Selenocysteine

L-selenocysteineSeC 3 NO 2 H 7 selenoamino acid
Selenocysteine (symbol Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid.wikipedia
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Selenoprotein

selenoproteins
Selenocysteine exists naturally in all three domains of life, but not in every lineage, as a building block of selenoproteins.
In molecular biology a selenoprotein is any protein that includes a selenocysteine (Sec, U, Se-Cys) amino acid residue.

Proteinogenic amino acid

proteinogenicamino acidsList of standard amino acids
Selenocysteine (symbol Sec or U, in older publications also as Se-Cys) is the 21st proteinogenic amino acid.
Both eukaryotes and prokaryotes can incorporate selenocysteine into their proteins via a nucleotide sequence known as a SECIS element, which directs the cell to translate a nearby UGA codon as selenocysteine (UGA is normally a stop codon).

Cysteine

CysL-cysteinecystein
Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.
Cysteine has the same structure as serine, but with one of its oxygen atoms replaced by sulfur; replacing it with selenium gives selenocysteine.

Selenol

methylselenolSeH
Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.
The best known member of the group is the amino acid selenocysteine.

Thressa Stadtman

Thressa
Selenocysteine was discovered by biochemist Thressa Stadtman at the National Institutes of Health.
Thressa Campbell Stadtman (February 12, 1920 – December 11, 2016) was an American biochemist, notable for her discovery of selenocysteine, and her research on selenoproteins and bioenergetics.

Thioredoxin reductase

EC 1.8.1.9NADPH-thioredoxin reductase-thioredoxinthioredoxin glutathione reductase
Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).

Selenium

SeSe 3 selenium poisoning
Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.
This was followed by the discovery of selenocysteine in proteins.

Catalytic triad

triadactive sitecatalytic tryad
Selenoenzymes have been found to employ catalytic triad structures that influence the nucleophilicity of the active site selenocysteine.
The nucleophile is most commonly a serine or cysteine amino acid, but occasionally threonine or even selenocysteine.

Glutathione peroxidase

glutathione peroxidase 4glutathion peroxidases
Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).

Hydrogenase

hydrogenase enzymehydrogenases
Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).
In some [NiFe] hydrogenases, one of the Ni-bound cysteine residues is replaced by selenocysteine.

Protein

proteinsproteinaceousstructural proteins
Unlike other amino acids present in biological proteins, selenocysteine is not coded for directly in the genetic code.
In general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteine and—in certain archaea—pyrrolysine.

Stop codon

termination codonstop codonsamber stop codon
Instead, it is encoded in a special way by a UGA codon, which is normally a stop codon.
In 2007, the UGA codon was identified as the codon coding for selenocysteine (Sec) and found in 25 selenoproteins located in the active site of the protein.

SECIS element

SECISSelenocysteine insertion sequenceselenocysteine insertion sequence(SECIS)
The UGA codon is made to encode selenocysteine by the presence of a selenocysteine insertion sequence (SECIS) in the mRNA.
This structural motif (pattern of nucleotides) directs the cell to translate UGA codons as selenocysteines (UGA is normally a stop codon).

EF-Tu

EF-Tu proteinElongation Factor Thermo-Unstableelongation factor Tu
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 elongation factor (EF-Tu in bacteria, eEF1A in eukaryotes).
EF-Tu • GTP binds all correctly-charged aa-tRNAs with approximately identical affinity, except those charged with initiation residues and selenocysteine.

Genetic code

codoncodonsencoded
Unlike other amino acids present in biological proteins, selenocysteine is not coded for directly in the genetic code. Instead, it is encoded in a special way by a UGA codon, which is normally a stop codon.
For example, UGA can code for selenocysteine and UAG can code for pyrrolysine.

Selenocysteine lyase

Selenocysteine is decomposed by the enzyme selenocysteine lyase into L -alanine and selenide.
Thus, the two substrates of this enzyme are L-selenocysteine and reduced acceptor, whereas its 3 products are selenide, L-alanine, and acceptor.

SECISBP2

SBP2
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.
The incorporation of selenocysteine into a protein requires the concerted action of an mRNA element called a sec insertion sequence (SECIS), a selenocysteine-specific translation elongation factor and a SECIS binding protein.

Pyrrolysine

Pyl
Two unusual genetically-encoded amino acids are selenocysteine and pyrrolysine.

Selenomethionine

alkali diseaseL-selenomethionine
Biotechnological applications of selenocysteine include use of 73 Se-labeled Sec (half-life of 73 Se = 7.2 hours) in positron emission tomography (PET) studies and 75 Se-labeled Sec (half-life of 75 Se = 118.5 days) in specific radiolabeling, facilitation of phase determination by multiwavelength anomalous diffraction in X-ray crystallography of proteins by introducing Sec alone, or Sec together with selenomethionine (SeMet), and incorporation of the stable 77 Se isotope, which has a nuclear spin of 1⁄2 and can be used for high-resolution NMR, among others.

Domain (biology)

domaindomainsdomains of life
Selenocysteine exists naturally in all three domains of life, but not in every lineage, as a building block of selenoproteins.

Sulfur

sulphurSbrimstone
Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.

Thiol

mercaptansulfhydrylthiols
Selenocysteine is a cysteine analogue with a selenium-containing selenol group in place of the sulfur-containing thiol group.

Enzyme

enzymologyenzymesenzymatic
Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).

Deiodinase

Deiodinase enzymesdeiodinasestetraiodothyronine 5' deiodinase
Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).

Formate dehydrogenase

Selenocysteine is present in several enzymes (for example glutathione peroxidases, tetraiodothyronine 5′ deiodinases, thioredoxin reductases, formate dehydrogenases, glycine reductases, selenophosphate synthetase 2, methionine-R-sulfoxide reductase B1 (SEPX1), and some hydrogenases).