Download Carbohydrates Learning Objective Carbohydrates

Molecular & Cell Biology
Carbohydrates
Carbohydrates
Carbohydrates, one of the four major classes of biomolecules,
are aldehyde or ketone compounds with multiple hydroxyl
groups. They function as energy stores, metabolic
intermediates and important fuels for the body.
Learning Objective
After Interacting with this learning object, learner will be able
to:
Describe Monosaccharides.
Describe Disaccharides and Polysaccharides.
Define Glycoconjugates.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
All monosaccharide sugars are optically
active due to the presence of
asymmetric carbon atoms. The simplest
aldotriose, D-glyceraldehyde, has one
asymmetric centre giving rise to the D
and L enantiomers, which are mirror
images of each other. A compound will
have 2n isomers, where n is the number
of asymmetric centres..
Molecular & Cell Biology
Carbohydrates
Monosaccharides
Simple D-aldose sugars can have
anywhere from three to seven carbon
atoms with an aldehyde functional
group. All the D-sugars have the same
absolute configuration as that of Dglyceraldehyde at their asymmetric
centre that is farthest away from the
carbonyl carbon. The most commonly
observed sugars include D-glucose, Dmannose and D-galactose.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
Those sugars that differ in configuration
from each other about only one
asymmetric carbon atom are referred to
as epimers. D-Glucose and D-mannose are
epimers at the second carbon atom while
D-glucose and D-galactose are epimers at
the fourth carbon atom. D-mannose and
D-galactose
however
are
only
diastereomers since they differ in
configuration at two asymmetric centres.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
Ketoses can be three, four, five or six
carbon sugars with a ketone
functional group. Dihydroxyacetone,
the simplest ketose having 3 carbon
atoms
does
not
possess
an
asymmetric
centre.
The
D
configuration is therefore based on
the absolute configuration of Derythrulose, the four carbon ketose
and is designated based on the
asymmetric centre farthest away
from the ketone group.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
The aldehyde or ketone group of
monosaccharides reacts with the
alcohol
groups
to
form
intramolecular hemiacetals or ketals.
This gives rise to stable five or sixmembered rings known as furanose
and pyranose respectively. The
functional group carbon atom is
designated as C-1 and is known as
the
anomeric
carbon.
The
configuration of the groups about the
anomeric carbon gives rise to the
alpha and beta configurations. The
chair form of the six membered
pyranose ring is more stable due to
minimal steric hindrance at the axial
positions since they are occupied by
small hydrogen atoms.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
Although glucose is more stable in the six
membered
pyranose
configuration,
stability of fructose is greater as a fivemembered furanose ring.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
A simple test for identifying sugars
such as glucose is by the Fehling’s
test. The free aldehyde group
provides the sugars with a reducing
nature thereby bringing about
reduction of a solution of cupric ions.
This results in an easily identifiable
brown precipitate.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
The reactive anomeric carbon atom
can be modified by reaction with
alcohols or amines to form adducts.
Reaction of glucose with methanol
gives the corresponding methyl
glucopyranoside with the formation
of an O-glycosidic bond. When the
anomeric carbon is linked to an
amine via its nitrogen atom, it
results in formation of an Nglycosidic bond. Several other
modified monosaccharides such as
fucose, N-acetyl glucosamine etc are
present
which
serve
various
structural and functional roles.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
Disaccharides are formed by the
condensation reaction between two
monosaccharide units. The release of a
molecule of water results in the formation
of a glycosidic bond between the two
residues. Shown in this example is the
formation of maltose, a disaccharide that
is composed of two units of glucose linked
by an -14 glycosidic bond. Maltose is
hydrolyzed into its individual units by the
enzyme maltase.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
Table sugar or sucrose is a disaccharide
composed of one unit of glucose and
one of fructose linked by a 1
− 2 bond.
It is a non-reducing sugar since the
aldehyde group of glucose and ketone
group of fructose are involved in
formation of the glycosidic linkage. It
can be cleaved by the enzyme sucrase.
Lactose, the sugar component of milk,
is made up of one unit of galactose and
one of glucose linked by an −14
linkage. It can be cleaved by the
enzyme lactase, also known as galactosidase.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
Starch granules, which form the major
nutritional reserve of plants, are
composed of two components – amylose
and amylopectin. Amylose consists of
linear, unbranched chains of D-glucose
residues linked by -1,4 glycosidic
linkages. Amylopectin, however, is a
branched polymer with an -1,6 glycosidic
linkage present around every 30 residues.
