Serpin
Template:Short description Template:Cs1 config Template:Featured article Template:Use dmy dates Template:Infobox protein family
Serpins are a superfamily of proteins with similar structures that were first identified for their protease inhibition activity and are found in all kingdoms of life.<ref name="Silverman_2001" /><ref>Template:Cite journal</ref> The acronym serpin was originally coined because the first serpins to be identified act on chymotrypsin-like serine proteases (serine protease inhibitors).<ref>Template:Cite book</ref><ref name="Silverman_2010">Template:Cite journal</ref><ref name="Whisstock_2010">Template:Cite journal</ref> They are notable for their unusual mechanism of action, in which they irreversibly inhibit their target protease by undergoing a large conformational change to disrupt the target's active site.<ref name="Huntington_2000">Template:Cite journal</ref><ref name="Gettins_2002">Template:Cite journal</ref> This contrasts with the more common competitive mechanism for protease inhibitors that bind to and block access to the protease active site.<ref name="Whisstock_2006">Template:Cite journal</ref><ref name="Law_2006"/>
Protease inhibition by serpins controls an array of biological processes, including coagulation and inflammation, and consequently these proteins are the target of medical research.<ref name="Stein_1995">Template:Cite journal</ref> Their unique conformational change also makes them of interest to the structural biology and protein folding research communities.<ref name="Gettins_2002"/><ref name="Whisstock_2006" /> The conformational-change mechanism confers certain advantages, but it also has drawbacks: serpins are vulnerable to mutations that can result in serpinopathies such as protein misfolding and the formation of inactive long-chain polymers.<ref name="Janciauskiene_2011">Template:Cite journal</ref><ref name="Carrell_1997" /> Serpin polymerisation not only reduces the amount of active inhibitor, but also leads to accumulation of the polymers, causing cell death and organ failure.<ref name="Stein_1995"/>
Although most serpins control proteolytic cascades, some proteins with a serpin structure are not enzyme inhibitors, but instead perform diverse functions such as storage (as in egg white—ovalbumin), transport as in hormone carriage proteins (thyroxine-binding globulin, cortisol-binding globulin) and molecular chaperoning (HSP47).<ref name="Law_2006"/> The term serpin is used to describe these members as well, despite their non-inhibitory function, since they are evolutionarily related.<ref name="Silverman_2001" />
HistoryEdit
Protease inhibitory activity in blood plasma was first reported in the late 1800s,<ref>Template:Cite journal</ref> but it was not until the 1950s that the serpins antithrombin and alpha 1-antitrypsin were isolated,<ref>Template:Cite journal</ref> with the subsequent recognition of their close family homology in 1979.<ref>Template:Cite book</ref><ref>Template:Cite journal</ref> That they belonged to a new protein family became apparent on their further alignment with the non-inhibitory egg-white protein ovalbumin, to give what was initially called the alpha1-antitrypsin-antithrombin III-ovalbumin superfamily of serine proteinase inhibitors,<ref>Template:Cite journal</ref> but was subsequently succinctly renamed as the Serpins.<ref>Template:Cite journal</ref> The initial characterisation of the new family centred on alpha1-antitrypsin, a serpin present in high concentration in blood plasma, the common genetic disorder of which was shown to cause a predisposition to the lung disease emphysema<ref name="Laurell_2013">Template:Cite journal</ref> and to liver cirrhosis.<ref>Template:Cite journal</ref> The identification of the S and Z mutations<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> responsible for the genetic deficiency and the subsequent sequence alignments of alpha1-antitrypsin and antithrombin in 1982 led to the recognition of the close homologies of the active sites of the two proteins,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> centred on a methionine<ref>Template:Cite journal</ref> in alpha1-antitrypsin as an inhibitor of tissue elastase and on arginine in antithrombin<ref>Template:Cite journal</ref> as an inhibitor of thrombin.<ref>Template:Cite journal</ref>
