Alanine is an α-amino acid with the chemical formula HO2CCH(NH2)CH3. The L-isomer is one of the 20 proteinogenic amino acids, i.e. the building blocks of proteins. Its three letter code is ala, its one letter code is A, and its codons are GCU, GCC, GCA, and GCG. It is classified as an nonpolar amino acid. L-alanine is second only to leucine, accounting for 7.8% of the primary structure in a sample of 1,150 proteins . D-alanine occurs in bacterial cell walls and in some peptide antibiotics.