BIO202 Official Handouts

BIO202 – Biochemistry-I

Topic no 1 to 112 in mid term and 113 to 225 final term syllabus

Lesson 1 Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis Many, but not all, carbohydrates have the empirical formula(CH2O)n; some also contain nitrogen, phosphorus or sulfur

Classification

• Carbohydrates are classified into four major groups:

1. Monosaccharides
2. Oligosaccharides
3. Disaccharides
4. Polysaccharides

Monosaccharides (simple sugars) are those which cannot be hydrolyzed further into simpler forms The backbones of common monosaccharides are unbranched carbon chains in which all the carbon atoms are linked by single bonds In the open-chain form, one of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group Each of the other carbon atoms has a hydroxyl group

If the carbonyl group is at an end of the carbon chain (that is, in an aldehyde group) the monosaccharide is an aldose If the carbonyl group is at any other position (in a ketone group) the monosaccharide is a ketose The simplest monosaccharides are the two three-carbon trioses: Glyceraldehyde,an aldotriose Dihydroxyacetone,a ketotriose The carbons of a sugar are numbered beginning at the end of the chain nearest the carbonyl group All common monosaccharides and disaccharides have names ending with the suffix ―-ose.‖ Ketoses are designated by inserting "ul" into the name of a corresponding aldose For example, ribulose is the ketopentose corresponding to the aldopentose ribose • There are a few exceptions however, for example, fructose and dihydroxyacetone Monosaccharides can be subdivided further depending upon: Number of carbon atoms Whether aldehyde or ketone groups are present as aldoses or ketoses

General formula Aldosugars Ketosugars Trioses C3H6O3 Glyceraldehyde Dihydroxyacetone Tetroses C4H8O4 Erythrose Erythrulose Pentoses C5H10O5 Ribose Ribulose Hexoses C6H12O6 Glucose Fructose The hexoses which include the aldohexose D-glucose and the ketohexose D-fructose, are the most common monosaccharides in nature The aldopentoses D-ribose and 2deoxy-D-ribose are components of nucleotides and nucleic acids

Lesson 2

Disaccharides are those sugars which yield two molecules of the same or different monosaccharides on hydrolysis Maltose - 2 molecules of glucose on hydrolysis Lactose – 1 glucose & 1 galactose on hydrolysis Sucrose – 1 glucose & 1 fructose on hydrolysis Oligosaccharides are short chains of monosaccharide units or residues (3-10) joined by characteristic linkages called glycosidic linkage Glycosidic bond of maltose is formed between the OH of carbon 1 and carbon 4 of 2nd glucose monomers. Thereforer, it forms an alpha (1– 4) glycosidic bond.

Lesson 3

Polysaccharides (Glycans) are those which yield more than 10 molecules of monosaccharides on hydrolysis Some have hundreds or thousands of units Some polysaccharides, such as cellulose, are linear chains; others, such as glycogen, are branched Both glycogen and cellulose consist of recurring units of D-glucose, but they differ in the type of glycosidic linkage and consequently have strikingly different properties and biological roles Polysaccharides are further divided into two groups:

1. Homopolysaccharides (homoglycans): Polymers of same monosaccharide

units e.g. starch, glycogen, inulin, cellulose, dextrins, dextrans

2. Heteropolysaccharides (heteroglycans): Polymer of different

monosaccharide units or their derivatives e.g. Mucopolysaccharides (glycosaminoglycans)

Biomedical Importance

Chief source of energy Constituents of compound lipids and conjugated proteins Lactose , the principal sugar of milk Degradation products utilized for synthesis of other substances such as fatty acids, amino acids, cholesterol, etc Constituents of mucopolysaccharides which form the ground substance of mesenchymal tissues Certain carbohydrate derivatives are used as drugs like cardiac glycosides. Inherited deficiency of certain enzymes in metabolic pathways of different carbohydrates can cause diseases e.g. galactosemia, glycogen storage diseases, lactose intolerance, etc Derangement of glucose metabolism is seen in Diabetes mellitus

Lesson 4

Isomerism

• The existence of two or more compounds having the same molecular formula but a different arrangement of atoms within the molecule

Isomerism

Compounds which are identical in composition and differ only in configuration (the fixed spatial arrangement of atoms) are called

stereoisomers

Stereoisomers that are mirror images of each other are called

enantiomers

Pairs of stereoisomers that are not mirror images of each other are called

diastereomers

The identifying characteristic of stereoisomers is that they cannot be interconverted without temporarily breaking one or more covalent bonds A carbon atom to which four different atoms or groups of atoms are attached is said to be asymmetric or chiral