Carbohydrates, also known as saccharides or carbs, are sugars or starches. They are important as
1.
Source of energey (sugar and starch).
2. Structural role in the cell walls of
bacteria (peptidoglycan) and plants (cellulose)
3. Storage role (glycogen and starch)
4. Glycoconjugates, are involved in cell–cell interactions, including cell–cell recognition and in cell–matrix interactions.
1. PROPERTIES
2. TYPES
Carbohydrates are classified into 1. Monossaccharides 2. Disaccharides 3. Oligosaccharides and 4. Polysaccharides
2.1 Monosaccharides
Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose and fructose. In monosaccharides, the number of carbons usually ranges from three to seven. Most monosaccharide names end with the suffix -ose.
a. Aldose and Ketose If the sugar has an aldehyde group (the functional group with the
structure R-CHO), it is known as an aldose,
and if it has a ketone group (the functional group with the structure
RC(=O)R'), it is known as a ketose.
Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons).
Although glucose, galactose, and fructose, all have the same chemical
formula (C6H12O6), they differ structurally
and chemically.
b. Chiral Center Except dihydroxyacetone, all the monosaccharides contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms.
The simplest aldose, glyceraldehyde, contain 1 chiral center and thus has 2 optical isomers, or enantiomers. These are mirror images and not super imposable.
In general, a molecule with n chiral centers can have 2n stereoisomers.
Glyceraldehyde has 21 = 2; the aldohexoses (glucose), with four chiral centers, have 24 =16 stereoisomers.
Those in which the configuration at this
reference carbon is the same as that of D-glyceraldehyde are designated D
isomers, and those with the same configuration as L-glyceraldehyde are L
isomers (Fischer System).
Dextrorotation (d-, right-handed rotation) and levorotation (l-, left-handed rotation) are early systems of designating optical isomers based on optical rotation of plane-polarized light. One limitation of this system is not all optical isomers can rotate light.
The (R)- and (S)- system (from the Cahn–Ingold–Prelog) also used in organic chemistry for optical isomers.
c. Epimer
Two sugars that differ only in the configuration around one carbon atom are called epimers; D-glucose and D-mannose, which differ only in the stereochemistry at C-2, are epimers, as are D-glucose and D-galactose (which differ at C-4).
d. Anomers [a and b isomers] In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms occur predominantly as cyclic (ring) structures in which the carbonyl group forms a covalent bond with the oxygen of a hydroxyl group along the chain.
The monosaccharide isomers that differ only in their configuration about the hemiacetal or hemiketal carbon atom are known as anomers and the hemiacetal carbon atom is called the anomeric carbon.
D-glucose exists in solution as an intramolecular hemiacetal in which with the aldehydic at C-1 reactes with free hydroxyl group at C-5, rendering a carbon asymmetric and producing two stereoisomers, designated a and b.
These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran.
The systematic names for the two ring forms of D-glucose are a-D-glucopyranose and b -D-glucopyranose.
Ketohexoses (D-fructose) also exist in cyclic forms having five membered rings, which, because they resemble the five membered ring compound furan, are called furanoses.
Mutarotation
The α-D-glucose and β-D-glucose exist in separate crystalline forms and specific roations. For example α-D-glucose has a specific rotation of +112° while β-D-glucose has a specific rotation of +19°. However, when either of these two forms is dissolved in water and allowed to stand, it gets converted into an equilibrium mixture of α-and β-forms through a small amount of the open chain form.
As a result of this equilibrium, the specific rotation of a freshly prepared solution of α-D-glucose gradually decreases from of +112° to +52.7° and that of β-D-glucose gradually increases from +19° to +52.7°.
This change in specific rotation of an optically active compound in solution with time, to an equilibrium value, is called mutarotation. During mutarotation, the ring opens and then recloses either in the inverted position or in the original position giving a mixture of α-and-β-forms. All reducing carbohydrates, i.e., monosaccharides and disacchardies (maltose, lactose etc.) undergo mutarotation in aqueous solution.
e. Monosaccharide
Derivatives They
are a number of sugar derivatives in which -OH group in the parent compound is
replaced with another substituent, or a carbon atom is oxidized to a carboxyl
group.
