The building blocks of proteins are amino acids. Amino acid is an organic compound containing an amino group (-NH2) and a carboxyl group (-COOH). It was discovered by Emil Fischer and Franz Hofmeister in 1902. The number of protein amino acids in the human body is 20. Leucine, isoleucine, lysine, methionine, serine, valine. , proline, alanine, threonine, tyrosine, histidine, cysteine, asparagine, glycine, arginine, tryptophan, glutamic acid, glutamine, aspartic acid and phenylalanine.