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Selenocysteine
 
 
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Selenocysteine is an amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 that is present in several enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
s (for example glutathione peroxidase
Glutathione peroxidase

Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage....
s, tetraiodothyronine 5' deiodinase
Tetraiodothyronine 5 deiodinase

Tetraiodothyronine 5' deiodinase is an enzyme. The amino acid selenocysteine is one of its constituent parts.It creates the active form of triiodothyronine from thyroxine and is one of three selenocysteine-containing iodothyronine deiodinases that work in concert to guarantee adequate local and systemic levels of the active thyroid horm...
s, thioredoxin reductase
Thioredoxin reductase

Thioredoxin Reductases are the only known enzymes to reduce thioredoxin . Thioredoxin reductase is a homodimer of 316-residue subuints that catalyzes the reduction of thioredoxin using NADPH as a reducing agent....
s, formate dehydrogenase
Formate dehydrogenase

Formate dehydrogenases are a set of enzymes that catalyse the oxidation of formate to bicarbonate, donating the electrons to a second substrate, such as Nicotinamide adenine dinucleotide in formate:NAD+ oxidoreductase or to a cytochrome in formate:ferricytochrome-b1 oxidoreductase ....
s, glycine reductase
Glycine reductase

In enzymology, a glycine reductase is an enzyme that catalysis the chemical reactionThe 4 substrate of this enzyme are acetyl phosphate, ammonia, thioredoxin disulfide, and water, whereas its 3 product are glycine, phosphate, and thioredoxin....
s and some hydrogenase
Hydrogenase

A hydrogenase is an enzyme that Catalysis the reversible Redox of molecular hydrogen . Hydrogenases play a vital role in Glycolysis.Hydrogen uptake is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide, and fumarate, whereas proton reduction is essential in pyruvate fermentation and in the di...
s).

joint nomenclature committee of the IUPAC/IUBMB has officially recommended the three-letter symbol Sec and the one-letter symbol U for selenocysteine.

nocysteine has a structure similar to cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, but with an atom of selenium
Selenium

Selenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature....
 taking the place of the usual sulfur
Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
.






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Selenocysteine is an amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
 that is present in several enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
s (for example glutathione peroxidase
Glutathione peroxidase

Glutathione peroxidase is the general name of an enzyme family with peroxidase activity whose main biological role is to protect the organism from oxidative damage....
s, tetraiodothyronine 5' deiodinase
Tetraiodothyronine 5 deiodinase

Tetraiodothyronine 5' deiodinase is an enzyme. The amino acid selenocysteine is one of its constituent parts.It creates the active form of triiodothyronine from thyroxine and is one of three selenocysteine-containing iodothyronine deiodinases that work in concert to guarantee adequate local and systemic levels of the active thyroid horm...
s, thioredoxin reductase
Thioredoxin reductase

Thioredoxin Reductases are the only known enzymes to reduce thioredoxin . Thioredoxin reductase is a homodimer of 316-residue subuints that catalyzes the reduction of thioredoxin using NADPH as a reducing agent....
s, formate dehydrogenase
Formate dehydrogenase

Formate dehydrogenases are a set of enzymes that catalyse the oxidation of formate to bicarbonate, donating the electrons to a second substrate, such as Nicotinamide adenine dinucleotide in formate:NAD+ oxidoreductase or to a cytochrome in formate:ferricytochrome-b1 oxidoreductase ....
s, glycine reductase
Glycine reductase

In enzymology, a glycine reductase is an enzyme that catalysis the chemical reactionThe 4 substrate of this enzyme are acetyl phosphate, ammonia, thioredoxin disulfide, and water, whereas its 3 product are glycine, phosphate, and thioredoxin....
s and some hydrogenase
Hydrogenase

A hydrogenase is an enzyme that Catalysis the reversible Redox of molecular hydrogen . Hydrogenases play a vital role in Glycolysis.Hydrogen uptake is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide, and fumarate, whereas proton reduction is essential in pyruvate fermentation and in the di...
s).

