Amino Acids are the monomers that make up proteins by joining in condensation reactions to form peptide bonds between themselves. All amino acids have an alpha carboxylic acid group, an alpha amine group and a hydrogen atom bonded to a central carbon along with a fourth variable group. This group varies in the 20 essential amino acids and generally allows amino acids to exhibit sterioisomerism to create optical isomers D and L. The only exception to this being the simplest amino acid glycine with its variable group being another hydrogen atom. This prevents sterioisomerism as there aren't four different groups then bonded to the central carbon1.
1. Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto, Jr. , Lubert Stryer, Biochemistry 8th edition Freeman