|
Monosaccharide Derivatives |
|
|
Amino Sugars [Glucosamine, Galactosamine Mannosamine] |
|
|
N-acetylglucosamine (NAG) N-acetylmuramic acid (NAM) |
Bacterial cell wall (peptidoglycan) |
|
Deoxy Sugars |
|
|
L-fucose
or L-rhamnose |
found in plant
polysaccharides and glycoproteins and glycolipids |
|
Aldonic
Acids [glucuronic,
galacturonic, or mannuronic acid] |
|
|
N-acetylneuraminic acid (sialic acid), a
derivative of N-acetylmannosamine |
component of many
glycoproteins and glycolipids in animals |
f. Monosaccharides-Reducing Agents Monosaccharides readily loose electrons (oxidation) and can be oxidized by relatively mild oxidizing agents such as ferric (Fe3+) or cupric (Cu2+) ion and carbonyl carbon is oxidized to a carboxyl group, thus act as reducing agents.
Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars. This property is the basis of Fehling’s Test, a qualitative test for the presence of reducing sugar. For many years this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of Diabetes mellitus.
Now, more sensitive methods for measuring blood glucose employ glucose oxidase.
2.2 DIsaccharides
a. Glycosic
Bond
Disaccharides (maltose, lactose, sucrose
and trehalose) consist of two
monosaccharides joined covalently by an O-glycosidic
bond. This reaction represents
the formation of an acetal from a hemiacetal (one sugar
molecule) and an alcohol (-OH of the second sugar molecule). A
water molecule is released and O-linkage is formed between the two molecules
and O-glycosidic bond so it's an ether
linkage.
Glycosidic
bonds are readily hydrolyzed by acid
but resist cleavage by base.
The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s). When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar. In describing disaccharides or polysaccharides, the end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.
b. Examples
The maltose, contains two D-glucose residues joined by a glycosidic bond between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other. Since, maltose retains a free anomeric carbon (C-1 of the glucose residue on the right), maltose is a reducing sugar. The configuration of the anomeric carbon atom in the glycosidic linkage is a (1®4).
The lactose, occurs naturally only in milk, yields D-galactose and D-glucose on acid hydrolysis. The anomeric carbon of the glucose residue is available for oxidation, and thus lactose is a reducing disaccharide.
Sucrose is a disaccharide of glucose and fructose. In contrast to maltose and lactose, sucrose contains no free anomeric carbon atom; the anomeric carbons of both monosaccharide units are involved in the glycosidic bond. Sucrose is therefore a nonreducing sugar. Nonreducing disaccharides are named as glycosides.
Trehalose, a disaccharide of D-glucose that, like sucrose, is a nonreducing sugar—is a major constituent of the circulating fluid (hemolymph) of insects, serving as an energy-storage compound.
2.3 OLIGOsaccharides
An oligosaccharide is a saccharide polymer containing a small number of monosaccharides [3 to 10]. Oligosaccharides can have many functions including cell recognition and cell binding and we learn more in glycocongugates.
2.4 Polysaccharides
Most carbohydrates found in nature occur as polysaccharides (also called glycans).
a. Types
Homopolysaccharides contain only a single type of monomer; heteropolysaccharides contain two or more different kinds.
b. Examples
Starch (amylum)
is a homopolysaccharide is produced by most green plants as energy storage. It is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, maize (corn), rice, wheat and cassava.
The starch contains two types of glucose polymers, amylase and amylopectin.
Amylose
consists of long, unbranched chains of D-glucose residues
connected by (a1®4)
linkages.
Amylopectin also has a high molecular weight (up to 100 million) but unlike amylose is highly branched.
Glycogen
is a homopolysaccharide that serves as a form of energy storage in animals, fungi, and bacteria.
Like amylopectin, glycogen is a polymer of (a1®4)-linked subunits of glucose, with (a1®6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch.
Each branch in glycogen ends with a nonreducing sugar unit, a glycogen molecule has as many nonreducing ends as it has branches, but only one reducing end.
Dextrans
Made up of (a1®6)-linked poly-D-glucose; all have (a1®3) branches, and some have (a1®2) or (a1®4) branches. Dental plaque (formed by bacteria growing on the surface of teeth) is rich in dextrans.
Synthetic dextran, Sephadex used in size-exclusion chromatography.
Cellulose
Like amylose, it is a linear, unbranched homopolysaccharide, consisting of 10,000 to 15,000 D-glucose units. In cellulose, the glucose residues have the b configuration.
The glucose residues in cellulose are linked by (b1®4) glycosidic bonds. The a-amylases, enzymes of saliva and intestinal secretions hydrolyze glycogen and starch that break (a1®4) glycosidic bonds between glucose units.
