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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?

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  • $\begingroup$ Well, phosphorus is only present in some proteins which are subject to post translational modifications, such as phosphorylation. Sulphur can originate from either cystein or methionine present in the protein. So only proteins that are modified after translation will contain phosphorus. $\endgroup$ – Jeppe Nielsen Jul 9 '17 at 23:54
  • $\begingroup$ So, phosphorus is not characteristic in the structure of a protein, right? $\endgroup$ – DrPepper Jul 9 '17 at 23:57
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    $\begingroup$ Both phosphorus and sulfur are important to the functional structure of many proteins, even if phosphorylation happens later. $\endgroup$ – Bryan Krause Jul 10 '17 at 0:04
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    $\begingroup$ Bryan that is wrong, show me a source on that please. As the asker points out, phosphorus is not present in the amino acids which make up the protein. The phosphor present will have been added after translation by the ribosome, thus as post translational modifications. PTMs typically occur after folding. That phosphorylation occurs is true, but it is usually not essential for protein structures, unless it changes the conformation of the protein significantly. $\endgroup$ – Jeppe Nielsen Jul 10 '17 at 0:17
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    $\begingroup$ @JeppeNielsen I don't think you understand what he is saying. $\endgroup$ – canadianer Jul 10 '17 at 19:17
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Of the 22 proteinogenic amino acids, all contain hydrogen, carbon, nitrogen and oxygen. Some (methionine and cysteine) contain sulfur while one (selenocysteine) contains selenium. None are known to contain phosphorus, but this element can be incorporated by:

  1. Post translational phosphorylation of various residues (STYHRK). This often allows/prevents binding of other proteins and/or triggers a conformational change in the target protein. It is also necessary for proper folding of some proteins (example). Sulfur can also be incorporated in a similar manner (sulfation).

  2. Incorporation of phosphorus containing prosthetic groups. This can occur covalently and is necessary for the function of some proteins. Sulfur can also be incorporated as part of prosthetic groups.

You may also consider the many other elements necessary for the structure/function of many proteins: iodine in thyroglobulin, iron in hemoglobin, etc...

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