AMINO ACIDS ARE SUBUNITS OF PROTEINS

Amino acids are small organic molecules,generally colourless crystalline solids which are soluble in water but insoluble in organic solvents.Amino acids are formed of C,H,O & N.Some amino acids may contain sulphur(S).There are over 2000 amino acids out of which only 20 amino acids and their derivatives occur in proteins.Amino acids found in proteins are called proteins amino acids whereas others are called non-protein amino acids.


Each amino acid exhibits one definig property viz. they all posess a carboxylic acid group and an amino group both linked to their alpha-carbon atom.Each amino acid also has a side chain atched to its alpha-carbon.The identity of this side chain is what distinguishes one amino acid from the other.

Amino group occurs in alpha-posotion or carbon next to terminal carboxylic group.Therefore,protein amino acids are also called alpha-amino acids.

In the cell where pH is close to 7, free amino acids exist in their ionised form but when they are incorporated into a polypeptide chain , the charges on amino and carboxylic groups disappear.This ionised structure is termed as zwitter ion.

Cells use amino acids to build proteins—polymers made of amino acids, which are joined head-to-tail in a long chain that folds up into a three dimensional structure that is unique to each type of protein.The covalent bond between two adjacent amino acids in a protein chain is called a peptide bond; the chain of amino acids is also known as a polypeptide. Peptide bonds are formed by condensation reactions that link one amino acid to the next. Regardless of the specific amino acids from which it is made, the polypeptide always has an amino (NH2) group at one end—its N-terminus—and a carboxyl (COOH) group at its other end—its C-terminus). 

 Amino acids in a protein are held together by peptide bonds. the four amino acids shown are linked together by three peptide bonds, one of which is highlighted in yellow. One of the amino acids, glutamic acid, is shaded in gray. the amino acid side chains are shown in pink. The two ends of a polypeptide chain are chemically distinct. One end, the N-terminus, is capped by an amino

group, and the other, the c-terminus, ends in a carboxyl group. The sequence of amino acids in a protein is abbreviated using either a three-letter or a one-letter code, and the sequence is always read from the N-terminus .

Twenty types of amino acids are commonly found in proteins, each with a different side chain attached to the α-carbon atom .The same 20 amino acids are found in all proteins, whether they hail from bacteria, plants, or animals. How this precise set of 20 amino acids came to be chosen is one of the mysteries surrounding the evolution of life; there is no obvious chemical reason why other amino acids could not have served just as well. But once the selection had been locked into place, it could not be changed, as too much chemistry had evolved to exploit it. Switching the types of amino acids used by cells would require a living creature to retool its entire metabolism to cope with the new building blocks.