Types of Carbohydrates and its function

 Types of Carbohydrates and its function

    1.1  Monosaccharide

Monosaccharide consist of a single polyhydroxy aldehyde or ketone unit. Monosaccharides are the simplest sugars and they a general formula CnH2nOn. Monosaccharides are colorless, crystalline solids that are freely soluble in water but insoluble in nonpolar solvents. The most abundant monosaccharide in nature is the D-glucose.

      I.                 I. Number of carbon atoms

Monosaccharides can be named by a system that is based on the number of carbons with the suffix- ose added. Monosaccharides with four, five, six and seven carbon atoms are called tetroses, pentoses, hexoses and heptoses, respectively.

System for numbering the carbons : The carbon are numbered sequentially with the aldehyde or

 ketone group being on the carbon with the lowest possible number.

 

      I.                II. Types of functional groups

Monosaccharide can be classified into aldoses and ketoses.

Aldoses are monosaccharides with an aldehyde group.

Ketoses are monosaccharides containing ketone group.

 For example, the monosaccharide glucose is an aldohexoses; that is, it has six carbon (-hexose) and aldehyde group (aldo-). Similarly fructose is a ketohexoses that is, it has six carbon (-hexoses) and a ketone group (keto-). Trioses are simplest monosaccharides. There are two trioses- dihydroxyacetone and glyceraldehyde. Dihydroxyacetone is a ketose because it contains a keto group, whereas glyceraldehyde is an aldose because it contains an aldehyde group.

1.1.1       Cyclic forms

Monosaccharides having 5 or 6 carbons in the chain gives cyclic structure in aqueous solution via internal hemiacetal  or hemiketal formation.

Hemiacetal

            in general, an aldehyde can react with an alcohol to form a hemiacetal.

 

Hemiketal

            A ketone can react with an alcohol to form a hemiketal.

For an aldohexose such a glucose, the C-1 aldehyde group in the open chain form of glucose reacts with the C-5 hydroxy group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six- membered ring, is called pyranose because of its similarity to pyran.

Similarly, the C-2 keto group in the open chain- form of a ketohexose, such as fructose, can form an intramolecular hemiketal by reacting with either the C-6 hydroxyl group to form a six-membered cyclic hemiketal or the C-5 hydroxyl group to form a five membered cyclic hemiketal. The five- membered ring is called a furanose because of its similarity to furan, thus fructose forms both pyranose and furanose rings.

Aldopentose such as ribose can form furanose or pyranose rings. For the five carbon sugar ribose, the pyranose form arises when the carbonyl group reacts with the terminal hydroxyl group. The carbon 5 is incorporated into the ring. If the cyclization occurs between the hydroxyl group on carbon 4 and the carbonyl group, then furanose ring form. This place the carbon 5 outside the ring.      

Cyclic structure exists in two different configurational forms. If the hydroxyl on the anomeric carbon is below plane of the ring, it is said to be in the α-position; if the above plane of the ring, it is in the β-position. These two diastereoisomers are called anomers and the hemiacetal or hemiketal carbon is known as anomeric carbon.

 In glucose, the C-1 carbon atom is called is called the anomeric carbon atom, and the α and β forms are called anomers. An equilibrium mixture of glucose contains approximately 37% α-form and 63% β form and less than 1% of the open chain form. The anomers have different physical and chemical properties. For example, α-D- glucose has a specific rotation of +112^o whereas the β-D- glucose form has a specific rotation of the +19^o. When either of these pure substance is dissolved in water, the specific optical rotation of the solution slowly changes until it reaches an equilibrium value of specific rotation of +52.7^o. In aqueous solution the interconversion of α- and β- forms via the open chain structure, to give an equilibrium mixture is known as mutarotation. 

Figure: The α and β cyclic isomers of D- glucose can interconvert, with the open chain structure as the intermediate.

The same nomenclature applies to the furanose ring form of fructose, except that α and β refer to the hydroxyl groups attached to C-2, the anomeric carbon atom.     

1.1.1       Derivatives of monosaccharide

                 Glucosides

When hemiacetals reacts with alcohols, it forms acetals and if hemiacetal of the sugar reacts with an alcohol to form acetal, it is known as a glycoside. Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, and a second compound that may or may not be another monosaccharide. If the hemiacetal portion is glucose, the resulting compound is glucoside; if galactose, a galactoside; and so on. Glycosides are widely distributed in nature. A very common glycoside is ouabain which inhibite the action of enzymes that pump Na⁺ and K⁺ ions across cell membranes. Other glycosides include antibiotics such as streptomycin. 

Sugar acids

            The aldehyde group on aldose can be oxidized to produce a class of monosaccharide called aldonic acids (if glucose, it is gluconic acid) One important aldonic acid is L- ascorbic acid or vitamin C. Aldose can undergo selective oxidation also. If terminal -OH group oxidizes, it produce uronic acid (if glucose, it is glucuronic acid). If both the aldehyde group and terminal ­-OH oxidizes then aldaric acid (if glucose it is glucaric acid) is produced.

1.2 Disaccharides and glycosidic bond

Disaccharides are the simplest and almost common oligosaccharides containing three or more residues are relatively rare, occurring almost entirely in plants. A disaccharides consist of two monosaccharides joined by an O-glycosidic bond. A bond formed between the anomeric carbon atom of a monosaccharide and the oxygen atom of an alcohol is called a glycosidic bond. Glycosidic bonds are labelled α or β depending on the anomeric configuration of the carbon involved in the glycosidic bond. For example, in maltose two molecules of glucose are linked by an α1→ 4 glycosidic bond. The glycosidic bond forms between C-1 (the anomeric carbon) of one glucose residue and hydroxyl oxygen atom on C-4 of the other. The configuration of the anomeric carbon atom participates in this glycosidic bond formation is α.

