A protease (also called a peptidase, proteinase, or proteolytic enzyme)
In the absence of functional accelerants, proteolysis would be very slow, taking hundreds of .
Classification
Based on catalytic residue
Proteases can be classified into seven broad groups:
Proteases were first grouped into 84 families according to their evolutionary relationship in 1993, and classified under four catalytic types: Serine protease, cysteine, aspartic, and Metalloproteaseproteases.
Peptide lyases
A seventh catalytic type of proteolytic enzymes, asparagine peptide lyase, was described in 2011. Its proteolytic mechanism is unusual since, rather than
hydrolysis, it performs an elimination reaction.
During this reaction, the catalytic
asparagine forms a cyclic chemical structure that cleaves itself at asparagine residues in proteins under the right conditions. Given its fundamentally different mechanism, its inclusion as a peptidase may be debatable.
Based on evolutionary phylogeny
An up-to-date classification of protease evolutionary superfamilies is found in the MEROPS database.
In this database, proteases are classified firstly by 'clan' (superfamily) based on structure, mechanism and catalytic residue order (e.g. the
PA clan where P indicates a mixture of nucleophile families). Within each 'clan', proteases are classified into
protein family based on sequence similarity (e.g. the S1 and C3 families within the PA clan). Each family may contain many hundreds of related proteases (e.g.
trypsin,
elastase,
thrombin and
Streptogrisin A within the S1 family).
Currently more than 50 clans are known, each indicating an independent evolutionary origin of proteolysis.
Based on optimal pH
Alternatively, proteases may be classified by the optimal pH in which they are active:
-
Acid proteases
-
Neutral proteases involved in type 1 hypersensitivity. Here, it is released by and causes activation of complement and kinins.
-
Basic proteases (or alkaline proteases)
Enzymatic function and mechanism
Proteases are involved in
digestion long protein chains into shorter fragments by splitting the
peptide bonds that link
amino acid residues. Some detach the terminal amino acids from the protein chain (
exopeptidases, such as
, carboxypeptidase A); others attack internal peptide bonds of a protein (
endopeptidases, such as
trypsin,
chymotrypsin,
pepsin,
papain,
elastase).
Catalysis
Catalysis is achieved by one of two mechanisms:
-
Aspartic, glutamic, and metallo-proteases activate a water molecule, which performs a nucleophilic attack on the peptide bond to hydrolyze it.
-
Serine, threonine, and cysteine proteases use a nucleophilic residue (usually in a catalytic triad). That residue performs a nucleophilic attack to link the protease to the substrate protein, releasing the first half of the product. This covalent acyl-enzyme intermediate is then hydrolyzed by activated water to complete catalysis by releasing the second half of the product and regenerating the free enzyme
Specificity
Proteolysis can be highly promiscuous such that a wide range of protein substrates are hydrolyzed. This is the case for digestive enzymes such as
trypsin, which have to be able to cleave the array of proteins ingested into smaller peptide fragments. Promiscuous proteases typically bind to a single
amino acid on the substrate and so only have specificity for that residue. For example,
trypsin is specific for the sequences ...K\... or ...R\... ('\'=cleavage site).
Conversely some proteases are highly specific and only cleave substrates with a certain sequence. Blood clotting (such as thrombin) and viral polyprotein processing (such as TEV protease) requires this level of specificity in order to achieve precise cleavage events. This is achieved by proteases having a long binding cleft or tunnel with several pockets that bind to specified residues. For example, TEV protease is specific for the sequence ...ENLYFQ\S... ('\'=cleavage site).
Degradation and autolysis
Proteases, being themselves proteins, are cleaved by other protease molecules, sometimes of the same variety. This acts as a method of regulation of protease activity. Some proteases are less active after autolysis (e.g.
TEV protease) whilst others are more active (e.g.
trypsinogen).
Biodiversity of proteases
Proteases occur in all organisms, from
to
to
viruses. These enzymes are involved in a multitude of physiological reactions from simple digestion of food proteins to highly regulated cascades (e.g., the [[blood-clotting
cascade|coagulation]], the complement system, [[apoptosis]] pathways, and the invertebrate prophenoloxidase-activating cascade). Proteases can either break specific peptide bonds (''limited proteolysis''), depending on the [[amino acid]] sequence of a protein, or completely break down a peptide to amino acids (''unlimited proteolysis''). The activity can be a destructive change (abolishing a protein's function or digesting it to its principal components), it can be an activation of a function, or it can be a signal in a signalling pathway.
Plants
Plant genomes encode hundreds of proteases, largely of unknown function. Those with known function are largely involved in developmental regulation.
Plant proteases also play a role in regulation of
photosynthesis.
