Carbohydrates are the most abundant bio-molecules on earth made up of carbon, hydrogen and oxygen. Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones or substances that yield such compounds on hydrolysis. Carbohydrates have the empirical formula of (CH2O)n . Carbohydrates serve as energy stores and are the structural elements in the cell walls of bacteria and plants and exoskeletons of invertebrates. Carbohydrates are the starting materials for many organic compounds like fats and amino acids. Monosaccharides consist of a single polyhydroxy aldehyde or ketone unit. Disaccharides consist of two monosaccharide units. Oligosaccharides consists of three to twelve monosaccharide units. Polysaccharides consist of more than twelve monosaccharide units.
Monosaccharide containing an aldehyde group is called an aldose sugar. Monosaccharide containing a keto group is called a ketose sugar. Based on the number of carbon atoms, monosaccharides are classified. A triose has three carbons, a tetrose has four carbons, a pentose has five carbons, a hexose has six carbons, and a heptose has seven carbons. The prefixes D and L designate the absolute configuration and refer to the position of the last chiral carbon. D is the OH on the right. L is the OH on the left. Epimers of Monosaccharides differ in configuration at a single asymmetric carbon. Cyclization of monosaccharides usually occur in aqueous solution as cyclic rings. In the process, carbonyl group (aldehyde or keto) forms a covalent bond with oxygen of a hydroxyl group along the chain. The ring structure of an aldose is a hemiacetal. The ring structure of a ketose is a hemiketal. Anomerism is the formation of rings that results in the creation of an asymmetric carbon which is called anomeric carbon. Mutarotation is the process in which cyclic alpha and beta anomers produce equilibrium in a solution. The cyclic structure is retained in solution but isomerism takes place about the anomeric carbon. Monosaccharides are important in several ways. A constant source of blood glucose is compulsory for human life. Glucose is the preferred energy source of the brain.
Glycogen stored in muscles 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. Glycogen undergoes glycogenolysis and releases glucose units. Glucose undergoes various reactions in glycolysis, which takes place in cytoplasm of all cells and a series of reactions in citric acid cycle in mitochondria. Glycolysis also provides the main pathway for the metabolism of fructose and galactose derived from the diet. Glucose undergoes glycolysis. It is degraded in a series of enzyme catalyzed reactions to yield pyruvate. Pyruvate is oxidatively decarboxylated to acetyl Coenzyme A, which enters Kreb=s cycle for its complete oxidation to yield ATP, water and carbon dioxide.
Reactions of Monosaccharides
The reducing action of monosaccharides depends on the free anomeric carbon in their molecules. They are capable of reducing cupric to cuprous ions and themselves get oxidized to sugar acids. In an example of the oxidation-reduction reaction, oxidation of glucose takes place, giving glucuronic acid. In glycoside formation, two monosaccharides join covalently when hydroxyl group of one reacts with anomeric carbon of the other. The glycosidic bond is the linkage between the two monosaccharide units. In the deoxygenation of sugars, the hydroxyl group in a monosaccharide is replaced by hydrogen. In the formation of aminosugars, one or more OH groups are replaced by amino groups. Uronic acids are produced by oxidation of CH2OH group to COOH group. Sugar alcohols are produced by reduction of aldoses or ketoses.
Disaccharides are classified as reducing sugars or nonreducing sugars. In a reducing sugar, the anomeric carbon of second glucose residue is free. In a non-reducing sugar, anomeric carbons of glucose and fructose are involved in linkage. Hence, no free anomeric carbons.
Polysaccharides consist of repeat units of monosaccharides or their derivatives held together by glycosidic bonds. They are classified as homopolysaccharides and heteropolysaccharides. Polysaccharides function as storage and structural components in plants and animals. Homopolysaccharides are the polysaccharides which contain only a single type of monosaccharides. Examples are starch, amylose, amylopectin, cellulose, and glycogen. Starch is nonreducing because most of the anomeric carbons of its glucose residues remain bound by glycosidic bonds. Starch is made up of two types of glucose polymers, amylose and amylopectin. Amylose consists of long unbranched chains of D-glucose connected by alpha(1-4) linkages. Amylopectin is highly branched chain with alpha(1-6) bonds at branching points and alpha(1-4) bonds everywhere else. Cellulose is present in plant cell wall and is a polymer of glucose, linked by β(1-4)glycosidic linkages. Glycogen is the main storage polysaccharide of animal cells and abundant in liver. Like amylopectin, glycogen is a polymer of alpha (1-4) linked subunits of glucose with alpha (1-6) linked branches. Glycogen is more extensively branched and more compact than starch. Heteropolysaccharides are composed of different types of monosaccharides or their derivatives. Mucopolysaccharides: also known as glycosaminoglycans. These are large complexes of negatively charged heteropolysaccharides. They are made up of repeating disaccharide units, generally composed of an amino sugar and a sugar acid. Glycoproteins contain highly variable amounts of carbohydrate and perform a variety of functions, which include enzymes, hormones, receptors, structural proteins and transport proteins. Proteoglycans are also known as Mucoproteins or Mucoids. They are those glycoproteins in which the carbohydrate content is mucopolysaccharides.