The critical role of the active centre residue in determining the specificity of inhibition of serpins was unequivocally confirmed by the finding that a natural mutation of the active centre methionine in alpha1-antitrypsin to an arginine, as in antithrombin, resulted in a severe bleeding disorder.<ref name="Mutation of antitrypsin to antithro">Template:Cite journal</ref> This active-centre specificity of inhibition was also evident in the many other families of protease inhibitors<ref name="Gettins_2002" /> but the serpins differed from them in being much larger proteins and also in possessing what was soon apparent as an inherent ability to undergo a change in shape. The nature of this conformational change was revealed with the determination in 1984 of the first crystal structure of a serpin, that of post-cleavage alpha1-antitrypsin.<ref name="Huber_1984">Template:Cite journal</ref> This together with the subsequent solving of the structure of native (uncleaved) ovalbumin<ref name="Stein_1990">Template:Cite journal</ref> indicated that the inhibitory mechanism of the serpins involved a remarkable conformational shift, with the movement of the exposed peptide loop containing the reactive site and its incorporation as a middle strand in the main beta-pleated sheet that characterises the serpin molecule.<ref name="Carrell_1991">Template:Cite journal</ref><ref>Template:Cite journal</ref> Early evidence of the essential role of this loop movement in the inhibitory mechanism came from the finding that even minor aberrations in the amino acid residues that form the hinge of the movement in antithrombin resulted in thrombotic disease.<ref name="Carrell_1991" /><ref>Template:Cite journal</ref> Ultimate confirmation of the linked displacement of the target protease by this loop movement was provided in 2000 by the structure of the post-inhibitory complex of alpha1-antitrypsin with trypsin,<ref name="Huntington_2000" /> showing how the displacement results in the deformation and inactivation of the attached protease. Subsequent structural studies have revealed an additional advantage of the conformational mechanism<ref name="Huntington_2006" /> in allowing the subtle modulation of inhibitory activity, as notably seen at tissue level<ref>Template:Cite journal</ref> with the functionally diverse serpins in human plasma.
Over 1000 serpins have now been identified, including 36 human proteins, as well as molecules in all kingdoms of life—animals, plants, fungi, bacteria, and archaea—and some viruses.<ref name="Irving_2000" /><ref name="Irving_2002" /><ref name="Steenbakkers_2008">Template:Cite journal</ref> The central feature of all is a tightly conserved framework, which allows the precise alignment of their key structural and functional components based on the template structure of alpha1-antitrypsin.<ref>Template:Cite journal</ref> In the 2000s, a systematic nomenclature was introduced in order to categorise members of the serpin superfamily based on their evolutionary relationships.<ref name="Silverman_2001">Template:Cite journal</ref> Serpins are therefore the largest and most diverse superfamily of protease inhibitors.<ref name="Rawlings_2004">Template:Cite journal</ref>
ActivityEdit
Most serpins are protease inhibitors, targeting extracellular, chymotrypsin-like serine proteases. These proteases possess a nucleophilic serine residue in a catalytic triad in their active site. Examples include thrombin, trypsin, and human neutrophil elastase.<ref>Template:Cite journal</ref> Serpins act as irreversible, suicide inhibitors by trapping an intermediate of the protease's catalytic mechanism.<ref name="Huntington_2000" />
Some serpins inhibit other protease classes, typically cysteine proteases, and are termed "cross-class inhibitors". These enzymes differ from serine proteases in that they use a nucleophilic cysteine residue, rather than a serine, in their active site.<ref>Template:Cite journal</ref> Nonetheless, the enzymatic chemistry is similar, and the mechanism of inhibition by serpins is the same for both classes of protease.<ref>Template:Cite journal</ref> Examples of cross-class inhibitory serpins include serpin B4 a squamous cell carcinoma antigen 1 (SCCA-1) and the avian serpin myeloid and erythroid nuclear termination stage-specific protein (MENT), which both inhibit papain-like cysteine proteases.<ref name="Schick_1998">Template:Cite journal</ref><ref name="McGowan_2006">Template:Cite journal</ref><ref>Template:Cite journal</ref>
Biological function and localizationEdit
Protease inhibitionEdit
Approximately two-thirds of human serpins perform extracellular roles, inhibiting proteases in the bloodstream in order to modulate their activities. For example, extracellular serpins regulate the proteolytic cascades central to blood clotting (antithrombin), the inflammatory and immune responses (antitrypsin, antichymotrypsin, and C1-inhibitor) and tissue remodelling (PAI-1).<ref name="Law_2006"/> By inhibiting signalling cascade proteases, they can also affect development.<ref name="Acosta 1146–1158">Template:Cite journal</ref><ref name="Hashimoto 945–950">Template:Cite journal</ref> The table of human serpins (below) provides examples of the range of functions performed by human serpin, as well as some of the diseases that result from serpin deficiency.
