Carbohydrates and Classification
Carbohydrates are the most abundant biomolecules on earth. They have either an aldehyde or a keto group in addition to many hydroxyl groups. Dihydroxyacetone is the simplest carbohydrate with a keto group. Carbohydrates are defined as the polyhydroxy aldehydes or polyhydroxy ketones or substances that yield such compounds on hydrolysis. The empirical formula of carbohydrates is (CH2O) n. Carbohydrates serve as energy stores, structural elements and they are precursors for many organic compounds like fats and amino acids. Carbohydrates are classified into four categories. Monosaccharides (simple sugars) containing a single polyhydroxy aldehyde or ketone unit. Ex: Glucose. Disaccharides consist of two monosaccharides linked by glycosidic linkage. Ex: Maltose. Oligosaccharides consist of three to twelve monosaccharides. Ex: Maltotriose which is a trisaccharide, made up of three glucose units. Polysaccharides consist of more than twelve monosaccharides. Ex: Cellulose.
Monosaccharides
Monosaccharides with an aldehyde group are called an aldose sugar. Ex: Glucose and with keto group called ketose sugar. Ex: Fructose. On the basis of number of carbon atoms, monosaccharides are classified as trioses, tetroses, pentoses, hexoses and heptoses. Glyceraldehyde is an aldrotriose with a single asymmetric carbon atom. Fischer projection of a molecule with asymmetric carbon is discussed. D and L glyceraldehdye are the mirror images of each other, also called enantiomers. Monosaccharides exhibit isomerism. Glucose and fructose are aldose-ketose isomers of one another. Glucose and mannose are epimers. They differ in configuration at the 2nd carbon of these aldohexoses. Mostly monosaccharides occur in aqueous solution as aqueous ring. In the process, carbonyl group (-CHO/-C=O) forms a covalent bond with oxygen of a hydroxyl group along the chain. The ring structure hemiacetal is formed by the combination of an aldehyde and an alcohol group. The ring has six members and is called pyranose ring. Ex: α-D-Glucopyranose. With the ketose group, hemiketal is formed. The ring contains five members and is called furanose ring. Ex: α-D-Fructofuranose. Anomerism is the process in which there is a creation of asymmetric carbon, called anomeric carbon. Ex: α-D-Glucopyranose and β-D-Glucopyranose. Mutarotation is the process in which cyclic α and β anomers produce equilibrium in a solution.
A constant source of blood glucose is compulsory for human life. Glucose is the preferred energy source of the brain. Glycogen stored in muscle releases hexose units for glycolysis within the muscle itself. Glycogen in the liver is concerned with export of hexose units for maintenance of the blood glucose, particularly between meals. Monosaccharides are degraded by a process called glycolysis which takes place in cytosol and a series of reactions in citric acid cycle in mitochondria. When one molecule of glucose is oxidized to carbon dioxide and water, 38 ATP molecules are generated. Reducing action of monosaccharides depends on the free anomeric carbon in their molecules, which are capable of reducing cupric to cuprous ions and themselves get oxidized to sugar acids. Oxidation of glucose gives rise to glucuronic acid. Two monosaccharides join covalently (glycosidic bond) when hydroxyl group of one reacts with anomeric carbon of the other forming a glycoside. Deoxy sugars, amino sugars, sugar acids and sugar alcohols are derivatives of monosaccharides.
Disaccharides
Maltose is a reducing disaccharide. It is made up of 2 glucose units linked by α (1-4) glycosidic linkage. Anomeric carbon of second glucose residue is free. It is produced during starch digestion and it exhibits mutarotation. Lactose is also a reducing disaccharide made up of one glucose and one galactose, linked by β (1-4) glycosidic linkage. It is present in milk and exhibits mutarotation. Sucrose is a non-reducing sugar. It is composed of one glucose and one fructose linked by α1- β2 glycosidic linkage. Anomeric carbon units are involved in linkage; hence no free anomeric carbons are available.
Polysaccharides
Consist of repeating units of monosaccharides or their derivatives held together by glycosidic bonds. Ex: Starch, Hyaluronic acid, homopolysaccharides contain only a single type of monosaccharide. Ex: Starch, glycogen, cellulose, chitin. Starch is found in cereals, legumes, potatoes. Here, glucose is the monosaccharide unit. They are made up of amylase and amylopectin with α (1-4) linage in linear structure of amylase and α (1-4) and α (1-6) linkage in branched structure amylopectin. In cellulose, polymer of glucose is linked by β (1-4) glycosidic linkage. Cellulose is present in plant cell wall and functions as a dietary fiber. Glycogen is the main storage polysaccharide in animals and is abundant in the liver. Glycogen is more extensively branched. Heteropolysaccharides are composed of different types of monosaccharides or their derivatives. Mucopolysaccharides are known as glycosaminoglycans. They are negatively charged large complexes. Ex: Hyaluronic acid, heparin, chondroitin sulfate, dermatan sulfate, keratin sulfate etc. Glycoproteins mostly contain oligosaccharides which are tightly bound to proteins. Ex: Immunoglobulins, mucin of stomach. They function as enzymes, hormones, receptors, structural proteins, transport proteins. In proteoglycans, proteins are covalently bound with mucopolysaccharides. Found in the bone, elastin and collagen.
