Biomolecules of the Cell - Carbohydrates (Part 1)

Carbohydrates are the most abundant biomolecules belonging to the class of the organic compounds consisting of carbon (C), hydrogen (H) and oxygen (O). The term 'carbohydrate' generated from 'carbon' and 'hydrate' though some also have nitrogen, phosphorous or sulphur. If you want to define carbohydrate, then the definition goes like this - ‘Carbohydrates are polyhydroxylated aldehydes or ketones.’ They generally have as many Os as Cs (meaning they are highly oxidised). The general formula depicting the carbohydrate is (CH2O)n  where ‘n’ is the number of carbon atoms.
From where do these carbohydrates originate? Yes, one of the origins is - the product of photosynthesis (sucrose) which is a reductive condensation of carbon dioxide and water in the presence of light and the chlorophyll pigment.

Classification of Carbohydrates:

Carbohydrates are called ‘saccharides’ or sugars. The carbohydrates can be broadly divided into the following groups depending on the number of carbon atoms:

1. Monosaccharides: It is the simplest and smallest unit of carbohydrate containing 3-7 carbon atoms. The one with three carbon atoms are called trioses (ex. glyceraldehyde); the one with four carbon atoms are tetroses (ex. erythrose); the one with five carbon atoms are pentoses (ex. ribose), the one with six carbon atoms are hexoses (ex. glucose shown in figure) and the one with seven carbon atoms are heptoses (ex. sedoheptulose). These are the very basic carbohydrates from which the below described disaccharides, oligosaccharides and polysaccharides are formed.

2. Disaccharides:  A disaccharide consists of two monosaccharides joined together by a glycosidic linkage. Disaccharides can be homo (consisting of two same monomers; ex. mannose which consists of two glucose molecules) or hetero-disaccharide (consisting of two different monomers; ex. lactose which consists of  galactose + glucose as depicted in the adjacent figure).

3. Oligosaccharides: An oligosaccharide contains upto 10 monosaccharide units joined by a glycosidic linkage. They are generally found linked to amino acid chains in proteins or lipid moieties. 

4. Polysaccharides: They are the complex sugars; polymers consisting of many monosaccharide units joined together by glycosidic linkages. They are very large, maybe branched or unbranched biomolecules. Polysaccharides can be homo-polysaccharides as well as hetero-polysaccharides wherein, in the former, all the monosaccharides are of the same type and the latter contains more than one type of monosaccharide.

In this post, we will discuss only about monosaccharides.

Characteristics of Monosaccharides:

Lets have a detailed look at monosaccharides which form the basis of all the carbohydrates.

A. Families of Monosaccharides 

Each monosaccharide has a carbonyl group (one of the C-atom is double bonded to an O-atom). Where this carbonyl group is placed in the structure; indicates the type of the monosaccharide. When this carbonyl group is placed at the end of the molecule, then it is aldehyde and the aldehyde-containing monosaccharides are called aldoses. Remember that all aldoses have –CHO at the top and the CH2OH at the bottom (see adjacent figure). Alternatively, when this carbonyl group is present at any other position (except the end as shown in adjacent figure), then it forms a ketone and ketone-containing monosaccharide are called ketoses. Remember that the ketoses have CH2OH at the top as well as the bottom of the molecule.

B. Chiral Center, Enantiomers, Diastereoisomers, Epimers: 

Let us take an example of the smallest and simplest monosaccharide, glyceraldehyde which is a triose to understand what is a chiral center. The central carbon atom here is referred to as the 'chiral center'. A carbon atom is called ‘chiral’ when that carbon atom has four different functional groups attached to it (see the figure below). The compound having a chiral carbon atom is called the chiral compound. These chiral compounds lack a plane of symmetry and exist as two optical isomers which are called as enantiomers. Thus, enantiomers are those isomers that lack the plane of symmetry and exist as two forms in nature as right handed (D-form) or left-handed (L-form) which are non-superimposable. These compounds also have the property of differentially reflecting polarized light. To make it more clear, the two forms of glyceraldehyde (D- and L- glyceraldehyde) are depicted in the figure below. These two isomers are mirror images of each other.  

D and L-sugar: When a monosaccharide or a sugar can be called D-sugar or L-sugar? Monosaccharides are often represented by a Fischer Projection, a shorthand notation particularly useful for showing stereochemistry in straight chained organic compounds.  By convention, when the hydroxyl (-OH) group in the chiral carbon is on the right hand side of the Fischer projection, it is called the D-sugar (or D-isomer) and when the hydroxyl group is on the left hand side, it is the L-sugar (or L-isomer).

Diastereoisomers: Another type of isomerism seen in carbohydrates is diastereoisomers. When the chiral carbons are connected to the exactly same substrates but the configurations are different (R or S), then they are diastereoisomers. Diastereoisomers are NOT mirror images like that of enantiomers. For example, in the adjacent figure, D-glucose and D-altrose are diastereoisomers.

Epimers: The last type of isomerism shown by carbohydrates is epimerism. Epimers are two diastereomers that contain more than one chiral center but differ from each other in the absolute configuration at only one chiral center. For example, in the figure, D-glucose and D-mannose are examples of epimers.

 C. Cyclic Monosaccharides:

Monosaccharides can exist as three different forms - open chain (Fischer projection, mentioned above) and two cyclic forms as alpha (α) and beta (β) sugar. Monosaccharides that have 5, 6 or 7 carbon atoms are often stable in aqueous solutions as they can form cyclic structures as against the open chains. 

How are cyclic structures formed? Cyclic monosaccharides are spontaneously formed by a covalent linkage between the carbonyl carbon and a  hydroxyl group in the carbon backbone. This covalent bond is the result of the reaction between an alcohol group and an aldehyde group of an aldose sugar to a form a hemiacetal or between an alcohol group and the ketone group of a ketose sugar to form a hemiketal. Confused? Okay! To make it more clear, lets take an example as to how

D-glucose cyclization reaction takes place?
Here, the C-5 hydroxyl group of D-glucose attacks the oxygen atom of C-1 aldehyde to form a cyclic hemiacetal (adjacent figure). In this conformation, the C-1 carbon of D-glucose becomes the new chiral center and cyclic forms exists as either β-D-glucose where the hydroxyl group at C-1 is above the plane of the ring or as α-D-glucose where the hydroxyl group is below the plane of the ring (figure at the adjacent makes it clear).
The hemiacetal C-1 carbon atom of cyclic D-glucose is called the anomeric carbon. α-D-glucose and β-D-glucose which are two different forms of cyclic sugars are called anomers as they differ only at anomeric carbon. Cyclic conformations of hexose sugars are called pyranoses (for ex. glucose in cyclic forms will be either α-D-glucopyranose or β-D-glucopyranose)  because the 6-membered ring is similar to a pyran compound. Ketose (like fructose) also forms cyclic structure but the carbonyl is at the C-2 position of the open chain and thus, the ring form contains only 5 carbon atoms. These sugars are called furanoses because they resemble the compound, furan (fructose in cyclic form will be referred to as α-D-fructofuranose or β-D-fructofuranose).

Remember that pyranose rings are much more stable in solution than furanose rings. 

This was all about monosaccharides. In the next post, I have discussed about disaccharides, oligosaccharides and polysaccharides.