The protease targets of intracellular inhibitory serpins have been difficult to identify, since many of these molecules appear to perform overlapping roles. Further, many human serpins lack precise functional equivalents in model organisms such as the mouse. Nevertheless, an important function of intracellular serpins may be to protect against the inappropriate activity of proteases inside the cell.<ref>Template:Cite journal</ref> For example, one of the best-characterised human intracellular serpins is Serpin B9, which inhibits the cytotoxic granule protease granzyme B. In doing so, Serpin B9 may protect against inadvertent release of granzyme B and premature or unwanted activation of cell death pathways.<ref>Template:Cite journal</ref>
Some viruses use serpins to disrupt protease functions in their host. The cowpox viral serpin CrmA (cytokine response modifier A) is used in order to avoid inflammatory and apoptotic responses of infected host cells. CrmA increases infectivity by suppressing its host's inflammatory response through inhibition of IL-1 and IL-18 processing by the cysteine protease caspase-1.<ref>Template:Cite journal</ref> In eukaryotes, a plant serpin inhibits both metacaspases<ref name="Vercammen_2006">Template:Cite journal</ref> and a papain-like cysteine protease.<ref name="Lampl_2010">Template:Cite journal</ref>
Non-inhibitory rolesEdit
Non-inhibitory extracellular serpins also perform a wide array of important roles. Thyroxine-binding globulin and transcortin transport the hormones thyroxine and cortisol, respectively.<ref name="cbg">Template:Cite journal</ref><ref name="Zhou_2006"/> The non-inhibitory serpin ovalbumin is the most abundant protein in egg white. Its exact function is unknown, but it is thought to be a storage protein for the developing foetus.<ref>Template:Cite journal</ref> Heat shock serpin 47 is a chaperone, essential for proper folding of collagen. It acts by stabilising collagen's triple helix whilst it is being processed in the endoplasmic reticulum.<ref name="Mala_2010">Template:Cite journal</ref>
Some serpins are both protease inhibitors and perform additional roles. For example, the nuclear cysteine protease inhibitor MENT, in birds also acts as a chromatin remodelling molecule in a bird's red blood cells.<ref name="McGowan_2006"/><ref>Template:Cite journal</ref>
StructureEdit
All serpins share a common structure (or fold), despite their varied functions. All typically have three β-sheets (named A, B and C) and eight or nine α-helices (named hA–hI).<ref name="Huber_1984" /><ref name="Stein_1990"/> The most significant regions to serpin function are the A-sheet and the reactive centre loop (RCL). The A-sheet includes two β-strands that are in a parallel orientation with a region between them called the 'shutter', and upper region called the 'breach'. The RCL forms the initial interaction with the target protease in inhibitory molecules. Structures have been solved showing the RCL either fully exposed or partially inserted into the A-sheet, and serpins are thought to be in dynamic equilibrium between these two states.<ref name="Whisstock_2006" /> The RCL also only makes temporary interactions with the rest of the structure, and is therefore highly flexible and exposed to the solvent.<ref name="Whisstock_2006" />
The serpin structures that have been determined cover several different conformations, which has been necessary for the understanding of their multiple-step mechanism of action. Structural biology has therefore played a central role in the understanding of serpin function and biology.<ref name="Whisstock_2006"/>
Conformational change and inhibitory mechanismEdit
Inhibitory serpins do not inhibit their target proteases by the typical competitive (lock-and-key) mechanism used by most small protease inhibitors (e.g. Kunitz-type inhibitors). Instead, serpins use an unusual conformational change, which disrupts the structure of the protease and prevents it from completing catalysis. The conformational change involves the RCL moving to the opposite end of the protein and inserting into β-sheet A, forming an extra antiparallel β-strand. This converts the serpin from a stressed state, to a lower-energy relaxed state (S to R transition).<ref name="Gettins_2002"/><ref name="Whisstock_2006"/><ref>Template:Cite journal</ref>
Serine and cysteine proteases catalyse peptide bond cleavage by a two-step process. Initially, the catalytic residue of the active site triad performs a nucleophilic attack on the peptide bond of the substrate. This releases the new N-terminus and forms a covalent ester-bond between the enzyme and the substrate.<ref name="Gettins_2002"/> This covalent complex between enzyme and substrate is called an acyl-enzyme intermediate. For standard substrates, the ester bond is hydrolysed and the new C-terminus is released to complete catalysis. However, when a serpin is cleaved by a protease, it rapidly undergoes the S to R transition before the acyl-enzyme intermediate is hydrolysed.<ref name="Gettins_2002"/> The efficiency of inhibition depends on fact that the relative kinetic rate of the conformational change is several orders of magnitude faster than hydrolysis by the protease.
