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Threonine proteases are a family of
proteolytic Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called prote ...
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s harbouring a
threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO� ...
(Thr) residue within the active site. The prototype members of this class of enzymes are the
catalytic Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
subunits of the
proteasome Proteasomes are protein complexes which degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks peptide bonds. Enzymes that help such reactions are called proteases. Proteasomes are part of a major mechanism by whi ...
, however the acyltransferases convergently evolved the same
active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate ( binding site) ...
geometry and mechanism.


Mechanism

Threonine proteases use the
secondary alcohol In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol (ethyl alcohol), which is ...
of their
N-terminal The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
threonine as a nucleophile to perform catalysis. The threonine must be N-terminal since the terminal amine of the same residue acts as a general base by polarising an ordered water which deprotonates the alcohol to increase its reactivity as a nucleophile. Catalysis takes place in two steps: * Firstly the nucleophile attacks the substrate to form a covalent acyl-enzyme intermediate, releasing the first product. * Secondly the intermediate is hydrolysed by water to regenerate the free enzyme and release the second product. ** In ornithine
acyltransferase Acyltransferase is a type of transferase enzyme that acts upon acyl groups. Examples include: * Glyceronephosphate O-acyltransferase * Lecithin-cholesterol acyltransferase * Long-chain-alcohol O-fatty-acyltransferase See also * Acetyltransferas ...
, instead of water, the substrate
ornithine Ornithine is a non-proteinogenic amino acid that plays a role in the urea cycle. Ornithine is abnormally accumulated in the body in ornithine transcarbamylase deficiency. The radical is ornithyl. Role in urea cycle L-Ornithine is one of the produ ...
(the acceptor) performs the second nucleophilic attack and so leaves with the acyl group.


Classification and evolution

Five families belonging to two separate superfamilies are currently recognised: the Ntn fold proteosomes (superfamily PB) and the DOM fold ornithine acyltransferases (superfamily PE). The two superfamilies represent two independent,
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
s of the same active site.


See also

*
Protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
** serine- ** cysteine- ** aspartic- ** metallo- *
Enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
*
Proteolysis Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...
*
Catalytic triad A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, acylases, li ...
*
Convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
*
The Proteolysis Map The Proteolysis MAP (PMAP) is an integrated web resource focused on proteases. Rationale PMAP is to aid the protease researchers in reasoning about proteolytic networks and metabolic pathways. History and funding PMAP was originally created ...
*
Protease inhibitor (pharmacology) Protease inhibitors (PIs) are medications that act by interfering with enzymes that cleave proteins. Some of the most well known are antiviral drugs widely used to treat HIV/AIDS and hepatitis C. These protease inhibitors prevent viral replic ...
*
Protease inhibitor (biology) In biology and biochemistry, protease inhibitors, or antiproteases, are molecules that inhibit the function of proteases ( enzymes that aid the breakdown of proteins). Many naturally occurring protease inhibitors are proteins. In medicine, ' ...
*
TopFIND TopFIND is the Termini oriented protein Function Inferred Database (TopFIND) is an integrated knowledgebase focused on protein termini, their formation by proteases and functional implications. It contains information about the processing and t ...
- database of protease specificity, substrates, products and inhibitors *
MEROPS MEROPS is an online database for peptidases (also known as proteases, proteinases and proteolytic enzymes) and their inhibitors. The classification scheme for peptidases was published by Rawlings & Barrett in 1993, and that for protein inhibitors ...
- database of protease evolutionary groups


References

{{Portal bar, Biology, border=no Proteases EC 3.4