Organic Chemistry

Module 5. Amino acids and peptides

Lesson 14
GENERAL PROPERTIES OF AMINO ACIDS

14.1 Introduction

Fig. 14.1 General structure of amino acid

R- Side chain-aliphatic, aromatic or heterocyclic
Have both an amino and a carboxylic acid groups attached to the same carbon atom

• Amino group is on the carbon atom adjacent to carboxyl group (α- carbon atom) - therefore known as α- amino acids
• All naturally occurring amino acids have ‘L’ configuration therefore α-L-amino acid- obtained by acid or enzymatic hydrolysis of proteins
• On hydrolysis with alkali - L form converted into mixture of ‘D’ and ‘L’ form (Racemization).
• D-amino acids occurs in cells and some peptides (e.g. antibiotics containing peptide group) but not in proteins
• About 20 amino acids are usually found as constituents in most of the naturally occurring proteins
• More than 80 amino acids exist in small concentration - specific for proteins derived from specific source-not usual and not always found

e.g. Citrulline= in watermelon
Lanthionine= in sheepwool
Hydroxylysine= in collagen of body tissue

• Different amino acids have different taste -bitter (arginine), sweet (glycine), tasteless (tyrosine), aspartame (substitute for sugar, known as sugar free gold).
• Classified according to chemical nature of side chain ‘R’
• Amino acids (naturally occurring)

Fig. 14.2

Note on the given Table: (Fig. 14.3-a Types of Amino Acids), (Fig. 14.3-b structure of Amino Acids)

• Most of the amino acids contain C,H,O and N elements, but a few contain ‘S’ also

• All amino acids, except glycine, contain an asymmetric carbon atom = therefore they are optically active
• Most of the α-amino acids are Laevo (-) rotator, but some are Dextro (+) rotator as well.
• Except proline and hydroxy proline, all other amino acids have common denominations and free carboxylic group and a free unsubstituted amino group on the ‘α’ carbon atom
• Proline and hydroxyproline are immino acids
• Some amino acids contain free or potentially free second carboxyl group, such amino acids -called acidic amino acids
• Some amino acids contain second basic group like amine, guanidine, amidazole, etc - known as basic amino acids.

14.2 Essential Amino Acids

• “Amino acids which are not synthesized in human body and these must be supplied in diet from exogenous sources for normal physiological functions of the body”
• Indispensable (essential)amino acids –include-

1. Argninie, 2. Histidine, 3. Isoleucine, 4. Leucine, 5. Lysine, 6. Methionine, 7. Phenylalanine , 8. Threonine, 9. Tryptophane and 10. Valine
“A HILL MP TTV”

• Ten amino acids which the human body can synthesize are called non-essential or dispensable amino acids.