligosacharides
- oligosaccharides is a chain containing
2-10 sugar units.
- Two sugar units also called Disaccharides.
- The name is derived from the Greek word oligos,
meaning "a few", and from the Latin/Greek word
sacchar which means "sugar"
- These sugar units join to one another by glycosidic
linkages.
- Maltose and Lactose: Alternative Glycosidic
Linkages (1,4)
- Sucrose: glucose-α-1,2-fructose
Examples
• Maltose, a cleavage product of starch (e.g.,
amylose, is a disaccharides with an α(1,4)
glycosidic linkage between the C1 hydroxyl of
one glucose and the C4 hydroxyl of a second
glucose. Maltose is the α anomer, because
the O at C1 points down from the ring.
• Sucrose, common table sugar, has a glycosidic
bond linking the anomeric hydroxyls of glucose
and fructose. Because the configuration at the
anomeric carbon of glucose is α (O points down
from the ring), the linkage is designated α(1,2).
• Lactose, milk sugar, is composed of glucose and
galactose with β(1,4) linkage from the anomeric
hydroxyl of galactose.
Disaccharide Synthesis
Energy +
The reactions, the names of the sugars, and whether they are mono- or
disaccharides is what you should know (also, “Glycosidic linkage”)
(a)
Dehydration reaction
in the synthesis of
maltose. The bonding
of two glucose units
forms maltose. The H
glycosidic link joins
the number 1 carbon
of one glucose to the HO
number 4 carbon of
the second glucose.
Joining the glucose
monomers in a
different way would
result in a different
disaccharides.
CH2OH
CH2OH
O
H
OH
H
H
H
H
OH
HO
(b)
H
OH
H
H
OH
H
OH
OH
O
H
H
OH
H
CH2OH
H
1–4
glycosidic
1
linkage
HO
4
H
OH
H
H
OH
O
H
OH
O
H
H
OH
OH
H2O
Glucose
Glucose
CH2OH
H
O
CH2OH
O
H
OH
H
Dehydration reaction
in the synthesis of HO
sucrose. Sucrose is
a disaccharide formed
H
OH
from glucose and fructose.
Notice that fructose,
Glucose
though a hexose like
glucose, forms a
five-sided ring.
CH2OH
H
OH
HO
H
Maltose
CH2OH
O
H
OH
H
OH
HO
CH2OH
H
O
H
H
CH2OH
1–2
glycosidic
1
linkage
O
H
H
2
H
HO
O
HO
H
OH
CH2OH
OH
H
H2O
Fructose
Sucrose
Figure 5.5
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Reducing & Non Reducing Sugar
• Sugars exist in solution as an equilibrium mixture of
open-chain and closed-ring (or cyclic) structures.
• In the open-chain form, the carbon atom that
contains the C=O bond is called the carbonyl carbon.
• Sugars that can be oxidised by mild oxidising agents
such as Benedict's Solution, Fehling's Solution,and
Tollen's Reagen are called reducing sugars because
the oxidising agent is reduced in the reaction.
• Reducing sugar: one that has a free aldehyde group
and this aldehyde is easily oxidized.
Tollens’ agent
+ Ag (NH3)2
Being
reduced
• A non-reducing sugar is not oxidised by mild
oxidising agents.
• All common monosaccharides are reducing
sugars.
• The disaccharides maltose and lactose are
reducing sugars.
• The disaccharides sucrose is a non-reducing
sugar (both anomeric groups are involved in
glycosidic linkage)
Polysaccharides
 are the complex carbohydrates
 is
a
macromolecule,
containing
>1000
sugar
units/monomers.
They are made up of chains of monosaccharides (the
sugars) which are linked together by glycosidic bonds,
which are formed by the condensation reaction.
The linkage of monosaccharides into chains creates
chains of greatly varying length, ranging from chains
of just two monosaccharides, which makes a
disaccharide to the polysaccharides, which consists
of many thousands of the sugars.
 the molecule can be straight chain or branched.
 e.g amylose, cellulose are straight.
 e.g xanthan gum, amylopectin are branched.
 the monomers can be of 1 type (homopolysaccharide)
e.g starch ( poly D-glucose) or
 Mixed types (heteropolymers)
e.g xanthan gum which are glucose-glucose-mannoseglu.acid-mannose
 the importance of glycosidic linkage:
 β-glycosidic linkage: cellulose and chitin –structural
material
 α-glycosidic linkage: starch and glycogen – carbohydrate
storage polymers in plants and animals
Xantham gum
Cellulose
• Cellulose is a major component of plant cell
walls. It is an unbranched polymer with about
ten thousand glucose units per chain.