Similarly, Sucrose is a disaccharide of glucose and fructose residue joined by an α1 ↔ 2β glycosidic bond between C-1 (anomeric carbon) of glucose residue and C-2 (the anomeric carbon) of the fructose residue the anomeric carbons of both monosaccharide units are involved in the glycosidic bond. The configuration of the anomeric carbon atom involved in the glycosidic bond formation are α for glucose and β for fructose. The abbreviated name of sucrose is either Glc(1α↔2β) Fru or Fru (2β↔α1) Glc. 

Figure: The bond connecting the anomeric carbon to the hydroxyl oxygen atom is termed a glycosidic bond. Sucrose is a disaccharide of glucose and fructose residues joined by α1↔2β glycosidic bond. The disaccharide maltose contains two glucose residues joined by an α1→4 glycosidic bond between C-1 ( the anomeric carbon) of one glucose residue and C-4 of the other. Lactose is a disaccharide of galactose and glucose residue joined by a β1→4 glycosidic bond.  

Oligosaccharide (as well as polysaccharides) have a directionally which is defined by their reducing and nonreducing ends. The monosaccharide unit at the reducing end has a free anomeric carbon atom that has reducing activity because it can form the open- chain form whereas monosaccharide unit at the non-reducing end has no free anomeric carbon due to its participation in the glycosidic bond formation.

Disaccharides	Structure	Physiological role
Sucrose	Glucose (α1↔ 2β) Fructose	A product of photosynthesis.
Lactose	Galactose (β1→ 4) Glucose	A major animal energy source.
Trehalose	Glucose (α1 ↔ 1α) Glucose	A major circulatory sugar in insects; used for energy.
Maltose	Glucose (α1 → 4) Glucose	The dimer derived from the starch and glycogen.
Cellobiose	Glucose (β1 → 4) Glucose	The dimer of the cellulose polymer.
Gentiobiose	Glucose (β1 → 6) Glucose	Constituent of plant glycosides and some polysaccharides.

 1.3  Polysaccharides

Polysaccharides are ubiquitous in nature. They are also called glycans. They can be classified into two separate groups, based on their functions: Structural and storage polysaccharides. Structural polysaccharides provide mechanical stability to cells, organs and organisms. Storage polysaccharide serve as carbohydrate stores that release monosaccharide as per required. Polysaccharides may be homopolysaccharides (contain only single type of monomeric unit) or heteropolysaccharides (contain two or more different kind of monomeric units).

1.1.1                    1.3.1Homopolysaccharides

Starch is a branched chain of D-glucose units. It is the storage form of glucose in plants. It contains mixture of amylose and amylopectin. Amylopectin is a branched polymer of α-D glucose with α1→4- glycosidic bonds with α1→6 branching points that occur at intervals of approximately 25 to 30 α-D glucose residues. Amylose is a linear unbranched polymer of α-D- glucose units in a repeating sequence of α1→4- glycosidic bonds. 

Figure: The storage polysaccharide in plant is starch. Amylose, the unbranched fraction of starch, consist of glucose residues joined by α1→4 bond. Amylopectin, the branched fraction, has branching at intervals of approximately 25 to 30 glucose residues.    

The iodine test is used to test for the presence of starch.  Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The amylose forms helical structure. The iodine slip inside of the helical structure, forming a deep blue color. Amylopectin having a branched structure, reacts with iodine to give a reddish purple color. Since amylopectin is highly branched, it only binds a small amount of iodine and produce a reddish purple color.

Glycogen is the major storage form of carbohydrate in animals, found mostly in liver and muscle. It is a highly branched form of amylopectin; branching occurs at intervals of eight to ten glucose residues.

Cellulose is a linear, unbranched homopolysaccharide of D-glucose residue joined by β1→4 glycosidic bonds. Cellulose is a structural polysaccharide of plant cells. Although cellulose forms a part of the human diet (e.g in vegetables and fruits), it is not hydrolysed by human enzyme systems. Cellulose is one of the most abundant organic compound in the biosphere.

Chitin is linear homopolysaccharide composed of N-acetyl-D-glucosamine residues joined by β1→4glycosidic bonds. The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. It is structural polysaccharides present the cell wall of fungi and also in the exoskeleton of insects and crustaceans.   

       

1.1.1            1.3.2  Heteropolysaccharides

Glycosaminoglycans are negatively charged, unbranched heteropolysaccharide composed of repeating disaccharide units,[acidic sugar – amino sugar]_n. amino sugar always either N-acetylglucosamine or N-acetylgalactosamine and the acidic sugar in most cases is a uronic acid, usually glucuronic acid. The simplest glycosaminoglycan hyaluronan (hyaluronic acid) contains alternating residues of D-glucuronic acid and N-acetylglucosamine. Chondroitin sulfate, keratan sulfate, heparin, heparan sulfate, dermatan sulfate and hyaluronate are the major glycosaminoglycans. These polysaccharide are unique to animals and bacteria and are not found in plants. With the exception of hyaluronic acid, all the GAGs contain sulfate groups, either as O-esters or as N-sulfate. All of the glycosainoglycans except hyaluronic acid are found covalently attached to protein forming proteoglycan.

Figure: Glycosaminoglycans are made up of disaccharide repeating units in which one of the two monosaccharide units is a uronic acid (keratan sulfate is an exception) and the other an N- acetylated amino sugar. Glycosaminoglycans are usually attached to proteins through link tetrasaccharide to form proteoglycans. In a typical link tetrasaccharide, the xylose residue at the reducing end of the linker