Since the RCL is still covalently attached to the protease via the ester bond, the S to R transition pulls protease from the top to the bottom of the serpin and distorts the catalytic triad. The distorted protease can only hydrolyse the acyl enzyme intermediate extremely slowly and so the protease remains covalently attached for days to weeks.<ref name="Huntington_2000" /> Serpins are classed as irreversible inhibitors and as suicide inhibitors since each serpin protein permanently inactivates a single protease, and can only function once.<ref name="Gettins_2002" />
Allosteric activationEdit
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The conformational mobility of serpins provides a key advantage over static lock-and-key protease inhibitors.<ref name="Huntington_2006"/> In particular, the function of inhibitory serpins can be regulated by allosteric interactions with specific cofactors. The X-ray crystal structures of antithrombin, heparin cofactor II, MENT and murine antichymotrypsin reveal that these serpins adopt a conformation wherein the first two amino acids of the RCL are inserted into the top of the A β-sheet. The partially inserted conformation is important because co-factors are able to conformationally switch certain partially inserted serpins into a fully expelled form.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> This conformational rearrangement makes the serpin a more effective inhibitor.
The archetypal example of this situation is antithrombin, which circulates in plasma in a partially inserted relatively inactive state. The primary specificity determining residue (the P1 arginine) points toward the body of the serpin and is unavailable to the protease. Upon binding a high-affinity pentasaccharide sequence within long-chain heparin, antithrombin undergoes a conformational change, RCL expulsion, and exposure of the P1 arginine. The heparin pentasaccharide-bound form of antithrombin is, thus, a more effective inhibitor of thrombin and factor Xa.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Furthermore, both of these coagulation proteases also contain binding sites (called exosites) for heparin. Heparin, therefore, also acts as a template for binding of both protease and serpin, further dramatically accelerating the interaction between the two parties. After the initial interaction, the final serpin complex is formed and the heparin moiety is released. This interaction is physiologically important. For example, after injury to the blood vessel wall, heparin is exposed, and antithrombin is activated to control the clotting response. Understanding of the molecular basis of this interaction enabled the development of Fondaparinux, a synthetic form of Heparin pentasaccharide used as an anti-clotting drug.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Latent conformationEdit
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Certain serpins spontaneously undergo the S to R transition without having been cleaved by a protease, to form a conformation termed the latent state. Latent serpins are unable to interact with proteases and so are no longer protease inhibitors. The conformational change to latency is not exactly the same as the S to R transition of a cleaved serpin. Since the RCL is still intact, the first strand of the C-sheet has to peel off to allow full RCL insertion.<ref name="Lindahl_1989">Template:Cite journal</ref>
Regulation of the latency transition can act as a control mechanism in some serpins, such as PAI-1. Although PAI-1 is produced in the inhibitory S conformation, it "auto-inactivates" by changing to the latent state unless it is bound to the cofactor vitronectin.<ref name="Lindahl_1989"/> Similarly, antithrombin can also spontaneously convert to the latent state, as an additional modulation mechanism to its allosteric activation by heparin.<ref>Template:Cite journal</ref> Finally, the N-terminus of Template:Proper name, a serpin from Thermoanaerobacter tengcongensis, is required to lock the molecule in the native inhibitory state. Disruption of interactions made by the N-terminal region results in spontaneous conformational change of this serpin to the latent conformation.<ref>Template:Cite journal</ref><ref name="Zhang_2008">Template:Cite journal</ref>
