Selenocysteine amino acid is the analog of cysteine having the same structure as that of cysteine. But here sulfur atom is replaced by selenium. It is the 21st amino acid found in the ribosome-mediated protein synthesis. It is present in the number of enzymes like tetraidothyronine deiodinase, glutathione perxidase, thioredoxine reductase, glycine reductase, formate dehydrogenase and in hydrogenases. Proteins having more than one selenocysteine residues are called selenoprotein. There is no single free pool of selenocysteine amino acid that exists within cells to be used. So it means it is an essential amino acid which is needed to be provided through food.
Selenocysteine is found in proteins and in a variety of foods of either animal origin or plant origin.
Animal origin: Selenocysteine animal sources are meat, poultry, chicken, egg, cheese fish, seafood, and turkey.
Plant origin: Selenocysteine of plant origin contains wheat, oats, corn, rice, nuts especially of Brazil nuts, soybeans.
Selenocysteine is available in the food in the form of selenomethionine. In some foodstuff, it occurs in the form of selenate.
The biosynthesis of this amino acid occurs on the tRNA which needs to be first acylated with serine and then is subsequently transformed into selenocysteine by an enzyme synthase that further utilizes selenophosphate in the form of selenium donor and the cofactor pyridoxal phosphate.
- The important functions of selenocysteine in proteins are its antioxidant activity. This is due to its lower pKa and higher reduction potential.
- It is also used in the preparation of a variety of vitamins and lots of other supplements.
- It is also fortified with livestock feeds.
- Our body utilizes selenocysteine to form selenium, which is believed to play important role in preventing mercury toxicity as well as enhance liver functions.
- People deficient with selenium have lean body mass, prone to premature aging or to heart diseases.
Selenocysteine is not coded for in the genetic code directly. It is encoded by the UAG codon in a very special way. UAG is a stop codon.