Selenoproteins

Synthesis

Mammalian selenoproteins contain selenocysteine residues, usually at their active sites. The importance of the selenocysteine residue lies in the fact that, at physiological pH, the residue is fully ionized, allowing it to participate effectively in redox-type reactions. In contrast, cysteine residues, which may also participate in redox reactions, are only approximately 10% ionized at physiological pH. Selenocysteine residues are incorporated at specific sites in the selenoproteins through a co-translational event directed by the UGA codon. Although the UGA codon can be recognized by the cell as a termination codon, in selenoprotein synthesis the UGA codon also signals the insertion of a selenocysteine residue. The recognition of the UGA codon to signal the insertion of a selenocysteine residue requires selenocysteine insertion sequence (SECIS) elements. In eukaryotes, the SECIS elements are located in the 3' untranslated region of the mRNA and comprise a small number of conserved nucleotides, which form a stem-loop structure. These SECIS elements are different for each selenoprotein but form a similar-shaped stem loop and are functionally interchangeable (Berry, 1991).

The synthesis of selenocysteine and its insertion into specific selenoproteins in prokaryotes involves the products of four genes (selA, selB, selC and selD). The products are: a selenocysteine-specific tRNA species (tRNASec) (selC), which carries the anticodon for UGA, the enzymes selenocysteine synthase (selA) and selenophosphate synthetase (selD), which are essential for the formation of selenocysteine-tRNASec from seryl-tRNASec, and the elongation fac tor, which specifically recognizes the selenocysteine-tRNA (selB) (Bermano et al., 1995; Allan et al., 1999).

The lack of a series of mutants has delayed the description of the mechanism of selenocysteine incorporation and selenoprotein synthesis in eukaryotes. However, two forms of the tRNASec have been isolated in eukaryotes and both contain the UGA anticodon, which is functional in Escherichia coli (Kollmus et al., 1996; Low and Berry, 1996). Like bacterial tRNASec, in eukaryotes tRNASec is esterified with serine and is subsequently converted to seryl-tRNASec. The major steps of this co-translational event in eukaryotes are illustrated in Fig. 12.1, highlighting the role of SECIS-binding protein 2 (SBP2) and multiple selenophosphate synthetases (Mansell and Berry, 2001).

Selenoproteins that have been characterized

Although 30-40 selenoproteins have been demonstrated using isotopic labelling, only approximately 20 have been further characterized. The biological actions of some of these selenoproteins are known and are summarized in Table 12.1 (see also Holmgren, 1985, 1989; Beckett and Arthur, 1994; Burk,

  1. 12.1. The synthesis of specific selenoproteins. AtRNAthat recognizes the stop codon UGA on mRNA forms a complex with the selenocysteine insertion sequence (SECIS) loop in the 3' untranslated region of the mRNA. Selenophosphate converts a serine bound to the tRNA to selenocysteine; this is then incorporated into the backbone of the protein. The part of the protein complex that carries out this reaction on the ribosome is SBP2; SEL C is the tRNA and SEL D is one of the selenophosphate synthetases.
  2. 12.1. The synthesis of specific selenoproteins. AtRNAthat recognizes the stop codon UGA on mRNA forms a complex with the selenocysteine insertion sequence (SECIS) loop in the 3' untranslated region of the mRNA. Selenophosphate converts a serine bound to the tRNA to selenocysteine; this is then incorporated into the backbone of the protein. The part of the protein complex that carries out this reaction on the ribosome is SBP2; SEL C is the tRNA and SEL D is one of the selenophosphate synthetases.

Table 12.1. Properties of selenoproteins.

Selenoprotein family

Member

Where found

Structural features

Role

Glutathione peroxidase (GPX)

Cytoplasmic GPX (cyGPX)

Phospholipid hydroperoxide GPX (PHGPX)

Extracellular GPX

Gastrointestinal GPX

lodothyronine deiodinase (ID) ID I

All cells

Cytosol and membranes of many cells. High activity in testis

Plasma. Synthesized Tetramer mainly in proximal tubules of kidney;

also by thyrocytes

Closely related to cyGPX

Liver and kidney provide 80% of plasma T3

Tetramer: four identical subunits (molecular weight 19-25 kDa). Each subunit has a glutathione-binding site and a single selenocysteine residue at the active site Monomer (molecular weight 19 kDa)

Catalyses reduction of a variety of hydroperoxides, including H2O2 and fatty-acid hydroperoxides

Catalyses reduction of a variety of hydroperoxides, especially fatty-acid and cholesterol hydroperoxides

  • Accounts for all hydroperoxide-reducing activity of plasma
  • Protection of gastrointestinal tract against ingested hydroperoxides

Thyroid hormone metabolism: catalyses 5- and 5'-monodeiodination of iodothronines (including thyroid hormones). 5'-Deiodination promotes conversion of T4 to its active form T3; 5-deiodination promotes conversion of T4 to the inactive reverse T3

Iodothyronine deiodinase (ID) ID II

ID III

Selenoprotein P

Thioredoxin reductase

At least three isoforms exist

Selenoprotein W

Brain, CNS, Placenta, piturity

Brain, CNS, Placenta, piturity

Extracellular (accounts for 40% of plasma Se)

All tissues

Four forms identified (in the rat)

Up to ten selenocysteine residues, nine of which are located at the carboxyl terminus

FAD-containing homodimer with a single selenocysteine residue near the carboxyl terminus of each subunit chain

Intracellular. High amounts in brain, muscle, testis and spleen; low amount in liver

Catalyses 5'-monodeiodination of iodothyronines (including thyroid hormones)

Catalyses 5-monodeiodination of iodothyronines (including thyroid hormones)

Function not known

Along with thioredoxin (substrate) and NADPH (cofactor) forms a powerful dithiol-disulphide oxidoreductase system. Can catalyse the reduction of a variety of chain substrates. Thioredoxin involved as hydrogen donor for ribonucleotide reductase (key step of DNA synthesis). Involved in many cell functions (cell growth, apoptosis inhibition, maintenance of cellular redox state)

? Antioxidant

CNS, central nervous system; NADPH, nicotinamide adenine dinucleotide phosphate; FAD, flavin adenine dinucleotide.

1994; Sunde, 1994; Arthur et al., 1997; St Germain and Galton, 1997; Burk and Hill, 1999; Arner and Holmgren, 2000).

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