There are 20 naturally occurring amino acids. The 20 common amino acids are all alpha-amino acids and all have similar structural features. They contain a carboxylate (-COO-) group, an amino group (-NH3+), and a side chain (R group), all attached to the α-carbon. 

The general structure of Amino acids is H2NCH RCOOH. It is denoted as:

COOH

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H2N – – C – – H

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R

Amino acids differ from each other in their side-chain, which is called the R group. Each amino acid has 4 different groups attached to α- carbon – COOH, Amino group, Hydrogen atom, and Sidechain (R).