Biochemistry and Human Nutrition

Module 1. Bio-molecules

Lesson 3
CARBOHYDRATES

3.1 Introduction

• Carbohydrates are polyhydroxy aldehydes or ketones
• Classified into three categories
  

1. Monosaccharides- a single polyhydroxy aldehyde or ketone unit cannot be hydrolysed further into monomers. Example – Glucose (Dextrose),
2. Oligosaccharides- made up of 2-6 monosaccharides linked together by glycosidic bonds. Example - Sucrose
3. Polysaccharides – made up of more than six monosaccharides Example- Starch, Glycogen, Cellulose

3.2 Major Functions

• They form major organic matter on earth because of their extensive roles in all forms of life.
• They serve as energy stores (starch and glycogen), fuels, and metabolic intermediates.
• Ribose and deoxyribose sugars are component of RNA and DNA
• Polysaccharides are structural elements in the cell walls of bacteria and plants.
• Carbohydrates are linked to many proteins and lipids, where they play key roles in mediating interactions among cells and interactions between cells and other elements in the cellular environment.

3.3 Structure of Important Carbohydrates

3.3.1 Monosaccharides

• Monosaccharides with three, four, five, six, and seven carbon atoms in their backbones are called, triose tetroses, pentoses, hexoses, and heptoses respectively.
• The carbons of a sugar are numbered beginning at the end of the chain nearest the carbonyl group.
• All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms.(Fig. 3.1 Basic Structure of Monosaccharides and Fig. 3.2 Representative Monosaccharides).

• In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain.
• The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals which contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms. For example, D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers, designated α and β.
• Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. The α and β anomers of D-glucose interconvert in aqueous solution by a process called mutarotation. (Fig. 3.3 Pyranoses and furanoses)
  

• Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe³⁺) or cupric (Cu²⁺) ion. The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar. By measuring the amount of oxidizing agent reduced by a solution of a sugar, it is also possible to estimate the concentration of that sugar.
• Two sugars that differ only in the configuration around one carbon atom are called epimers. D-Mannose differs from D-glucose only in its configuration around carbon 2. D-Galactose differs from D-glucose only in its configuration around carbon 4 . D-Galactose and D-Mannose are not epimers. (Fig. 3.4)
  

3.3.2 Disaccharides

• Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other.
• The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s). When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a nonreducing sugar. In describing disaccharides or polysaccharides, the end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end. (Fig. 3.5)
  

3.3.3 Polysaccharides

• Homopolysaccharides contain only a single type of monomer; heteropolysaccharides contain two or more different kinds
• Some homopolysaccharides serve as storage forms of monosaccharides that are used as fuels; starch and glycogen are homopolysaccharides of this type.
• Other homopolysaccharides (cellulose and chitin for example) serve as structural elements in plant cell walls and animal exoskeletons. Heteropolysaccharides provide extracellular support for organisms of all kingdoms. For example, the rigid layer of the bacterial cell envelope (the peptidoglycan) is composed in part of a heteropolysaccharide built from two alternating monosaccharide units. In animal tissues, the extracellular space is occupied by several types of heteropolysaccharides, which form a matrix that holds individual cells together and provides protection, shape, and support to cells, tissues, and organs. (Fig. 3.6)