The Essential Poison: Connecting the Chemistry of Selenium with Biology

Selenium is an essential trace element for humans and is referred to as the “essential poison” because it is needed at very low levels, about 50 micrograms per day, while only slightly elevated levels, 800 micrograms per day, are toxic. Selenium replaces sulfur in some rare instances in the form of selenocysteine, the 21^st amino acid in the genetic code. The complexity of the selenocysteine-insertion machinery at the ribosome suggests that selenium performs some chemistry that sulfur cannot.  Here, I attempt to answer the question “Why Nature chose selenium?” by connecting its chemical properties with its importance to biology. Selenium is a Janus-faced element because it is a good donor of electrons and a good acceptor of electrons. Both of these properties can explain its use in biology. Selenium is a good antioxidant because it quickly reacts with reactive oxygen species to form selenium-oxides. These selenium-oxides are highly electrophilic. The selenium atom of the oxide then accepts electrons from a donor to become reduced. The chemistry-biology connection for selenium is that it confers enzymes with the ability to resist inactivation by oxidation.