Selenocysteine is an amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 that is present in several enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

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' deiodinases, thioredoxin reductase

Thioredoxin reductase

Thioredoxin Reductases are the only known enzymes to reduce thioredoxin . Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. Both classes are flavoproteins which function as homodimers...

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 NAD+ in formate:NAD+ oxidoreductase or to a cytochrome in formate:ferricytochrome-b1 oxidoreductase .-Biological function:NAD-dependent formate...

s, glycine reductase

Glycine reductase

In enzymology, a glycine reductase is an enzyme that catalyzes the chemical reactionThe 4 substrates of this enzyme are acetyl phosphate, NH3, thioredoxin disulfide, and H2O, whereas its 3 products are glycine, phosphate, and thioredoxin....

s, and some hydrogenase

Hydrogenase

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....

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 that of cysteine

Cysteine

Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...

, but with an atom of selenium

Selenium

Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

 taking the place of the usual sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

, forming a selenol

Selenol

Selenols are organic compounds that contain the functional group with the connectivity C-Se-H. Selenols are sometimes also called selenamercaptans, selenathiols, and selenothiols. Selenols are one of the principal classes of organoselenium compounds...

 group. Proteins that contain one or more selenocysteine residues are called selenoprotein

Selenoprotein

In molecular biology a selenoprotein is any protein that includes a selenocysteine amino acid residue. Among functionally characterized selenoproteins are five glutathione peroxidases and three thioredoxin reductases, which both contain only one Se-Cys. Selenoprotein P is the most common...

s.

Biology

Selenocysteine has both a lower pKa (5.47) and a higher reduction potential than cysteine. These properties make it very suitable in proteins that are involved in anti-oxidant activity.

Unlike other amino acids present in biological protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

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 translated into proteins by living cells....

. Instead, it is encoded in a special way by a UGA codon, which is normally a stop codon. When cells are grown in the absence of selenium, translation of selenoproteins terminates at the UGA codon, resulting in a truncated, nonfunctional enzyme. The UGA codon is made to encode selenocysteine by the presence of a SECIS element

SECIS element

In biology, the SECIS element is an RNA element around 60 nucleotides in length that adopts a stem-loop structure. This structural motif directs the cell to translate 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

Bacteria are a large domain of prokaryotic 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...

 and in eukaryote

Eukaryote

A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

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 .

Unlike the other amino acids, no free pool of selenocysteine exists in the cell. Its high reactivity would incur damage to cells. Instead, cells store selenium in the less reactive selenide form (H₂Se). Selenocysteine synthesis occurs on a specialized tRNA, which also functions to incorporate it into nascent polypeptides. 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, eEF1A in eukaryotes). Rather, the tRNA-bound seryl residue is converted to a selenocysteine-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 (a.k.a. eEFSec)), 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 bacteria, SelB) or an extra subunit (SBP2 for eukaryotic mSelB/eEFSec) which bind to the corresponding RNA secondary structures formed by the SECIS elements in selenoprotein mRNAs.

There are more than 20 human proteins that contain selenocysteine.