Selenocysteine, synthesis

Figure 6 summarizes the steps of selenocysteine synthesis and incorporation, as elucidated for the E. coli system. 

Amberg R, Mizutani T, Wu XQ, Gross HJ. Selenocysteine synthesis in mammalia an identity switch from tRNA(Ser) to tRNA(Sec). J. Mol. Biol. 1996 263 8-19. 

Selenophosphate synthetase Synthesizes selenophospate from selenide + ATP as first step in selenocysteine synthesis during Se incorporation into selenoproteins... 

The stem-loop structure in the noncoding 3 region of selenoprotein mRNAs has also been termed a SECTS element in mammals although it has a different overall structure. ° In silica analysis of the human genome sequence, using this consensus SECTS element along with the presence of the characteristic UGA codon within an exon, has led to the discovery of several new selenoproteins, including a selenium-dependent methionine sulfoxide reductase. It has been shown that a specific complex exists for selenoprotein synthesis that shuttles between the nucleus and the cytosol. This possibly protects the preformed complex for nonsense-mediated decay to allow for more efficient selenoprotein synthesis. The specific tRNA needed for selenocysteine... 

The synthesis in many extant organisms of these two amide residues from their respective precursors glutamate and aspartate esterified to tRNA (the indirect aminoacylation pathways described in Sections 5.14.3 and 5.14.4) and that of other amino acid residues, such as selenocysteine (which is also synthesized from a precursor esterified on a tRNA °) support the model of prebiotic metabolism taking place at the surface of solid particles, " analogous to ancestral RNAs. 

For the synthesis of selenocysteine derivatives that are suitable for peptide synthesis essentially two approaches have been used to date (1) conversion of p-chloroalanine 23 or serine-O-tosylate derivatives 24 into the desired selenocysteine derivatives by a nucleophilic displacement reaction with an areneselenol and (2) full reduction of selenocystine and in situ reaction with aryl halides to produce the aryl selenides. 7 25 In this context, reduction of selenocystine in 2 M NaOH with 2-methyl-2-propanethiol for concomitant formation of the mixed selenide/sulfide derivative 5e-(tert-butylsulfanyl)selenocysteine in analogy to the formation of 5-(fett-butylsulfanyl)cysteine 26 fails as a consequence of the difficult reduction of the diselenide with monothiols. 27 ... 

The Se-(4-methoxybenzyl)selenocysteine is obtained by reduction of selenocystine with NaBH4 and in situ reaction with 4-methoxybenzyl chloride. 7 The optimized procedure of Tanaka and Soda 32 is preferentially used for the synthesis of the starting selenocystine, which involves reaction of (1-chloroalanine with a 2.3-fold excess of disodium diselenide in aqueous solution at pH 9. Alternatively, the significantly less selenium demanding synthesis of Stocking et al. 33 is used for the preparation of expensive 77Se-selenocystine, this consists of the reaction of methyl (2R)-2-[(/ert-butoxycarbonyl)amino]-3-iodopropanoate with equivalent amounts of dilithium diselenide. Subsequent conversion of SeC(Mob) into the M -Fmoc derivative 7 and finally into the pentafluorophenyl ester 10 is performed following standard procedures. 

Table 1 Optimized Protocol of the Synthesis of Selenocysteine Peptides on Rink-Amide-MBHA Resin According to the Fmoc Strategy 10 ...

The advantage of this synthetic approach over the total synthesis of selenocysteine peptides is the reduced number of synthetic steps to which the Se-protected selenocysteine is exposed, thus, largely preventing deselenation as an undesired side reaction. It also allows access to larger peptides and even proteins containing selenocysteine to exploit the Se atom for X-ray crystallography, 13 or as 77Se nucleus for NMR spectroscopy. 60 ... 

Selenocysteine is a special case. This rare amino acid residue is introduced during protein synthesis rather than created through a postsynthetic modification. It contains selenium rather than the sulfur of cysteine. Actually derived from serine, selenocysteine is a constituent of just a few known proteins. 

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... 

A cotranslational mode of selenocysteine insertion necessitates the existence of a tRNA that carries this amino acid. This tRNA was identified during a screening for E. coli mutants blocked in selenoprotein formation. The genes and the function of the gene products found to be involved in selenoprotein synthesis are listed in Table 2. 

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. ... 

Gieseknan MD, Xie L, van Der Donk WA. Synthesis of a selenocysteine-containing peptide by native chemical ligation. 

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. 

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