Selenium or tellurium as an alternative to protein sulfur

Selenium or tellurium as an alternative to protein sulfur

Selenoproteins, in which sulfur is replaced by selenium, are found in all major forms of life, and are essential to many of them. All methionines in bacterial cells can be replaced by selenomethionines - this technique is commonly used when analyzing protein structures via X-ray crystallography. Some fungi can substitute tellurium for sulfur and selenium in their amino acids.