Starch is rapidly degraded by the enzyme
amylase.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
Glycogen is the storage form of glucose in
animal cells. It is structurally similar to
start with glucose residues being joined
by -1,4 glycosidic linkages. Glycogen has
more extensive branching than starch
with branch points being observed around
every 10 residues. These allow the
molecules to be stored in a very compact
way in the cells.
Molecular & Cell Biology
Carbohydrates
Glycoconjugates
Glycosaminoglycans
are
heteropolysaccharide components of
extracellular matrix spaces along with
other fibrous proteins. They are linear
polymers that are composed of
disaccharide repeating units of which
one residue is always a derivative of an
amino
sugar
such
as
N-acetyl
glucosamine or N-acetyl galactosamine.
They also contain a negatively charged
sulphate or carboxylate group.Common
glycosaminoglycans include chondrotin
sulphate, hyaluronate, heparin etc.
Molecular & Cell Biology
Carbohydrates
Glycoconjugates
Glycosyl transferases are specific enzymes
that bring about transfer of activated
sugar residues onto other substrates. The
activated sugar linked to a nucleotide
moiety such as UDP is cleaved and
covalently linked with the substrate which
could either be another monosaccharide
unit, a polysaccharide or the serine or
aspargine side chains of proteins.
Molecular & Cell Biology
Carbohydrates
Glycoconjugates
Glycosaminoglycans can be linked to
proteins
to
form
various
proteoglycans. These molecules have
a variety of functions in tissue
organization,
development
of
specialized
tissues
and
for
modulation of ligand interactions
with cell surface receptors. Aggrecan
is
a
proteoglycan
aggregate
consisting of many core proteins
bound to a single hyaluronate
molecules; they serve as shock
absorbers in cartilage.
Molecular & Cell Biology
Carbohydrates
Glycoconjugates
Carbohydrate groups are often
covalently attached to proteins to
form glycoproteins. The sugar
residues are typically attached to the
amide nitrogen atom of the
aspargine side chain or to the oxygen
atom of the serine or threonine side
chain. These glycoproteins are
components of cell membranes and
have a variety of functions in cell
adhesion processes. The ABO blood
groups arise due to differing
carbohydrate structures on the
surface of the blood cells.
Molecular & Cell Biology
Carbohydrates
Glycoconjugates
Carbohydrate moieties can also be
covalently linked with various lipids.
Glycolipids are membrane components
bearing a hydrophilic head group and a
hydrophobic lipid tail. Lipopolysaccharides
are a predominant feature on the outer
membrane of gram negative bacteria,
which consist of fatty acid chains bound
to sugar residues.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
1. Monosaccharide: Monosaccharides comprise
the simplest group of carbohydrates with the
empirical formula (C-H2O)n. They can be either
aldehydes or ketones having two or more
hydroxyl groups. These monosaccharides serve
as important fuel molecules and as the basic
building unit for nucleic acids.
2. Aldotriose: The smallest monosaccharide
having an aldehyde group and a total of 3
carbon atoms is referred to as an aldotriose.
Glyceraldehyde is the simplest aldotriose.
3. Ketotriose: The smallest monosaccharide
that has a ketone group with a total of 3
carbon atoms is referred to as a ketotriose.
Dihydroxyacetone is the simplest ketotriose.
4. Asymmetric centre: A carbon atom that has
four different groups attached to it in a
tetrahedral arrangement is said to be asymmetric
or chiral and gives rise to the phenomenon of
optical isomerism. All monosaccharides have
multiple asymmetric carbon atoms, giving rise to
2n isomers for each monosaccharide, where n
refers to the number of asymmetric centres.
5. Enantiomers: Molecules with a chiral centre
have a non-superimposable mirror image and the
two forms of this molecule are known as
enantiomers. They are designated as D and L or R
(rectus) and S (sinister) depending on the
arrangement of groups around the asymmetric
carbon atom. R and S nomenclature is based on
priority of atomic numbers of atoms directly
attached to the central asymmetric centre.
Molecular & Cell Biology
Carbohydrates
Monosaccharides
6. Aldopentose: A monosaccharide having an
aldehyde functional group with a total of five
carbon atoms is referred to as an aldopentose.
D-ribose, which is an important component of
all nucleic acids is one such aldopentose.
9. Epimers: Sugars that differ in configuration
from each other at just one asymmetric carbon
atom, are referred to as epimers. Glucose and
mannose are epimers at C-2 while glucose and
galactose are epimers at C-4.
7. Aldohexose: A monosaccharide having an
aldehyde functional group with a total of six
carbon atoms is referred to as an aldohexose.
D-glucose is one of the most common
aldohexoses.