• Hydroxyl groups (-OH) project out from each
chain, forming hydrogen bonds with
neighbouring chains which creates a rigid
cross-linking between the chains, making
cellulose the strong support material that it is.
• Despite the combined strength of cellulose, it
is fully permeable to water and solutes which
makes it ideal for allowing water and solutes
into and out of the cell.
• It is the most abundant organic substance in
the living world and it has been estimated that
more than half the total organic carbon on the
planet is in cellulose.
Cellulose
Know the
difference
Cellulose
Cellulose is a
Structural
polysaccharide
Glycogen
• Glycogen is a branched polysaccharide found in nearly all
animal cells and in certain protozoa and algae.
• In humans and other vertebrates it is principally stored in
the liver and muscles and is the main form of stored
carbohydrate in the body, acting as a reservoir of glucose
(which the glycogen can be broken down into) for when the
body is being starved of food.
• The structure of glycogen consists of long polymer chains
of glucose units connected by an alpha acetal linkage.
• All of the monomer units are alpha-D-glucose, and all the
alpha acetal links connect C 1 of one glucose to C 4 of the
next glucose.
• The no. of branch points is
significance for two
reasons:
i. More branched
polysaccharides, more
water soluble.
ii. More branched
polysaccharides, more
potential targets of
enzyme to allow a
quicker the mobilization
of glucose.
Starch
• Starch is similar to glycogen, however it is found in
plant cells, protists and certain bacteria. The starch
granules can be seen in the cell through a light
microscope. The starch granules are made up of two
polysaccharides, amylose and amylopectin.
• Amylose is an unbranched molecule made up of
several thousand glucose units, coiled helically into a
more compact shape. Amylopectin is also compact but
has a branched structure and is made up of twice as
many glucose units as amylose.
Chitin
• Chitin is closely related in structure to cellulose, also
being an unbranched polysaccharides. However,
instead of the hydroxyl groups (-OH), the chains have
the following structure –NH.CO.CH3 replacing it.
• Chitin is a polysaccharide found in the outer
skeleton of insects, crabs, shrimps, and lobsters and
in the internal structures of other invertebrates.
• It is the main source of production of chitosan, which
is used in a number of applications, such as a
flocculating agent, a wound healing agent, a sizing
and strengthening agent for paper, and a delivery
vehicle for pharmaceuticals and genes.
• Chitin, another important structural
polysaccharides
– Is found in the exoskeleton of arthropods
– Can be used as surgical thread
CH2OH
O OH
H
H
OH H
OH
H
H
NH
C
O
CH3
(a) The structure of the
chitin monomer.
Figure 5.10 A–C
(b) Chitin forms the exoskeleton
of arthropods. This cicada
is molting, shedding its old
exoskeleton and emerging
in adult form.
(c) Chitin is used to make a
strong and flexible surgical
thread that decomposes after
the wound or incision heals.
59
arthropods
FUNCTIONS ( GENERAL )
• Carbohydrates are one of the most important components
especially in foods. Carbohydrates may be present by itself as
monosaccharides, oligosaccharides or polysaccharides, or
they may be physically associated or covalently bound to
other molecules as glycoproteins ( to proteins), as glycolipids(
to lipids ) .
• Some carbohydrates are digestible by humans and therefore
provide an important source of energy, whereas others are
indigestible. Indigestible carbohydrates form part of a group
of substances known as dietary fiber, which also includes
lignin.
• Consumption of significant quantities of dietary fiber has
been shown to be beneficial to human nutrition, helping
reduce the risk of certain types of cancer, coronary heart
disease, diabetes and constipation.
• Carbohydrates also contribute to the sweetness, appearance
and textural characteristics of many foods. It is important to
determine the type and concentration of carbohydrates in
foods for a number of reasons.
Summary
• Monosaccharides ,the simplest carbohydrates,are classified as
aldoses or ketoses.
• The cyclic hemiacetal ( have aldehyde group) and hemi ketal ( have
ketone group)forms of monosaccharide have either the α or β
configuration at their anomeric carbon but are conformationally
variable.
• Sugars that can be oxidised by mild oxidising agents are called
reducing sugars because the oxidising agent is reduced in the
reaction.
• Polysaccharides consist of monosaccharide linked by glcosidic bond.
• The storage polysaccharides starch and glycogen consist of αglycosidically linked glucose residues.