MoBio Building Blocks - Amino Acids Chapter 2

An amino acid is defined as the molecule containing an amino group (NH2), a carboxyl group (COOH) and an R group. It has the following general formula,


The R group differs among various amino acids. In a protein, the R group is also called a sidechain.


Figure 2-A-1. The structure of an amino acid (tyrosine).

There are over 300 naturally occurring amino acids on earth, but the number of different amino acids in proteins is only 20.