Animals
Proteases are used throughout an organism for various metabolic processes. Acid proteases secreted into the stomach (such as
pepsin) and serine proteases present in the
duodenum (
trypsin and
chymotrypsin) enable us to digest the protein in food. Proteases present in blood serum (
thrombin,
plasmin,
Hageman factor, etc.) play an important role in blood-clotting, as well as lysis of the clots, and the correct action of the immune system. Other proteases are present in leukocytes (
elastase,
cathepsin G) and play several different roles in metabolic control. Some
snake venoms are also proteases, such as
pit viper haemotoxin and interfere with the victim's blood clotting cascade. Proteases determine the lifetime of other proteins playing important physiological roles like hormones, antibodies, or other enzymes. This is one of the fastest "switching on" and "switching off" regulatory mechanisms in the physiology of an organism.
By a complex cooperative action, proteases can catalyze cascade reactions, which result in rapid and efficient amplification of an organism's response to a physiological signal.
Bacteria
Bacteria secrete proteases to
hydrolyse the peptide bonds in proteins and therefore break the proteins down into their constituent
. Bacterial and fungal proteases are particularly important to the global
carbon and
nitrogen cycles in the recycling of proteins, and such activity tends to be regulated by nutritional signals in these organisms.
The net impact of nutritional regulation of protease activity among the thousands of species present in soil can be observed at the overall microbial community level as proteins are broken down in response to carbon, nitrogen, or sulfur limitation.
Bacteria contain proteases responsible for general protein quality control (e.g. the AAA+ proteasome) by degrading unfolded or misfolded proteins.
A secreted bacterial protease may also act as an exotoxin, and be an example of a virulence factor in bacterial pathogenesis (for example, exfoliative toxin). Bacterial exotoxic proteases destroy extracellular structures.
Viruses
The genomes of some
viruses encode one massive
polyprotein, which needs a protease to cleave this into functional units (e.g. the hepatitis C virus and the
).
These proteases (e.g.
TEV protease) have high specificity and only cleave a very restricted set of substrate sequences. They are therefore a common target for protease inhibitors.
Archaea
Archaea use proteases to regulate various cellular processes from
Cell signaling,
metabolism,
secretion and protein quality control.
Only two ATP-dependent proteases are found in archaea: the membrane associated LonB protease and a soluble
Proteasome complex .
Uses
The field of protease research is enormous. Since 2004, approximately 8000
Scientific paper related to this field were published each year.
Proteases are used in industry,
medicine and as a basic biological research tool.
Digestive proteases are part of many laundry detergents and are also used extensively in the bread industry in bread improver. A variety of proteases are used medically both for their native function (e.g. controlling blood clotting) or for completely artificial functions ( e.g. for the targeted degradation of pathogenic proteins). Highly specific proteases such as TEV protease and thrombin are commonly used to cleave and in a controlled fashion.
Protease-containing plant-solutions called vegetarian rennet have been in use for hundreds of years in Europe and the Middle East for making kosher and halal Cheeses. Vegetarian rennet from Withania coagulans has been in use for thousands of years as a Ayurveda remedy for digestion and diabetes in the Indian subcontinent. It is also used to make Paneer.
Inhibitors
The activity of proteases is inhibited by protease inhibitors.
One example of protease inhibitors is the
serpin superfamily. It includes alpha 1-antitrypsin (which protects the body from excessive effects of its own
inflammation proteases), alpha 1-antichymotrypsin (which does likewise), C1-inhibitor (which protects the body from excessive protease-triggered activation of its own complement system),
antithrombin (which protects the body from excessive
coagulation), plasminogen activator inhibitor-1 (which protects the body from inadequate coagulation by blocking protease-triggered
fibrinolysis), and
neuroserpin.
Natural protease inhibitors include the family of lipocalin proteins, which play a role in cell regulation and differentiation. Lipophilic ligands, attached to lipocalin proteins, have been found to possess tumor protease inhibiting properties. The natural protease inhibitors are not to be confused with the protease inhibitors used in antiretroviral therapy. Some viruses, with HIV/AIDS among them, depend on proteases in their reproductive cycle. Thus, protease inhibitors are developed as Antiviral drug therapeutic agents.
Other natural protease inhibitors are used as defense mechanisms. Common examples are the trypsin inhibitors found in the seeds of some plants, most notable for humans being soybeans, a major food crop, where they act to discourage predators. Raw soybeans are toxic to many animals, including humans, until the protease inhibitors they contain have been denatured.
See also
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Ligase
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Protease
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PA clan
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Convergent evolution
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Proteolysis
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Catalytic triad
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The Proteolysis Map
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Proteases in angiogenesis
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Intramembrane proteases
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Protease inhibitor (pharmacology)
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Protease inhibitor (biology)
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TopFIND - database of protease specificity, substrates, products and inhibitors
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MEROPS - Database of protease evolutionary groups
External links
