Naturally Occurring Selenoproteins

This is an area which has only very recently attracted intensive study. In fact, as recently pointed out by Stadtman detailed studies of the specific biochemical role of selenium at the enzyme level dates only from 1972. However, research is progressing rapidly. To put this into perspective, a 1979 report stated that there are at least three, and possibly four , selenoproteins. A 1980 article listed eight known selenoproteins Table 2 lists these selenoproteins and their source. Interestingly, the form of selenium which has been determined in all animal and bacterial proteins examined thus far appears to be selenocysteine. Both bacterial and mammalian enzymes apparently are involved in redox reactions (an exception may be thiolase see below). Selenomethionine, as mentioned previously, is a predominant form of selenium in many grains and grasses, but it has not been detected in mammalian enzymes. 

It is important to remember that only minute amounts of selenium are required for the synthesis of these proteins and whatever mechanism(s) accounts for the incorporation of the selenium, there must be an extremely effective discrimination between it and sulfur. Otherwise sulfur, being so much more abundant, in any kind of competitive mechanism, would completely overwhelm the selenium incorporation. In this regard it is interesting to note that even when bacteria are cultured in the presence of a tremendous molar excess of sulfur, clostridial glycine reductase is synthesized normally, containing the single selenocysteine in the same polypeptide chain as two cysteine and three methionine residues  

The mechanism of incorporation of selenium, particularly into mammalian proteins, in a relatively controversial issue and this is an area which deserves, and will undoubtedly 

Xanthine Dehydrogenase Clostridium acidiurici and Clostridium cylindrosporum 

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At this time it would appear prudent for a vibrational spectroscopist to undertake a careful and detailed experimental vibrational study of solutions of selenocysteine under various conditions since selenocysteine residues seem to occur in most, if not all, naturally occurring selenoproteins. The facile oxidation of this selenoamino acid must be kept in mind and any vibrational study must be carried out under carefully controlled conditions. 

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