Shared features in the structure of amino acids

Proteins are macromolecules, polymeric molecules. The monomers they are made of are alpha amino acids; on the left, you can see the generic model for them. All of them have the amino group bound to the alpha carbon, which is the carbon adjacent to the carboxyl group. The alpha carbon is also bound to a hydrogen atom and a sidechain or R group, which differs among amino acids.

structural formula for a generic alpha amino acid

At physiological pH, the amino group is protonated (-NH3+) and the carboxyl group is not (-COO).