Protein synthesis selenocysteine incorporation

Although known for its toxicity, but unlike antimony and arsenic, selenium is an essential element which has been identified as part of several prokaryotic and eukaryotic proteins in the form of the amino acid, selenocysteine. Selenocysteine has been referred to as the 21st amino acid since gene products required for its incorporation into protein were discovered in bacteria (Stadtman, 1996). Aspects of the mechanism of selenocysteine insertion during protein synthesis in eukaryotes are currently being investigated (Low and Berry, 1996). The two strands of current selenium research are... 

How have selenium-accumulators been able to absorb so much Se without any damage to themselves These plants are able to separate inorganic S (as sulphate) from inorganic Se (as selenate or selenite), when they enter in the plants, and to channel the Se into the synthesis of nonprotein amino acid analogues, which are not therefore incorporate into protein synthesis. The adapted plants then sequester them in the vacuoles of the leaves, where they are perfectly harmless to the plants but intensely harmful to any unsuspecting grazing animals. In the non-adapted plants selenium toxicity may be attributed to the replacement of cysteine by selenocysteine and the production of disfunctional proteins in which S-S bond between polypeptide chains are replaced by the more labile Se-Se bonds. Fig (14). 

In summary, the codon TGA, which normally codes for termination of protein synthesis is programmed from a distance to encode the 21st amino acid selenocysteine a special tRNA is loaded in a unique and unorthodox way, incorporating a selenocysteine residue which is synthesised de novo at the tRNA level by special enzymes highly specialised proteins are required to recognise and bind the secondary mRNA structures and the tRNA specialised elongation factors have to compete with canonical ones and with release factors. No wonder that chemists and biochemists ask what are the unique properties of selenocysteine compared to cysteine (Amer, 2010) to justify the involvement of so many molecular partners (Allmang, Wurth, Krol, 2009). 

In addition to the conventional 20 amino acids in Tables 26-1 and 26-3, two others, selenocysteine (Sec) and pyrrolysine (Pyl), are incorporated into proteins using the nueleie acid-based eell machinery described in Section 26-10. The three-base codes for Sec and Pyl are UGA and HAG, respectively. These codes normally serve to terminate protein synthesis. However, if they are preceded by certain specific base sequences, they instead cause incorporation of these unusual amino adds and continued growth of the peptide chain. 

The constmction of synthetic selenocysteine-containing proteins or selenium-containing proteins attracts considerable interest at present, mainly for the reason that it can be used to solve the phase problem in X-ray crystallography. Selenomethionine incorporation has been used mostly uutil now for this purpose. There are also two reports ou uew synthetic selenocysteine-containing proteins. In one case, the active site serine of subtUisin has been converted into a selenocysteine residue by chemical means, with the result that the enzyme gains a predominant esterase instead of protease activity. In the second case, automated peptide synthesis was carried out to produce a peptide in which all seven-cysteine residues of the Neurospora crassa metallothioueiu (Cu) were replaced by selenocysteine. The replacement resulted iu au alteration of both the stoichiometry and the affinity of copper binding. ... 

Selenium is incorporated into Se-requiring enzymes by the modification of serine. This serine is not modified when it is in the free state or when it occurs in a polypeptide chain. The serine residue in question is modified when it occurs boimd to transfer RNA, that is, eis the aminoacyl-tRNA derivative. Seryl-tRNA is converted to selenocysteinyl-tRNAby the action of selenocysteine synthase (Stur-chler et al, 1993). The codon for selenocysteine is UGA (TGA in DNA UGA in mRNA). The fact that this particular triplet of bases codes for an amino acid is very imusual, as UGA normally is a stop codon. Stop codons occur in mRNA and signal the termination of synthesis of the protein however, in the case of the UGA codons that code for selenocysteine residues, regions of the mRNA that lie beyond the coding sequence somehow convert the UGA from a codon that halts translation to one that codes for selenocysteine (Figure 10.55). The structure of selenocysteine is shown in Figure 10.56. 

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 ... 

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