Conformational change in non-inhibitory functionsEdit
Certain non-inhibitory serpins also use the serpin conformational change as part of their function. For example, the native (S) form of thyroxine-binding globulin has high affinity for thyroxine, whereas the cleaved (R) form has low affinity. Similarly, transcortin has higher affinity for cortisol when in its native (S) state, than its cleaved (R) state. Thus, in these serpins, RCL cleavage and the S to R transition has been commandeered to allow for ligand release, rather than protease inhibition.<ref name="cbg"/><ref name="Zhou_2006">Template:Cite journal</ref><ref>Template:Cite journal</ref>
In some serpins, the S to R transition can activate cell signalling events. In these cases, a serpin that has formed a complex with its target protease, is then recognised by a receptor. The binding event then leads to downstream signalling by the receptor.<ref name="Cao_2006">Template:Cite journal</ref> The S to R transition is therefore used to alert cells to the presence of protease activity.<ref name="Cao_2006"/> This differs from the usual mechanism whereby serpins affect signalling simply by inhibiting proteases involved in a signalling cascade.<ref name="Acosta 1146–1158"/><ref name="Hashimoto 945–950"/>
DegradationEdit
When a serpin inhibits a target protease, it forms a permanent complex, which needs to be disposed of. For extracellular serpins, the final serpin-enzyme complexes are rapidly cleared from circulation. One mechanism by which this occurs in mammals is via the low-density lipoprotein receptor-related protein (LRP), which binds to inhibitory complexes made by antithrombin, PA1-1, and neuroserpin, causing cellular uptake.<ref name="Cao_2006" /><ref name="Jensen_2009">Template:Cite journal</ref> Similarly, the Drosophila necrotic serpin is degraded in the lysosome after being trafficked into the cell by the Lipophorin Receptor-1 (homologous to the mammalian LDL receptor family).<ref name="Soukup_2009">Template:Cite journal</ref>
Disease and serpinopathiesEdit
Serpins are involved in a wide array of physiological functions, and so mutations in genes encoding them can cause a range of diseases. Mutations that change the activity, specificity or aggregation properties of serpins all affect how they function. The majority of serpin-related diseases are the result of serpin polymerisation into aggregates, though several other types of disease-linked mutations also occur.<ref name="Whisstock_2006" /><ref>Template:Cite journal</ref> The disorder alpha-1 antitrypsin deficiency is one of the most common hereditary diseases.<ref name="Janciauskiene_2011"/><ref name="de_Serres_2002">Template:Cite journal</ref>
Inactivity or absenceEdit
Since the stressed serpin fold is high-energy, mutations can cause them to incorrectly change into their lower-energy conformations (e.g. relaxed or latent) before they have correctly performed their inhibitory role.<ref name="Stein_1995"/>
Mutations that affect the rate or the extent of RCL insertion into the A-sheet can cause the serpin to undergo its S to R conformational change before having engaged a protease. Since a serpin can only make this conformational change once, the resulting misfired serpin is inactive and unable to properly control its target protease.<ref name="Stein_1995"/><ref>Template:Cite journal</ref> Similarly, mutations that promote inappropriate transition to the monomeric latent state cause disease by reducing the amount of active inhibitory serpin. For example, the disease-linked antithrombin variants wibble and wobble,<ref>Template:Cite journal</ref> both promote formation of the latent state.
