Catalytic triad
A catalytic triad is a set of three coordinated
As well as
History
The enzymes
Since their initial discovery, there have been increasingly detailed investigations of their exact catalytic mechanism. Of particular contention in the 1990s and 2000s was whether
Function
Enzymes that contain a catalytic triad use it for one of two reaction types: either to
Mechanism
Catalytic triads perform
Catalysis is performed in two stages. First, the activated nucleophile attacks the
The second stage of catalysis is the resolution of the acyl-enzyme intermediate by the attack of a second substrate. If this substrate is water then the result is hydrolysis; if it is an organic molecule then the result is transfer of that molecule onto the first substrate. Attack by this second substrate forms a new tetrahedral intermediate, which resolves by ejecting the enzyme's nucleophile, releasing the second product and regenerating free enzyme.[25]
Identity of triad members
Nucleophile
The side-chain of the nucleophilic residue performs covalent catalysis on the
Use of oxygen or sulfur as the nucleophilic atom causes minor differences in catalysis. Compared to
Very rarely, the selenium atom of the uncommon amino acid selenocysteine is used as a nucleophile.[29] The deprotonated Se− state is strongly favoured when in a catalytic triad.[29]
Base
Since no natural amino acids are strongly nucleophilic, the base in a catalytic triad
The base is most commonly histidine since its pKa allows for effective base catalysis, hydrogen bonding to the acid residue, and deprotonation of the nucleophile residue.
Acid
The acidic triad member forms a hydrogen bond with the basic residue. This aligns the basic residue by restricting its side-chain rotation, and polarises it by stabilising its positive charge.[3] Two amino acids have acidic side chains at physiological pH (aspartate or glutamate) and so are the most commonly used for this triad member.[3] Cytomegalovirus protease[b] uses a pair of histidines, one as the base, as usual, and one as the acid.[1] The second histidine is not as effective an acid as the more common aspartate or glutamate, leading to a lower catalytic efficiency. In some enzymes, the acid member of the triad is less necessary and some act only as a dyad. For example, papain[c] uses asparagine as its third triad member which orients the histidine base but does not act as an acid. Similarly, hepatitis A virus protease[d] contains an ordered water in the position where an acid residue should be.[citation needed]
Examples of triads
Ser-His-Asp
The Serine-Histidine-Aspartate motif is one of the most thoroughly characterised catalytic motifs in biochemistry.
The same triad has also convergently evolved in
Cys-His-Asp
The second most studied triad is the Cysteine-Histidine-Aspartate motif.[2] Several families of cysteine proteases use this triad set, for example TEV protease[a] and papain.[c] The triad acts similarly to serine protease triads, with a few notable differences. Due to cysteine's low pKa, the importance of the Asp to catalysis varies and several cysteine proteases are effectively Cys-His dyads (e.g. hepatitis A virus protease), whilst in others the cysteine is already deprotonated before catalysis begins (e.g. papain).[36] This triad is also used by some amidases, such as N-glycanase to hydrolyse non-peptide C-N bonds.[37]
Ser-His-His
The triad of cytomegalovirus protease[b] uses histidine as both the acid and base triad members. Removing the acid histidine results in only a 10-fold activity loss (compared to >10,000-fold when aspartate is removed from chymotrypsin). This triad has been interpreted as a possible way of generating a less active enzyme to control cleavage rate.[26]
Ser-Glu-Asp
An unusual triad is found in
Cys-His-Ser
The endothelial protease vasohibin[g] uses a cysteine as the nucleophile, but a serine to coordinate the histidine base.[38][39] Despite the serine being a poor acid, it is still effective in orienting the histidine in the catalytic triad.[38] Some homologues alternatively have a threonine instead of serine at the acid location.[38]
Thr-Nter, Ser-Nter and Cys-Nter
Threonine proteases, such as the
Ser-cisSer-Lys
