Selenium (Se) is an element required in the human body in trace amounts. Selenium is obtained through the diet and levels of selenium found in food depend greatly on the soil where it is grown

Question

Once ingested and absorbed as selenate, it can become incorporated into a small number of polypeptides. These selenoproteins are formed when selenium replaces an element that is found in two of the twenty "standard" amino acids. Using your knowledge of atomic structure, the periodic table in Figure 2–7, and the structure of amino acids found in Panel 2–5, deduce which two amino acids may be converted to "seleno" amino acids and used to make selenoproteins.

Answer 1

Sulfur is the only element found exclusively in two of the twenty amino acids. This element is located directly above selenium in the periodic table, indicating that these elements have the same number of electrons in their outer shell and both prefer to form bonds with other atoms to fill their outer orbital. If selenium instead of sulfur is incorporated into cysteine or methionine, the altered "seleno" amino acids will be produced (selenocysteine and selenomethionine). We can expect that this substitution will alter the nature of the proteins in which these amino acids are incorporated because selenium is a larger atom than sulfur.