Carbohydrates have many important functions in living organisms. They serve as a storage form of energy (glycogen), act as structural components (cellulose in plants), and are involved in cell signaling. Glucose is the most abundant and important carbohydrate, being produced by photosynthesis in plants and used as an energy source. Carbohydrates can be classified as monosaccharides, disaccharides, oligosaccharides, or polysaccharides depending on their size. Monosaccharides like glucose exist in both open-chain and ring forms and have isomers that differ in structure. Disaccharides are formed by linking two monosaccharides, while polysaccharides are long chains of monosaccharides that can be either homopolym
2. Introduction
• Hydrates of carbon
• OR
• Polyhydroxy aldehydes or ketones are called carbohydrates
• The emperical formula for many of the simpler carbohydrates is (CH2O)n, hence
the name “hydrates of carbon.”
3. Significance of carbohydrates
• They have a wide range of functions,
• a significant fraction of the dietary calories for most organisms (Starch, Glucose)
• act as a storage form of energy in the body ( Glycogen)
• serve as cell membrane components that mediate some forms of intercellular
communication. (Glycosaminoglycans)
• Carbohydrates also serve as a structural component of many organisms, including
the cell walls of bacteria, the exoskeleton of many insects, and the fibrous cellulose
of plants.
4. • Glucose is most important carbohydrate in living organisms
• It is synthesized by plants in a process called photosynthesis and stored as Starch
and cellulose
• Glucose is precursor for synthesis of all other carbohydrates in body such as
glycogen ( storage form of carbohydrates), Ribose and deoxyribose in nucleic
acids, Proteoglycans, glycoproteins and glycolipids.
6. Monosaccharides
• Monosaccharides are those carbohydrates that cannot be hydrolyzed into simpler
carbohydrates:
• They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses,
depending upon the number of carbon atoms; and as aldoses or ketoses
depending upon whether they have an aldehyde or ketone group.
7. Isomers and Epimers
• Compounds that have the same chemical formula but have different
structures are called isomers. For example, fructose, glucose, mannose, and
galactose are all isomers of each other, having the same chemical formula,
C6H12O6 but differ in structures.
• Carbohydrate isomers that differ in configuration around only one specific
carbon atom (with the exception of the carbonyl carbon) are defined as
epimers of each other.
• For example, glucose and galactose are C-4 epimers—their structures differ
only in the position of the –OH group at carbon 4.
• [Note: The carbons in sugars are numbered beginning at the end that
contains the carbonyl carbon—that is, the aldehyde or keto group.]
• Glucose and mannose are C-2 epimers. However, galactose and mannose
are NOT epimers—they differ in the position of –OH groups at two carbons
(2 and 4) and are, therefore, defined only as isomers
8. Enantiomers
• A special type of isomerism is found in the pairs of structures that
are mirror images of each other.
• These mirror images are called enantiomers, and the two members of
the pair are designated as a D- and an L-sugar.
• The vast majority of the sugars in humans are D-sugars.
• In the D isomeric form, the –OH group on the asymmetric carbon (a
carbon linked to four different atoms or groups) farthest from the
carbonyl carbon is on the right, whereas in the L-isomer it is on the left.
• Enzymes known as racemases are able to interconvert D- and L-
isomers.
9. • Less than 1% of each of the monosaccharides with five or more carbons exists in the open-chain
(acyclic) form.
• They are predominantly found in a ring (cyclic) form, in which the aldehyde (or keto) group has
reacted with an alcohol group on the same sugar, making the carbonyl carbon (carbon 1 for an
aldose or carbon 2 for a ketose) asymmetric.
• [Note: Pyranose refers to a six-membered ring consisting of five carbons and one oxygen, for
example, glucopyranose, whereas furanose denotes a five membered ring with four carbons and
one oxygen.]
Cyclization of Monosaccharides
10. Anomeric Carbon
•
Cyclization creates an anomeric carbon (the former carbonyl
carbon), generating the α and β configurations of the sugar,
• For example, α-D-glucopyranose and β-D-glucopryanose.
• These two sugars are both glucose but are anomers of each
other.