This unusual triad occurs only in one superfamily of amidases. In this case, the lysine acts to polarise the middle serine.[40] The middle serine then forms two strong hydrogen bonds to the nucleophilic serine to activate it (one with the side chain hydroxyl and the other with the backbone amide). The middle serine is held in an unusual cis orientation to facilitate precise contacts with the other two triad residues. The triad is further unusual in that the lysine and cis-serine both act as the base in activating the catalytic serine, but the same lysine also performs the role of the acid member as well as making key structural contacts.[40][41]
Sec-His-Glu
The rare, but naturally occurring amino acid selenocysteine (Sec), can also be found as the nucleophile in some catalytic triads.[29] Selenocysteine is similar to cysteine, but contains a selenium atom instead of a sulfur. An example is in the active site of thioredoxin reductase, which uses the selenium for reduction of disulfide in thioredoxin.[29]
Engineered triads
In addition to naturally occurring types of catalytic triads, protein engineering has been used to create enzyme variants with non-native amino acids, or entirely synthetic amino acids.[42] Catalytic triads have also been inserted into otherwise non-catalytic proteins, or protein mimics.[citation needed]
Subtilisin (a serine protease) has had its oxygen nucleophile replaced with each of sulfur,[43][44] selenium,[45] or tellurium.[46] Cysteine and selenocysteine were inserted by mutagenesis, whereas the non-natural amino acid, tellurocysteine, was inserted using auxotrophic cells fed with synthetic tellurocysteine. These elements are all in the 16th periodic table column (chalcogens), so have similar properties.[47][48] In each case, changing the nucleophile reduced the enzyme's protease activity, but increased a different activity. A sulfur nucleophile improved the enzymes transferase activity (sometimes called subtiligase). Selenium and tellurium nucleophiles converted the enzyme into an oxidoreductase.[45][46] When the nucleophile of TEV protease was converted from cysteine to serine, it protease activity was strongly reduced, but was able to be restored by directed evolution.[49]
Non-catalytic proteins have been used as scaffolds, having catalytic triads inserted into them which were then improved by directed evolution. The Ser-His-Asp triad has been inserted into an antibody,[50] as well as a range of other proteins.[51] Similarly, catalytic triad mimics have been created in small organic molecules like diaryl diselenide,[52][53] and displayed on larger polymers like Merrifield resins,[54] and self-assembling short peptide nanostructures.[55]
Divergent evolution
The sophistication of the active site network causes residues involved in catalysis (and residues in contact with these) to be highly
Reaction changes
Catalytic triads perform
Additionally, an alternative transferase mechanism has been evolved by
Nucleophile changes
Divergent evolution of active site residues is slow, due to strong chemical constraints. Nevertheless, some protease superfamilies have evolved from one nucleophile to another. This can be inferred when a superfamily (with the same fold) contains families that use different nucleophiles.[49] Such nucleophile switches have occurred several times during evolutionary history, however the mechanisms by which this happen are still unclear.[17][49]
Within protease superfamilies that contain a mixture of nucleophiles (e.g. the
Superfamily | Families | Examples |
---|---|---|
PA clan
|
C3, C4, C24, C30, C37, C62, C74, C99 | TEV protease (Tobacco etch virus) |
S1, S3, S6, S7, S29, S30, S31, S32, S39, S46, S55, S64, S65, S75 | Bos taurus )
| |
PB clan | C44, C45, C59, C69, C89, C95 | Homo sapiens )
|
S45, S63 | Penicillin G acylase precursor (Escherichia coli )
| |
T1, T2, T3, T6 | Archaean proteasome, beta component (Thermoplasma acidophilum) | |
PC clan | C26, C56 | Rattus norvegicus )
|
S51 | Dipeptidase E (Escherichia coli) | |
PD clan | C46 | Hedgehog protein (Drosophila melanogaster )
|
N9, N10, N11 | Intein-containing V-type proton ATPase catalytic subunit A (Saccharomyces cerevisiae )
| |
PE clan | P1 | DmpA aminopeptidase (Brucella anthropi) |
T5 | Ornithine acetyltransferase precursor (Saccharomyces cerevisiae) |
Pseudoenzymes
A further subclass of catalytic triad variants are
Superfamily | Families containing pseudoenzymes | Examples |
---|---|---|
