Selenium versus Sulphur:
Almost all chemical reactions involving selenium are faster in comparison to the same reaction with sulfur.
For this reason, it is tempting to conclude that nature chose selenium to replace sulfur for this enhanced chemical reactivity in order to accelerate enzymatic reactions.
In contrast, our answer to this question is that nature has chosen selenium due to its unique ability to react with oxygen and related ROS in a readily reversible manner.
Both sulfur and selenium are good nucleophiles that react with ROS in two-electron oxidation events and, in so doing, become oxidized.
The S-oxides and Se- oxides that are formed in this process show very strong divergence in their chemical reactivities, due in large part to very weak π-bonding in the Se-oxide.
As a result, Se-oxides have a much stronger ability to be rapidly reduced back to the original state in comparison to S-oxides.
The ability of selenium to both rapidly become oxidized and then be rapidly reduced has been referred to as the “selenium paradox.”
Evidence for this “selenium paradox” comes from the gain of function that selenium confers to both natural and artificial selenoenzymes; selenium confers resistance to inactivation by oxidation as the numerous examples discussed above show.
Closely related to the reversibility of two-electron oxidation events is the enhanced stability of the selenayl radical compared to the thiyl radical.
This means that selenium-containing proteins are much better able to withstand one-electron oxidation events.
There is a reason eumelanin is selenium based as it was chosen by NaTuRe (Ntr).
Here is a paper on the subject... https://www.chem.wisc.edu/areas/reich/papers/Reich-2016-Why-Selenium-ACS-Chem-Biol.pdf
Almost all chemical reactions involving selenium are faster in comparison to the same reaction with sulfur.
For this reason, it is tempting to conclude that nature chose selenium to replace sulfur for this enhanced chemical reactivity in order to accelerate enzymatic reactions.
In contrast, our answer to this question is that nature has chosen selenium due to its unique ability to react with oxygen and related ROS in a readily reversible manner.
Both sulfur and selenium are good nucleophiles that react with ROS in two-electron oxidation events and, in so doing, become oxidized.
The S-oxides and Se- oxides that are formed in this process show very strong divergence in their chemical reactivities, due in large part to very weak π-bonding in the Se-oxide.
As a result, Se-oxides have a much stronger ability to be rapidly reduced back to the original state in comparison to S-oxides.
The ability of selenium to both rapidly become oxidized and then be rapidly reduced has been referred to as the “selenium paradox.”
Evidence for this “selenium paradox” comes from the gain of function that selenium confers to both natural and artificial selenoenzymes; selenium confers resistance to inactivation by oxidation as the numerous examples discussed above show.
Closely related to the reversibility of two-electron oxidation events is the enhanced stability of the selenayl radical compared to the thiyl radical.
This means that selenium-containing proteins are much better able to withstand one-electron oxidation events.
There is a reason eumelanin is selenium based as it was chosen by NaTuRe (Ntr).
Here is a paper on the subject... https://www.chem.wisc.edu/areas/reich/papers/Reich-2016-Why-Selenium-ACS-Chem-Biol.pdf
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