• Because the α and β forms are not mirror images, they are
referred to as diastereomers (Not Enantiomers)
• Enzymes are able to distinguish between these two structures
and use one or the other preferentially. For example, glycogen
is synthesized from α-D-glucopyranose, whereas cellulose is
synthesized from β-D-glucopyranose.
• The cyclic α and β anomers of a sugar in solution are in
equilibrium with each other, and can be spontaneously
interconverted (a process called mutarotation
11. • If the hydroxyl group on the anomeric
carbon (C1/C2) of a cyclized sugar is not
linked to another compound by a
glycosidic bond, the ring can open.
• The sugar can act as a reducing agent, and
is termed a reducing sugar.
• A colorimetric test can detect a reducing
sugar in urine.
• A positive result is indicative of an
underlying pathology because sugars are
not normally
present in urine, and can be followed up
by more specific tests to identify the
reducing sugar
Reducing and Non-Reducing Sugars
12. Modified Monosaccharides
1. Deoxy Sugars:
Deoxy sugars are those in which a hydroxyl group from C2 has been replaced by
hydrogen.
Deoxyribose in DNA and L-Fucose occurs in glycoproteins
2. Amino Sugars (Hexosamines)
If OH group at C2 is replaced by Amino group it is called amino sugar. Word amine
is added at the end of the sugar name.
The amino sugars include
D-glucosamine, a constituent of hyaluronic acid
D-galactosamine, a constituent of chondroitin Sulphate
Several antibiotics (eg, erythromycin) contain amino sugars believed to be
important for their antibiotic activity
13. Disaccharides
• Monosaccharides can be joined to form disaccharides. The
bonds that link sugars are called glycosidic bonds.
• These bonds are formed by enzymes known as
glycosyltransferases that use nucleotide sugars such as
UDP-glucose as substrates.
• Important disaccharides are
• Maltose: It contains Glucose and Glucose
• Sucrose: It contains Glucose and Fructose
• Lactose: It contains Glucose and Galactose
14. Naming glycosidic bonds:
• Glycosidic bonds between sugars are named according to the
numbers of the connected carbons, and with regard to the position of
the anomeric hydroxyl group of the
sugar involved in the bond.
• If this anomeric hydroxyl is in the α configuration, the linkage is an
α-bond.
• If it is in the β configuration, the linkage is a β-bond.
• Lactose, for example, is synthesized by forming a glycosidic bond
between carbon 1 of β-galactose and carbon 4 of glucose. The linkage
is, therefore, a β(1→4) glycosidic bond.
• Linkage in Maltose is a(1-4) while in sucrose is a(1-2)
• [Note: Because the anomeric end of the glucose residue is not
involved in the glycosidic linkage in it (and, therefore, lactose)
remains a reducing sugar while sucrose is non reducing sugar as
anomeric carbons of both glucose and fructose are involved in
linkage]
15. Oligosaccharides
• These are condensation products of three to ten monosaccharides.
• Maltotriose* is an example.
16. Polysaccharides
• These are condensation products of more than ten monosaccharide units and may
contain hundreds or thousands of sugars.
• Polysaccharides can be of two types.
• Homopolysaccharides:
• Heteropolysaccharies:
• Made up of more than one type of sugar subunits such as Hyaluronic acid, Heparin,
Chondroitin Sulphate etc.
• Polysaccharides are sometimes classified as hexosans or pentosans, depending upon the
identity of the constituent monosaccharides.
17. Homopolysaccharides
• Made up of only one type of sugar subunits. Examples are the
starch, dextrins, Glycogen, cellulose (All made of glucose only)
and Inuline (Polymer of fructose).
1. Starch is a homopolymer of glucose forming an α- glucosidic
chain, called a glucosan or glucan. It is the most abundant dietary
carbohydrate in cereals, potatoes, legumes, and other vegetables.
• The two main constituents are
• Amylose (15–20%), which has a non-branching, helical structure
• Amylopectin (80–85%), which consists of branched chains
composed of 24–30 glucose residues united by 1 → 4 linkages in
the chains and by 1 → 6 linkages at the branch points.
18. 2. Glycogen is the stored form of polysaccharides in animals.