Most animals cannot use cellulose, because they lack an enzyme to hydrolyze the (b1®4) linkages. Termites readily digest cellulose, because their intestinal tract harbors a symbiotic microorganism, Trichonympha, that secretes cellulase, which hydrolyzes the (b1®4) linkages. Wood-rot fungi and bacteria also produce cellulase.
Starch, Glycogen and Cellulose Folding
The three-dimensional structure of starch and glycogen is a tightly coiled helix, stabilized by interchain hydrogen bonds and have six residues per turn. For amylose, the core of the helix is of precisely the right dimensions to accommodate iodine, giving an intensely blue complex. This interaction is a common qualitative test for amylose and starch.
For cellulose, the most stable conformation
a straight, extended chain or sheet.
Hemicellulose
A heteropolysaccharide present along with cellulose in almost all plant cell walls. Diverse kinds of hemicelluloses are known. Important examples include xylan, glucuronoxylan, arabinoxylan, glucomannan and xyloglucan.
Pectin
A branched heteropolysaccharide contained in the plant primary and middle lamella and cell walls. Its main component is galacturonic acid segments and other neutral sugars such as rhamnose, arabinose, galactose, and xylose, a sugar acid derived from galactose. It is produced commercially and is used in food as a gelling agent, particularly in jams and jellies.
Chitin
A linear homopolysaccharide composed of N-acetyl glucosamine residues in b linkage.
Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs, for example— and is the second most abundant polysaccharide, next to cellulose.
Pepetidogycan
The rigid component of bacterial cell walls is a heteropolymer of alternating (b1®4)-linked N-acetylglucosamine and N-acetylmuramic acid residues. The linear polymers lie side by side in the cell wall, cross linked by short peptides.
The enzyme lysozyme kills bacteria by hydrolyzing the (b1®4) glycosidic bond between N-acetylglucosamine and N-acetylmuramic acid.
Agar
The marine red algae have cell walls that contain agar, a mixture of sulfated heteropolysaccharides made up of D-galactose and an L-galactose derivative ether-linked between C-3 and C-6.
The two major components of agar are the unbranched polymer agarose and a branched component, agaropectin.
Agarose gels are widely used as inert supports for the electrophoretic separation of nucleic acids. Agar is also used to form a surface for the growth of bacterial colonies.
Glycosaminoglycans
The extracellular matrix (ECM) space in the tissues composed of an interlocking meshwork of heteropolysaccharides and collagen, elastin, fibronectin, and laminin. These are heteropolysaccharides of the glycosaminoglycans (GAG), linear heteropolymers composed of repeating disaccharide units of either N-acetylglucosamine or N-acetylgalactosamine; the other is in most cases a uronic acid, usually D-glucuronic or L-iduronic acid.
GAGs are classified into four groups based on core disaccharide structures (hyaluronic acid, chondroitin sulfate, keratan sulfate and Heparin).
Hyaluronic
acid contains
alternating residues of D-glucuronic acid and N-acetylglucosamine serve
as lubricants in the synovial fluid of joints and give the vitreous humor of
the vertebrate eye its jellylike consistency.
Chondroitin sulfate is normally found in cartilage around joints in the body and composed of a chain of alternating sugars (N-acetylgalactosamine and glucuronic acid).
Keratan sulfate found especially in the cornea,
cartilage, and bone. It is composed of the basic repeating disaccharide D-galactose β1®4 linked to N-acetylglucosamine-sulfate.
Heparin made up of α-iduronic acid, β-D-glucuronic acid and a-D-glucosamine repeat units. It is is used as an anticoagulant.
2.5 Glycoconjugates
Glycoconjugates are biologically important
molecules with diverse functions. They consist of carbohydrates of varying size
and complexity, attached to a non-sugar moiety as a lipid or a protein (carbohydrate + protein or lipid).
Glycoconjugates are very important compounds in biology and consist of many different categories such as,
a.
proteoglycans
b.
glycolipids and
c. glycoproteins
Although the
important molecular species DNA, RNA,
ATP,
cAMP, cGMP, NADH, NADPH,
and coenzyme
A all contain a carbohydrate part, generally they are not considered
as glycoconjugates.
a. Proteoglycans
are macromolecules of the cell surface or extracellular matrix in which one or more glycosaminoglycan chains are joined covalently to a membrane protein or a secreted protein.
The basic proteoglycan unit consists of a “core protein” with covalently attached glycosaminoglycan(s). The point of attachment is a Serine residue, to which the glycosaminoglycan is joined through a tetrasaccharide bridge.
