I've heard that proteins generally contain six main elements - carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.
I know that proteins are made from amino acids. Amino acids are composed of an amino group, a carboxyl group, a lone hydrogen atom, and a side chain that varies between amino acids. Of the 20 major amino acids, none of them have a side chain that contains phosphorus. Only cysteine and methionine contain sulfur.
So, are sulfur and phosphorus atoms really characteristic in the structure of a protein?