[priyasaravanan_1406]

Though there are ‘n’ numbers of amino acids present there are only twenty amino acids which are proved as essential amino acids and their genetic codes have been derived successfully. To this existing list of twenty amino acids enters the “Selenocysteine” the 21st amino acid whose genetic code has been identified. Selenocysteine makes its presence in almost all forms of lives from prokaryotes to eukaryotes.

Selenocysteine codon: UGA

UGA is one among the three stop codons (UAG, UGA and UAA) and these three codons acts exclusively as stop signal in protein synthesis and do not code for any other existing 20 amino acids.
Hence there must be a unique pathway in the protein synthesis involving selenocysteine which requires the regulation of UGA codon in the synthesis of selenocysteine by inhibiting its original function of termination. Yes, there are separate mechanisms for the synthesis of selenoproteins (protein containing the amino acid selenocysteine) identified in prokaryotes and eukaryotes where presence of selenium is significant.

In prokaryotes, the incorporation of selenocysteine in the protein is made possible by two factors like,
1. Stem loop structure in the 3 prime region of mRNA (untranslated)
2. Sel operon with Sel A, Sel B, Sel C and Sel D genes each of which functions uniquely aiding the synthesis of selenocysteine incorporated proteins .

In Eukaryotes, the mechanism is regulated by three factors,
1. SECIS (SeC insertion sequence)
2. SBP2 (SECIS binding protein)
3. eEFSeC (a translational factor enhances ribosomal binding)

Different selenoproteins

Iodothyronine deiodinases (3 types like DIO1,DIO2 & DIO3)
Glutathione peroxidases (5 types)
Selenophosphate synthetase (2 types)
Thioredoxin reductases (3 types)
Selenoproteins like Sel-H, I, K, M, N, O, P, R, S, T, V, W

There are about 25 different selenoproteins (contains selenium) identified which signifies the requirement of selenium by the human body. Selenium is essential as it protects cardiovascular system and muscular system, acts as a hurdle to cancer cells and also boosts immune system. But again high intake of selenium will pave way for selenium poisoning.

Selenocysteine Biotechnological application

The differently labeled selenocysteine are used in the fields like positron emission tomography, labeling radioactive elements and X-ray crystallography.