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Selenocysteine (#21)

By moleculeoftheday on November 15, 2007.

Amino acids impart lots of functions to proteins, and a lot of the interesting chemistry happens at a few residues. Many hydrophobic residues like valine and leucine play a huge role - they don't like to touch water, and they help the protein fold, but the heavy lifting of catalysis happens in just a few reactive places. Two of these are cysteine, and its rare brother, selenocysteine.

Going down the periodic table, things at once change and stay the same. A chlorine will act a lot like a bromine, which will act a lot like an iodine, but there are distinct differences in how they react. Just adding a dash of potassium iodide to some reactions with a chlorinated carbon will speed things up considerably - iodine's both a better nucleophile and leaving group compared to chlorine.

Walking down a group can have some neat effects - serine, which has a -CH2-OH, is less reactive than its sulfrous mate, cysteine, which has a -CH2-SH. Keep on goin' down and you have selenocysteine, which has -CH2-SeH. A superlative nucleophile.

i-782bf0b7296b36af97a89f06d4ecf0c6-selenocysteine.png

Selenium is rare and stinky, and you don't find selenoproteins very much in nature. When you do, though, evolution has found that selenium really plays to its strengths - such as in its reaction with the anticancer compound found in curry, of all places - curcumin.

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biology

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One thing worth mentioning is that selenocysteins are encoded by the stop codon UGA. However, to be recogized as a selenocystein encoding triplet additional sequences in the 3'-UTR are needed.

it's also accumulated in Astragalus Bisulcatus up to about 20,000 ppm by dry weight. it is concentrated in trichomes for the most part. platns don't encode the amino acid specifically so they evolved mechanisms to keep as much of the selenium locked up rather than available to substitute for cysteine in proteins. selenium isn't found as selenomethionine in any significant amount as selenocysteine is the major metabolite. plants use it for defense against bugs for the most part.