Chemistry of Carbohydrates
Classification and Structure
Classification
There are three major classes of carbohydrates
⦁ Monosaccharides (Greek, mono = one)
⦁ Oligosaccharides (Greek, oligo= few) 2-10 monosaccharide units.
⦁ Polysaccharides (Greek, Poly = many) >10 monosaccharide units.
Monosaccharides
Monosaccharides also called simple sugars. They consist of a single polyhydroxy aldehyde or ketone units. The most abundant monosaccharides in nature are the 6-carbon sugars like D- glucose and fructose.
Structure
Monosaccharide has a backbone, which is un- branched, single bonded carbon chain. One of the carbon atoms is double bonded to an oxygen atom to form carbonyl group. Each of the other carbon atoms has a hydroxyl group. Example. Structure of Glucose
Fig.2.1. structures of D.Glucose
There are two families of monosaccharides. Monosaccharides having aldehyde groups are
called Aldoses and monosaccharides with Ketone group are Ketoses.
Depending on the number of carbon atoms, the monosaccharides are named trioses (C3),
tetroses (C4), pentoses (C5), hexoses (C6), heptoses (C7).
Table 1.1 Common Biologically important monosaccharides with their families.
Physical properties
Physical properties of Monosaccharides
They are colorless, crystalline compounds, readily soluble in water. Their solutions are optically active and exhibit the phenomenon of mutarotation. Carbohydrates spontaneously change between the and β configuration.
Asymmetric Center and Stereoisomerism
Asymmetric carbon is a carbon that has four different groups or atoms attached to it and having optically activity in solution.
All the monosaccharides except dihydroxyacetone contain one or more asymmetric or chiral carbon atoms and thus occur in optically active isomeric forms. Monosaccharides with n number of asymmetric centers will have (2n) isomeric forms. (n= number of asymmetric carbon atoms).
Fig 2.2: The two isomeric forms of glyceraldehyde.
The designation of a sugar isomer as the D- form or of its mirror images the L- form is determined by the spatial relationship to the parent compound of the carbohydrate family. The D and L forms of Glyceraldehyde are shown in the Figure 2.2.The orientation of- OH and- H groups around the carbon atom adjacent to the terminal primary alcohol carbon determines its D or L form .When the - OH group on this carbon is on the right, the sugar is a member of the D- series, when it is on the left, it is a member of the L-series. These D and L configuration are also called Enantiomers.
Optical Activity
The presence of asymmetric carbon atom causes optical activity. When a beam of plane- polarized light is passed through a solution of carbohydrate it will rotate the light either to right or to left. Depending on the rotation, molecules are called dextrorotatory (+) (d) or levorotatory (-) (l). Thus, D- glucose is dextrorotatory but D- fructose is levorotatory. When equal amounts of D and L isomers are present, the resulting mixture has no optical activity, since the activities of each isomer cancel one another. Such a mixture is called racemic or DL mixture.
Epimers
When sugars are different from one another, only in configuration with regard to a single carbon atom (around one carbon atom) they are called epimers of each other. For example glucose and mannose are epimers. They differ only in configuration around C2. Mannose and Galactose are epimers of Glucose
Fig 2.3: Structure of D - glucose, D-mannose and D- Galactose.
Anomers
The two stereoisomers at the hemiacetal (anomeric) carbon are:
- The alpha anomer: Where- OH group is down (Haworth)
- The beta anomer:Where- OH group is up (Haworth)
Cyclization of monosaccharides
Monosaccharides with five or more carbon atoms in the backbone usually occur in solution as
cyclic or ring structure, in which the carbonyl group is not free as written on the open chain
structure but has formed a covalent bond with one of the hydroxyl group along the chain to
form a hemiacetal or hemiketal ring. In general, an aldehyde can react with an alcohol to form
a hemiacetal or acetal.
The C-1 aldehyde in the open-chain form of glucose reacts with the -5th carbon atom containing
hydroxyl group to form an intramolecular hemiacetal. The resulting six membered ring is called
pyranose because of its similarity to organic molecule Pyran.
Two different forms of glucose are formed when the OH group extends to right it is α-D-Glucose
and when it extends to left, it is β-D-Glucose commonly called as Anomers.
Similarly, a ketone can react with an alcohol to form a hemiketal or ketal.
