zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases. Except glycine, all other naturally occurring a -amino acids are optically active, since the a -carbon atom is asymmetric. These exist both in ‘D’ and ‘L’ forms. Most naturally occurring amino acids have L-configuration.
L-Aminoacids are represented by writing the –NH group on left hand side. You have already read that proteins are the polymers of a -amino acids and they are connected to each other by peptide bond or peptide linkage . Chemically, peptide linkage is an amide formed between –COOH group and –NH group. The reaction between two molecules of similar or different amino acids, proceeds through the combination of the amino group of one molecule with the carboxyl group of the other.
This results in the elimination of a water molecule and formation of a peptide bond –CO–NH–. The product of the reaction is called a dipeptide because it is made up of two amino acids. For example, when carboxyl group of glycine combines with the amino group of alanine we get a dipeptide , glycylalanine. If a third amino acid combines to a dipeptide, the product is called a tripeptide .
A tripeptide contains three amino acids linked by two peptide linkages. Similarly when four, five or six amino acids are linked, the respective products are known as tetrapeptide, pentapeptide or hexapeptide , respectively. When the number of such amino acids is more than ten, then the products are called polypeptides . A polypeptide with more than hundred amino acid residues, having molecular mass higher than ,000u is called a protein.
However, the distinction between a polypeptide and a protein is not very sharp. Polypeptides with fewer amino acids are likely to be called proteins if they ordinarily have a well defined conformation of a protein such as insulin which contains amino acids. Proteins can be classified into two types on the basis of their molecular shape. (a) Fibrous proteins When the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, then fibre–