Biomolecules-III

Structures of Amino Acids Amino acids are the building blocks of proteins. There are 300 amino acids that occur in nature. Among these only 20 are known as standard amino acids that commonly occur in proteins. Amino acids contain two functional groups. They are - amino and carboxyl groups. The amino group (-NH2) is basic and the carboxyl group (-COOH) is acidic in nature. Nomenclature of Amino Acids All amino acids have trivial names. IUPAC names which even if not cumbersome are not used. For e.g., H2N CH2COOH is better known as glycine rather then a-amino acetic acid or 2-amino ethanoic acid. Classification of Amino Acids Base on Polarity Amino acids are classified into different ways based on polarity, structure, nutritional requirement, metabolic fate, etc. Generally used classification is based on polarity. Classification of Amino Acids Based on Nutrition The amino acids that are to be supplied through diet are called as essential amino acids. They cannot be produced by the body. The essential amino acids are arginine, valine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine and tryptophan. Physical Properties of a-Amino Acids Amino acids are colourless, crystalline solids. They are water-soluble high melting solids and behave like salts. The a-carbon atom has 'R' which is a side chain. This side chain is different for different amino acids. The carboxyl and amino groups interact resulting in the transfer of proton from carboxyl group to amino group. Thus the amino acid exists in ionised form known as zwitter ion. This explains the physical properties of these a-amino acids. Iso-electric Point It is defined as the point at which a molecule exists as zwitter ion with no net charge. Thus, at this point the molecule is electrically neutral. The molecules have minimum solubility, maximum precipitability and least buffer capacity. The acidic amino acids and basic amino acids strongly influence the iso-electric point (PI). The PI value of protein is determined by the nature of ionizable groups of amino acids. By taking average Pka values of ionisable groups PI value can be calculated. Chemical properties of a-amino acids When two amino acids combine with each other, the amino group of one amino acid combines with carboxyl group of other amino acid. This leads to peptide bond formation. The combination of the amino group of one molecule with the carboxyl group of the other results in the elimination of a water molecule and forms a -CO-NH-bond. Poly Peptides Peptides containing more than 10 amino acids are called poly peptides. Poly peptides are formed by the linear sequence of amino acids. Some proteins are composed of two or more poly peptide chains. Relatively shorter peptides are known as oligopeptides whereas longer polymers are called polypeptides. Polypeptides containing more than 100 amino acids having molecular mass higher then 10,000 are generally called as proteins. However the distinctive between a polypeptide and a protein is not sharp.