Types [Heparin sulfate containing proteoglycans and proteoglycan aggregates]
Heparin sulfate proteoglycans Examples: Syndecans and Glypicans
Proteoglycan aggregates Examples: Aggrecan has multiple chains of chondroitin sulfate and keratan sulfate, joined to Ser residues in the core protein through trisaccharide linkers.
b. Glycolipids
are membrane lipids in which the hydrophilic head groups are oligosaccharides, which act as specific sites for recognition by carbohydrate-binding proteins.
c. Glycoproteins
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains.
The carbohydrate is attached to the protein in a posttranslational modification. This process is known as glycosylation. Almost all the secreted and membrane-associated proteins of eukaryotic cells are glyco proteins.
There are several types of glycosylation, although the first two are the most common.
In N-glycosylation, sugars are attached to nitrogen, typically on the amide side-chain of asparagine.
In O-glycosylation, sugars are attached to oxygen, typically on serine or threonine,
but also on tyrosine.
In P-glycosylation, sugars are attached to phosphorus on
a phosphoserine.
In C-glycosylation, sugars are attached directly to carbon, such as in the addition
of mannose to tryptophan.
In S-glycosylation, a b-N-acetylglucosamine [GlcNAc] is attached to the sulfur atom of
a cysteine residue.
In glypiation, a GPI glycolipid is attached to the C-terminus of a polypeptide, serving as a membrane anchor.
In N-linked oligosaccharides, GlcNAc is invariably linked to the amide nitrogen of an Asn residue in the sequence Asn-X-Ser or Asn-X-Thr, where X is any amino acid except possibly Pro or Asp (Occurs in ER, further processing in Golgi apparatus).
Dolichols play a role in N-glycosylation in the form of dolichol phosphate. Dolichols function as a membrane anchor for the formation of the oligosaccharide. This oligosaccharide is transferred from the dolichol donor onto certain asparagine residues of newly forming polypeptide chains. The sugar group(s) can assist in protein folding or improve proteins stability.
In O-glycosylation, the addition of sugar chains can happen on the hydroxyl oxygen on the side-chain of hydroxylysine, hydroxyproline, serine, or threonine (Occurs in Golgi apparatus).
Examples Mucins, antibodies, major histocompatibility complex (MHC), Glycoprotein-41 (gp41) and glycoprotein-120 (gp120) are HIV viral coat proteins and Hormones (FSH, LH, TSH, HCG, Erythropoietin).
Methods of Carbohydrate Analysis For analysis of the oligosaccharide moieties of glycoproteins and glycolipids, the oligosaccharides are released by purified enzymes—glycosidases that specifically cleave O- or N-linked oligosaccharides or lipases that remove lipid head groups.
The resulting mixtures of carbohydrates are resolved into their individual components by a variety of methods, including the same techniques used in protein and amino acid separation: fractional precipitation by solvents, and ion-exchange and size exclusion chromatography.
Hydrolysis of oligosaccharides and polysaccharides in strong acid yields a mixture of monosaccharides, which may be identified and quantified by chromatographic techniques to yield the overall composition of the polymer. Oligosaccharide analysis relies increasingly on mass spectrometry and high-resolution NMR spectroscopy.
2.6 LECTINS
Lectins, found in all organisms, are proteins that bind carbohydrates with high specificity and with moderate to high affinity.
Lectins serve in a wide variety of cell-cell recognition, signaling, and adhesion processes and in intracellular targeting of newly synthesized proteins.
|
Lectin source and lectin |
|
Plant Concanavalin A Ricin Animal Mannose-binding protein A Viral Influenza virus hemagglutinin Polyoma virus protein 1 Bacterial Enterotoxin Cholera toxin |
2.7 Extra Points on carbohydrates
1. The Benedict's test is commonly
used to identify a reducing sugar.
2. The dolichol phosphate carries is a 14-residue precursor oligosaccharide
chain is synthesized in the ER.
3. Carrageenan
is composed of repeating units of galactose.
4.
Tunicamycin blocks N-linked glycosylation.
5. Sugar
alcohols are characteristically sweet tasting, and sorbitol, mannitol, and
xylitol are widely used to sweeten sugarless gum and mints.
6. Sorbitol buildup
in the eyes of diabetic persons is implicated in cataract formation.
7. Ribitol is
a constituent of flavin coenzymes.
8. The
L-rhamnose, deoxy sugar is a
component of a highly toxic cardiac glycoside.