The C-2 keto group in the open chain form of fructose can react with the 5th carbon atom
containing hydroxyl group to form an intramolecular hemiketal. This five membered ring is called
furanose because of its similarity to organic molecule furan
Fig 2.4. α and β forms of Fructose
Oligosaccharides
Oligosaccharides contain 2 to 10 monosaccharide units. The most abundant oligosaccharides
found in nature are the Disaccharides.
Disaccharides
When two monosaccharides are covalently bonded together by glycosidic linkages a
disaccharide is formed. Glycosidic bond is formed when the hydroxyl group on one of the
sugars reacts with the anomeric carbon on the second sugar.
Biologically important disaccharides are sucrose, maltose, and Lactose.
Maltose
Maltose contains two D glucose residues joined by a glycosidic linkage between OH at the first
carbon atom of the first glucose residues and OH at the fourth carbon atom of the second
glucose forming a α-(1,4) glycosidic linkage as shown in Figure below. Maltose is the major
degradative product of Starch.Maltose is hydrolyzed to two molecules of D- glucose by the
intestinal enzyme maltase, which is specific for the α- (1, 4) glycosidic bond.
Fig 2.5. Structure of Maltose
Sucrose (Cane sugar)
Sucrose is a disaccharide of α- D- glucose and β-D-fructose. It is obtained from cane sugar. It is
also present in various fruits. In contrast to other disaccharides sucrose contains no free
anomeric carbon atom. Since the anomeric carbons of both its component monosaccharide
units are linked to each other. For this reason sucrose is non reducing sugar.
Fig 2.7. Structure of sucrose α-(1, 2) β-Glycosidic bond
Polysaccharides
Most of the carbohydrates found in nature occur in the form of high molecular polymers called
polysaccharides.
There are two types of polysaccharides .
These are:
- Homopolysaccharides that contain only one type of monosaccharide building blocks.
- Heteropolysaccharides, which contain two or more different kinds monosaccharide building blocks.
Homopolysaccharides
Example of Homopolysaccharides: Starch, glycogen, Cellulose and dextrins.
Starch
It is one of the most important storage polysaccharide in plant cells.It is especially abundant in
tubers, such as potatoes and in seeds such as cereals.
Starch consists of two polymeric units made of glucose called Amylose and Amylopectin but
they differ in molecular architecture.
Amylose is unbranched with 250 to 300 D-Glucose units linked by α-(1, 4) linkages Amylopectin
consists of long branched glucose residue (units) with higher molecular weight.
The inner part of glucose units in amylopectin are joined by α-(1,4) glycosidic linkage as in
amylose , but the branch points of amylopectin are α- (1,6) linkages. The branch points repeat
about every 20 to 30 (1-4) linkages
Glycogen
Glycogen is the main storage polysaccharide of animal cells (Animal starch).
- It is present in liver and in skeletal muscle.
- Like amylopectin glycogen is a branched polysaccharide of D-glucose units in α - (1, 4) linkages, but it is highly branched.
- The branches are formed by α -(1,6) glycosidic linkage that occurs after every 8 -12 residues.
- Therefore liver cell can store glycogen within a small space. Multiple terminals of branch points release many glucose units in short time.
Cellulose
Cellulose is the most abundant structural polysaccharide in plants. It is fibrous, tough, water
insoluble. Cellulose is a linear unbranched homopolysaccharide of 10,000 or more D- glucose
units connected by β-(1, 4) glycosidic bonds. Humans cannot use cellulose because they lack of
enzyme (cellulase) to hydrolyze the β-( 1-4) linkages.
Figure: Structure of Cellulose
Dextrins
These are highly branched homopolymers of glucose units with α-(1, 6), α-(1, 4) and α-(1, 3)
linkages. Since they do not easily go out of vascular compartment they are used for intravenous
infusion as plasma volume expander in the treatment of hypovolumic shock.
Hetero polysaccharides
These are polysaccharides containing more than one type of sugar residues
Glycosaminoglycans, (GAGs or mucopolysaccharides)
They are long, usually unbranched, composed of a repeating disaccharide units
* They are negatively charged heteroplolysaccharid chains (polyanions)
- The amino sugar is either D-glucosamine or D-galactosamine in which the amino group is usually acetylated, thus eliminating its positive charges.
- The amino sugar may also be sulfated on carbon 4, 6, or on a monoacetylated nitrogen.
- The acidic sugar is either D-glucuronic acid or its carbon 6 epimer, L-uronic acid. For example Hayluronic acid, Heparin and chondatin sulphate.