• It is present in Liver (4-8%) and muscles (0.5-1%).
• Its structure is similar to amylopectin structure of starch.
• It is a more highly branched structure than amylopectin, with chains of 12–14 α-D-glucopyranose
residues (in α[1 → 4]-glucosedic linkage), with branching by means of α(1 → 6)-glucosidic bonds.
• The function of liver glycogen is to maintain the blood glucose level in the range of 60-100mgl/dl
• The function of muscle glycogen is to provide glucose for metabolism and release energy
19. 3. Cellulose is a homopolysaccharide of B-D-Glucose.
It is a chief constituent of plants cell wall.
• It is insoluble and consists of β-D-glucopyranose units linked by β(1 → 4) bonds to form
long, straight chains strengthened by cross-linked hydrogen bonds.
• Repeating disaccharide units in cellulose are called cellubiose.
• Cellulose cannot be digested by mammals because of the absence of an enzyme that
hydrolyzes the β linkage.
• Microorganisms in the gut of ruminants and other herbivores can hydrolyze the β
linkage and ferment the products to short-chain fatty acids as a major energy source.
• There is limited bacterial metabolism of cellulose in the human colon.a
20. 4. Dextrans:
• These are highly branched polymers of glucose.
• Linear chains are formed by a (1-6) bonds while branching occurs by a(1-2) (1-3) OR (1-4)
glycosidic linkages.These are produced by yeast and bacteria.
• They absorb water and thus are useful when administered intravenously in retaining water in
circulation for long periode (as they are not metabolized).
5. Dextrins:
Partially hydrolyzed/digested form of starch is called dextrin.
6. Inulin:
It is a homo-polysaccharide of fructose (and hence a fructosan) found in tubers and roots of some
plants. It is readily soluble in water and is used to determine the glomerular filtration rate.
7. Chitin:
It is a structural polysaccharide in the exoskeleton of crustaceans and insects and also in
mushrooms.
• It consists of N-acetyl-D-glucosamine units joined by β (1 →4)-glycosidic linkages
21. Heteropolysaccharides
Polysaccharides consisting of molecules of more than one sugar or sugar
derivative are called heteropolysaccharides or complex polysaccharides
(heteroglycans).
Most contain only two different units and are associated with proteins
(glycoproteins such as gamma globulin from blood plasma, acid
mucopolysaccharides) or lipids (glycolipids; e.g., gangliosides in the central
nervous system).
The complex nature of these substances has made detailed structural studies
extremely difficult.
22. 1. Glycosaminoglycans (GAGs)/Mucopolysaccharides
• These are Proteoglycans formed by repeating unbranched disaccharides.
• They contain amino sugars (glucosamine or galactosamine which may or may not be
sulphated) and uronic acid (glucuronic acid or iduronic acid)
• Disaccharide part is covalently attached to a protein molecule and is called
proteoglycan.
23.
24. • Glycosaminoglycans (mucopolysaccharides) are complex carbohydrates characterized
by their content of amino sugars and uronic acids.
• When these chains are attached to a protein molecule, the result is a proteoglycan.
• Proteoglycans provide the ground or packing substance of connective tissues.
•
• Their property of holding large quantities of water and occupying space, thus
cushioning or lubricating other structures, is due to the large number of OH groups
and negative charges on the molecules, which by repulsion, keep the carbohydrate
chains apart.
• Examples are hyaluronic acid, chondroitin sulfate, and heparin
27. Bacterial cell wall
• The cell wall of many bacteria is composed of peptidoglycan, which covers the entire
surface of the cell.
• It is made up of a combination of peptide bonds and carbohydrates. The wall of a
bacterium is classified in two ways:
• Gram-positive. A gram-positive cell wall has many layers of peptidoglygan that retain
the crystal of violet dye when the cell is stained. This gives the cell a purple color when
seen under a microscope.
• Gram-negative. A gram-negative cell wall is thin. The inside is made of peptidoglycan.
The outer membrane is composed of phospholipids and lipopolysaccharides.
• The cell wall does not retain the crystal of violet dye when the cell is stained. The cell
appears pink when viewed with a microscope.