9. RBC contains
glycoprotein on cell surface, which contain sialic acid and it's removal of
destruction of RBC.
10. An
oligosaccharide containing mannose 6-phosphate marks newly synthesized
proteins in the Golgi complex for transfer to the lysosome.
11. Cellobiose is
a disaccharide [reducing sugar] obtained
by enzymatic or acidic hydrolysis of cellulose and
cellulose-rich materials such as cotton, jute
or paper.
12. Barfoed's test is used for detecting the presence of monosaccharides.
13. Bial's test is for the presence of pentoses.
14. Seliwanoff's test distinguishes between aldose and ketose sugars.
15. Hyaluronidase, an enzyme secreted by some pathogenic bacteria, can hydrolyze the glycosidic linkages of hyaluronate, rendering tissues more susceptible to bacterial invasion. In many organisms, a similar enzyme in sperm hydrolyzes an outer glycosaminoglycan coat around the ovum, allowing sperm penetration.
Previous CSIR Questions on Carbohydrates
1. Sucrose does not occur in its anomeric form while its hydrolyzed product glucose and fructose have anomers. The reason is [CSIR 2007 Jun]
a.
C1 of glucose and C1 of fructose are bonded in glycosidic linkage
b.
C1 of glucose and C2 of fructose are bonded in glycosidic linkage
c.
Sucrose is polysaccharide
d. Sucrose is not soluble in water
2.
Chitin occurs in cell wall of [CSIR 2008 Dec]
a. Bacteria
b. Plants
c. Fungus
d. Animals
3.
The structure of carbohydrate is shown as below. In polymer the bonding will be
[CSIR 2009 Dec]
a. 1,2
b. 1,4
c. 4,6
d. 2,4
4. Maximum possible isomers for glucose are [CSIR 2009 Jun]
a. 4
b. 8
c. 16
d. 32
5. Sucrose is composed of [CSIR 2010 Jun]
a. Glucose and galactose
b. Fructose and galactose
c. Glucose and Fructose
d. Mannose and
fructose
6. Glucose residues in amylose are linked by [CSIR 2013 Dec, Part B]
7. Which one of the following statements is correct? [CSIR 2015 Dec, Part C]
a. In all L-amino acids, only the Ca carbon atom is chiral
b. Deoxyribose is optically inactive·
c. The
specific rotation of sucrose will be the sum of the specific rotations of
D-glucose and D-fructose
d. Phosphatidyl choline isolated from
biological membranes is optically active
8. Following are structures of aldohexoses which stereochemistry of stereoisomers differ in the
Based on above structures. following information was given below:
A. D-glucose and D-mannose are epimers because they
differ in the stereochemistry at C-2 position.
B. D-glucose and
D-galactose are epimers because they differ in the stereochemistry at C-4
position.
C. D-mannose and D-glucose
are epimers because they differ in the stereochemistry at C-3 position.
D. D-galactose and D-glucose are epimers because they differ in the stereochemistry at C-5 position.
Choose one of the correct combinations of above statements: [CSIR 2018 Jun, Part C]
a. A and B
b. C and D
c. B, C and D
d. A and D
9. A stoichiometric mixture of alpha and beta anomers of D-glucose in water exhibits [CSIR 2020 June, Part B]
a.
net optical rotation proportional to the sum of the optical activities of each
anomer
b. no
optical activity as the signs of optical rotation are opposite and they cancel
each other
c. no
optical activity as the alpha and beta anomers exist in the linear forms that are optically
inactive
d. no optical activity as they form a racemic mixture
10.
The following statements are made with reference to characteristics of
glycosaminoglycans and proteoglycans, which are major constituents of
extracellular matrix. Which one of them is INCORRECT? [CSIR 2020 June,
Part C]
a. Glycosaminoglycans are very long polysaccharide chains composed of repeating disaccharide units of an amino sugar and a uronic acid.
b.
Except for hyaluronic acid, all glycosaminoglycans are covalently attached to
protein as proteoglycans.
c.
Glycoproteins contain less carbohydrate usually in the form of relatively
short, branched oligosaccharide chains whereas proteoglycans contain more
carbohydrate in the form of long unbranched glycosaminoglycan chains.
d. Like O-linked and N-linked glycoproteins, in proteoglycan also glycosaminoglycans are linked to serine or threonine and asparagine residues.
Keys
1b 2c 3b 4c 5c 6b 7d 8a 9a 10d