CA clan | C1, C2, C19 | Calpamodulin
|
CD clan | C14 | CFLAR |
SC clan | S9, S33 | Neuroligin |
SK clan | S14 | ClpR |
SR clan | S60 | Serotransferrin domain 2 |
ST clan | S54 | RHBDF1 |
PA clan
|
S1 | Azurocidin 1 |
PB clan | T1 | PSMB3 |
Convergent evolution
The
Cysteine and serine hydrolases
The same triad geometries been converged upon by serine proteases such as the
Families of cysteine proteases
Superfamily | Families | Examples |
---|---|---|
CA | C1, C2, C6, C10, C12, C16, C19, C28, C31, C32, C33, C39, C47, C51, C54, C58, C64, C65, C66, C67, C70, C71, C76, C78, C83, C85, C86, C87, C93, C96, C98, C101 | Homo sapiens )
|
CD | C11, C13, C14, C25, C50, C80, C84 | Rattus norvegicus) and separase (Saccharomyces cerevisiae )
|
CE | C5, C48, C55, C57, C63, C79 | adenovirus type 2)
|
CF | C15 | Pyroglutamyl-peptidase I (Bacillus amyloliquefaciens) |
CL | C60, C82 | Sortase A (Staphylococcus aureus) |
CM | C18 | Hepatitis C virus peptidase 2 (hepatitis C virus) |
CN | C9 | Sindbis virus-type nsP2 peptidase (sindbis virus )
|
CO | C40 | Dipeptidyl-peptidase VI (Lysinibacillus sphaericus) |
CP | C97 | Mus musculus )
|
PA |
C3, C4, C24, C30, C37, C62, C74, C99 | TEV protease (Tobacco etch virus) |
PB | C44, C45, C59, C69, C89, C95 | Homo sapiens )
|
PC | C26, C56 | Rattus norvegicus )
|
PD | C46 | Hedgehog protein (Drosophila melanogaster )
|
PE | P1 | DmpA aminopeptidase (Brucella anthropi) |
unassigned | C7, C8, C21, C23, C27, C36, C42, C53, C75 |
Families of serine proteases
Superfamily | Families | Examples |
---|---|---|
SB | S8, S53 | Subtilisin (Bacillus licheniformis) |
SC | S9, S10, S15, S28, S33, S37 | Sus scrofa )
|
SE | S11, S12, S13 | D-Ala-D-Ala peptidase C (Escherichia coli) |
SF | S24, S26 | Signal peptidase I (Escherichia coli) |
SH | S21, S73, S77, S78, S80 | Cytomegalovirus herpesvirus 5)
|
SJ | S16, S50, S69 | Lon-A peptidase (Escherichia coli )
|
SK | S14, S41, S49 | Clp protease (Escherichia coli) |
SO | S74 | Phage GA-1 neck appendage CIMCD self-cleaving protein ( Bacillus phage GA-1 )
|
SP | S59 | Homo sapiens )
|
SR | S60 | Homo sapiens )
|
SS | S66 | Murein tetrapeptidase LD-carboxypeptidase (Pseudomonas aeruginosa) |
ST | S54 | Rhomboid-1 (Drosophila melanogaster) |
PA |
S1, S3, S6, S7, S29, S30, S31, S32, S39, S46, S55, S64, S65, S75 | Bos taurus )
|
PB | S45, S63 | Penicillin G acylase precursor (Escherichia coli )
|
PC | S51 | Dipeptidase E (Escherichia coli) |
PE | P1 | DmpA aminopeptidase (Brucella anthropi) |
unassigned | S48, S62, S68, S71, S72, S79, S81 |
Threonine proteases
Threonine proteases use the amino acid threonine as their catalytic nucleophile. Unlike cysteine and serine, threonine is a secondary hydroxyl (i.e. has a methyl group). This methyl group greatly restricts the possible orientations of triad and substrate as the methyl clashes with either the enzyme backbone or histidine base.[2] When the nucleophile of a serine protease was mutated to threonine, the methyl occupied a mixture of positions, most of which prevented substrate binding.[71] Consequently, the catalytic residue of a threonine protease is located at its N-terminus.[2]
Two evolutionarily independent
Families of threonine proteases
Superfamily | Families | Examples |
---|---|---|
PB clan | T1, T2, T3, T6 | Archaean proteasome, beta component (Thermoplasma acidophilum) |
PE clan | T5 | Ornithine acetyltransferase (Saccharomyces cerevisiae) |
See also
- Active site
- Convergent evolution
- Divergent evolution
- Enzyme catalysis
- Enzyme superfamily
- Functional groups
- PA clan
- Protease
- Proteolysis
References
Notes
- ^ a b c d TEV protease MEROPS: clan PA, family C4
- ^ a b Cytomegalovirus protease MEROPS: clan SH, family S21
- ^ a b c d Papain MEROPS: clan CA, family C1
- ^ Hepatitis A virus protease MEROPS: clan PA, family C3
- ^ a b c Chymotrypsin MEROPS: clan PA, family S1
- ^ Sedolisin protease MEROPS: clan SB, family 53
- ^ Vasohibin protease MEROPS: clan CA
- ^ a b Proteasome MEROPS: clan PB, family T1
- ^ a b Ornithine acyltransferases MEROPS: clan PE, family T5
- ^ Penicillin acylase G MEROPS: clan PB, family S45
- ^ Penicillin acylase V MEROPS: clan PB, family C59
- ^ amidophosphoribosyltransferase MEROPS: clan PB, family C44
- ^ Subtilisin MEROPS: clan SB, family S8
- ^ Prolyl oligopeptidase MEROPS: clan SC, family S9
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