Proteins are complex organic compounds with high molecular weight. Proteins are mainly composed of carbon, hydrogen, oxygen and nitrogen, and some of them contain other elements such as sulfur, phosphoric acid, etc.
Proteins are made up of building blocks called amino acids. Several hundred types of amino acids have been identified in nature, but (20-25) 22 types of them participate in the structure of proteins.
Animals are able to make some amino acids, which we know as non-essential amino acids. Regarding some amino acids, animals are not able to make them and we know these amino acids as essential amino acids.
Essential amino acids include lysine, methionine, histidine, threonine, valine, leucine, isoleucine, tryptophan, phenylalanine and arginine. Taurine is only necessary in cats.