by PhilipJ on 9 August 2008
[sorry for the lack of updates, I was on vacation!]
If you have visited your local health food store or looked closely at the ingredients in your daily multivitamin, you may have noticed that the element selenium is often listed as one of the beneficial supplements. Selenium is a double-edged sword, however. In general, selenium compounds are toxic and have an unpleasant garlicy odor, but in trace amounts, selenium is essential for our health. Selenium atoms are similar to sulfur atoms, with similar properties, except that selenium compounds tend to be more reactive. In a few specialized proteins, this extra reactivity is just what is needed. For instance, by using a selenium atom instead of sulfur, thioredoxin reductase improves its rate of catalysis by 100 times, and formate dehydrogenases act 300 times faster.
Proteins contain two types of amino acids with sulfur atoms, and both may be made in versions with selenium atoms. Selenomethionine is not made specifically by cells, but it is created occasionally by accident by the normal enzymes that make methionine. This is not a problem, however, since it is similar enough to normal methionine. It is, however, very useful to crystallographers to help with the determination of x-ray crystal structures, since it has more electrons than sulfur and is easily located in x-ray experiments. Selenocysteine, on the other hand, is more reactive than normal cysteine, and it is specifically added to special selenoproteins when it is needed.
More from David Goodsell here.