Nomenclature

The joint nomenclature committee of the IUPAC/IUBMB has officially recommended the three-letter symbol Sec and the one-letter symbol U for selenocysteine.

Structure

Selenocysteine has a structure similar to cysteine
Cysteine

Cysteine is an a-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that humans can synthesize it....
, but with an atom of selenium
Selenium

Selenium is a chemical element with the atomic number 34, represented by the chemical symbol Se, an atomic mass of 78.96. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature....
 taking the place of the usual sulfur
Sulfur

Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
. Proteins that contain one or more selenocysteine residues are called selenoprotein
Selenoprotein

A selenoprotein is any protein that includes a selenocysteine residue. Selenoproteins exist in all major forms of life, eukaryote, bacteria and archaea....
s.

Biology

Unlike other amino acids present in biological protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
s, Selenocysteine is not coded for directly in the genetic code
Genetic code

The genetic code is the set of rules by which information encoded in genetic material is Translation into proteins by living cell s. The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids....
. Instead, it is encoded in a special way by a UGA codon, which is normally a stop codon. The UGA codon is made to encode selenocysteine by the presence of a SECIS element
SECIS element

In biology, the SECIS element is an cis-regulatory element around 60 nucleotides in length that adopts a stem-loop structure. This structural motif directs the cell to translation UGA codons as selenocysteines....
 (SElenoCysteine Insertion Sequence) in the mRNA. The SECIS element is defined by characteristic nucleotide sequences and secondary structure base-pairing patterns. In bacteria
Bacteria

The Bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals....
, the SECIS element is located immediately following the UGA codon within the reading frame for the selenoprotein. In archaea
Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . Archaea, like bacteria, are prokaryotic....
 and in eukaryote
Eukaryote

Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
s, the SECIS element is in the 3' untranslated region (3' UTR) of the mRNA, and can direct multiple UGA codons to encode selenocysteine residues. When cells are grown in the absence of selenium, translation of selenoproteins terminates at the UGA codon, resulting in a truncated, nonfunctional enzyme.

Like the other amino acids used by cells, selenocysteine has a specialized tRNA. The primary and secondary structure of selenocysteine tRNA, tRNA(Sec), differ from those of standard tRNAs in several respects, most notably in having an 8-base (bacteria) or 10-base (eukaryotes) pair acceptor stem, a long variable region arm, and substitutions at several well-conserved base positions. The selenocysteine tRNAs are initially charged with serine by seryl-tRNA ligase, but the resulting Ser-tRNA(Sec) is not used for translation because it is not recognised by the normal translation factor (EF-Tu in bacteria, EF1-alpha in eukaryotes). Rather, the tRNA-bound seryl residue is converted to a selenothiomise-residue by the pyridoxal phosphate-containing enzyme selenocysteine synthase. Finally, the resulting Sec-tRNA(Sec) is specifically bound to an alternative translational elongation factor (SelB or mSelB) which delivers it in a targeted manner to the ribosomes translating mRNAs for selenoproteins. The specificity of this delivery mechanism is brought about by the presence of an extra protein domain (in bacterial SelB) or an extra subunit (SBP-5 for eukaryotic mSelB) which bind to the corresponding RNA secondary structures formed by the SecIS elements in selenoprotein mRNAs. The SecIS elements of bacterial selenoproteins (as far as analysed) are located within the coding sequences immediately following the UGA codons for selenocysteine, those of Eukarya and Archaea are located in the 3' UTR of the respective mRNAs. In addition, at least one case has been described for an archaeal selenoprotein mRNA containing its SecIS in the 5' UTR.

See also

  • Sodium selenite
    Sodium selenite

    Sodium selenite is a chemical compound with the chemical formula Sodium2SeleniumOxygen3. This salt is the most common water soluble forms of selenium....
  • Pyrrolysine
    Pyrrolysine

    Pyrrolysine is a naturally occurring, genetically coded amino acid used by some methanogenic archaea in enzymes that are part of their methane-producing metabolism....
    , another amino acid not in the basic set of 20.


Further reading