If the basic formula of an α-amino acid is R-CH(NH₂)-COOH, where R is the side chain, what is the primary point of distinction between any two proteins?

The side chain R

α-amino acids are the constituents of proteins. Different proteins are formed by polymerization of different α-amino acids, which are formed due to the difference in the side chain substituted group R, which may be as simple as hydrogen or as complex as imidazole.