All amino acids have a center carbon. The center carbon has four different molecules attached to it, these are: AMINO GROUP [NH2], CARBOXYL GROUP [COOH], AN R GROUP [R] AND A HYDROGEN ATOM. There are twenty different amino acids in existence, each amino acid differ from another one based on their R groups. The R group is specific and unique to each amino acid. The center carbon in amino acid structure is also called alpha carbon.