10. Anomers: The aldehyde or ketone functional
groups can react with an alcohol to form a
hemiacetal or ketal. This intramolecular reaction
occurs in sugars, thereby allowing them to form
cyclic
structures.
These
are
commonly
represented by means of the Haworth’s
projections. Upon cyclization, the aldehyde or
ketone carbon becomes C1 and is referred to as
the anomeric carbon atom. The configuration of
groups about the anomeric carbon can result in
either the alpha or beta structures, which are
referred to as anomers.
8. Ketohexose: A sugar having a ketone
functional group with a total of six carbon
atoms is referred to as a ketohexose. Dfructose is the most abundant ketohexose.
Ketoses have fewer asymmetric centres
compared to aldoses.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
1. Disaccharide: Two monosaccharide units
joined together by means of an O-glycosidic
linkage forms a disaccharide. Three of the
most common disaccharides include maltose,
sucrose and lactose.
2. Polysaccharide: Several monosaccharide
units joined together by glycosidic bonds form
a polysaccharide. These help in maintaining
structural integrity and serve as fuel reserves
in organisms. Some of the most common
polysaccharides include starch, the nutritional
reservoir in plants, glycogen – the fuel storage
form in animal cells and cellulose, which is the
most important structural elements in plants.
3. Glycosidic bond: The bond formed by
interaction between the hydroxyl group of one
monosaccharide with the hydroxyl group,
aldehyde or ketone group of another
monosaccharide, with the subsequent
elimination of water is known as the glycosidic
bond.
4. Hemiacetal: The interaction between an
aldehyde or ketone group with a hydroxyl group
with the elimination of water results in the
formation of a hemiacetal or ketal. In sugars,
intramolecular hemiacetals and ketals are formed
by cyclization of the sugar molecules.
5. Homopolymer: When all the monosaccharide
units of a polysaccharide are the same, it is said
to be a homopolymer. The most common
homopolymers are starch and glycogen.
6. Heteropolymer: When the repeating
monosaccharide units of the polysaccharide are
different, they are said to be heteropolymers.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
7. Starch: The nutritional reserve of plants is
starch, which is composed of two components
– amylose and amylopectin. Amylose consists
of linear, unbranched chains of D-glucose
residues linked by -1,4 glycosidic linkages.
Amylopectin, however, is a branched polymer
with an -1,6 glycosidic linkage present
around every 30 residues. Starch is rapidly
degraded by the enzyme amylase.
8. Cellulose:
Cellulose plays a major
structural role in plants and consists of linear
chains of glucose residues linked together by
-1,4 glycosidic bonds. These chains formed by
-linkages have very high tensile strength and
can be digested by the enzyme cellulase,
which is not inherent in mammals.
9. Glycogen: Glycogen is the storage form of
glucose in animal cells. It is structurally similar
to starch, with glucose residues being joined
by -1,4 glycosidic linkages. Glycogen has more
extensive branching than starch with branch
points being observed around every 10 residues.
These allow the molecules to be stored in a very
compact way in the cells.
10. Chitin: Chitin is another structural
polysaccharide that is a homopolymer of Nacetyl-D-glucosamine residues joined together by
-1,4 glycosidic linkages. It is commonly found on
the exoskeleton of insects.
Molecular & Cell Biology
Carbohydrates
Disaccharides & Polysaccharides
1. Glycosaminoglycan: Glycosaminoglycans
are heteropolysaccharide repeating units
found on animal cell surfaces and in the
extracellular matrix. They are composed of
disaccharide units that contain a derivative of
an amino sugar and at least one of the sugars
in the unit has a negatively charged
carboxylate or sulphate group.
2. Proteoglycan: These are structural
elements
that
are
composed
of
glycosaminoglyan units linked to proteins. The
proteoglycan, aggrecan, is a major component
of cartilage along with the protein, collagen,
where it serves as a shock absorber.
Degradation of aggrecan and collagen can lead
to osteoarthritis.
3. Glycoprotein: Carbohydrate groups are
often covalently attached to proteins to form
glycoproteins. The sugar residues are typically
attached to the amide nitrogen atom of the
aspargine side chain or to the oxygen atom of the
serine or threonine side chain. These
glycoproteins are components of cell membranes
and have a variety of functions in cell adhesion
processes.
4. Glycolipid: Carbohydrate moieties can also be
covalently linked with various lipids. Glycolipids
are membrane components bearing a hydrophilic
head group and a hydrophobic lipid tail.
5. Glycosyl transferase: These are a specific
class of enzymes that are responsible for the
transfer or sugar residues onto other substrates.
They transfer the sugar in its activated state such
as UDP-glucose to other molecules such as other
monosaccharides, polysaccharides or amino acid
side chains of proteins.