The structure of the disease-linked mutant of antichymotrypsin (L55P) revealed another, inactive "δ-conformation". In the δ-conformation, four residues of the RCL are inserted into the top of β-sheet A. The bottom half of the sheet is filled as a result of one of the α-helices (the F-helix) partially switching to a β-strand conformation, completing the β-sheet hydrogen bonding.<ref name="Gooptu_2000">Template:Cite journal</ref> It is unclear whether other serpins can adopt this conformer, and whether this conformation has a functional role, but it is speculated that the δ-conformation may be adopted by Thyroxine-binding globulin during thyroxine release.<ref name="Zhou_2006"/> The non-inhibitory proteins related to serpins can also cause diseases when mutated. For example, mutations in SERPINF1 cause osteogenesis imperfecta type VI in humans.<ref name="Homan_2011"/>
In the absence of a required serpin, the protease that it normally would regulate is over-active, leading to pathologies.<ref name="Stein_1995"/> Consequently, simple deficiency of a serpin (e.g. a null mutation) can result in disease.<ref>Template:Cite journal</ref> Gene knockouts, particularly in mice, are used experimentally to determine the normal functions of serpins by the effect of their absence.<ref name="Heit_2013"/>
Specificity changeEdit
In some rare cases, a single amino acid change in a serpin's RCL alters its specificity to target the wrong protease. For example, the Antitrypsin-Pittsburgh mutation (M358R) causes the α1-antitrypsin serpin to inhibit thrombin, causing a bleeding disorder.<ref name="Mutation of antitrypsin to antithro"/>
Polymerisation and aggregationEdit
The majority of serpin diseases are due to protein aggregation and are termed "serpinopathies".<ref name="Carrell_1997" /><ref name="Gooptu_2000"/> Serpins are vulnerable to disease-causing mutations that promote formation of misfolded polymers due to their inherently unstable structures.<ref name="Gooptu_2000"/> Well-characterised serpinopathies include α1-antitrypsin deficiency (alpha-1), which may cause familial emphysema, and sometimes liver cirrhosis, certain familial forms of thrombosis related to antithrombin deficiency, types 1 and 2 hereditary angioedema (HAE) related to deficiency of C1-inhibitor, and familial encephalopathy with neuroserpin inclusion bodies (FENIB; a rare type of dementia caused by neuroserpin polymerisation).<ref name="Janciauskiene_2011"/><ref name="Carrell_1997"/><ref name="lomas_1992">Template:Cite journal</ref>
Each monomer of the serpin aggregate exists in the inactive, relaxed conformation (with the RCL inserted into the A-sheet). The polymers are therefore hyperstable to temperature and unable to inhibit proteases. Serpinopathies therefore cause pathologies similarly to other proteopathies (e.g. prion diseases) via two main mechanisms.<ref name="Janciauskiene_2011"/><ref name="Carrell_1997">Template:Cite journal</ref> First, the lack of active serpin results in uncontrolled protease activity and tissue destruction. Second, the hyperstable polymers themselves clog up the endoplasmic reticulum of cells that synthesize serpins, eventually resulting in cell death and tissue damage. In the case of antitrypsin deficiency, antitrypsin polymers cause the death of liver cells, sometimes resulting in liver damage and cirrhosis. Within the cell, serpin polymers are slowly removed via degradation in the endoplasmic reticulum.<ref name="Kroeger_2009">Template:Cite journal</ref> However, the details of how serpin polymers cause cell death remains to be fully understood.<ref name="Janciauskiene_2011"/>
Physiological serpin polymers are thought to form via domain swapping events, where a segment of one serpin protein inserts into another.<ref name="Yamasaki_2008">Template:Cite journal</ref> Domain-swaps occur when mutations or environmental factors interfere with the final stages of serpin folding to the native state, causing high-energy intermediates to misfold.<ref name="Bottomley_2011">Template:Cite journal</ref> Both dimer and trimer domain-swap structures have been solved. In the dimer (of antithrombin), the RCL and part of the A-sheet incorporates into the A-sheet of another serpin molecule.<ref name="Yamasaki_2008"/> The domain-swapped trimer (of antitrypsin) forms via the exchange of an entirely different region of the structure, the B-sheet (with each molecule's RCL inserted into its own A-sheet).<ref name="Yamasaki_2011">Template:Cite journal</ref> It has also been proposed that serpins may form domain-swaps by inserting the RCL of one protein into the A-sheet of another (A-sheet polymerisation).<ref name="lomas_1992"/><ref>Template:Cite journal</ref> These domain-swapped dimer and trimer structures are thought to be the building blocks of the disease-causing polymer aggregates, but the exact mechanism is still unclear.<ref name="Yamasaki_2008"/><ref name="Bottomley_2011"/><ref name="Yamasaki_2011"/><ref name="Miranda_2010">Template:Cite journal</ref>
Therapeutic strategiesEdit
Several therapeutic approaches are in use or under investigation to treat the most common serpinopathy: antitrypsin deficiency.<ref name="Janciauskiene_2011"/> Antitrypsin augmentation therapy is approved for severe antitrypsin deficiency-related emphysema.<ref name="Sandhaus_2004">Template:Cite journal</ref> In this therapy, antitrypsin is purified from the plasma of blood donors and administered intravenously (first marketed as Prolastin).<ref name="Janciauskiene_2011"/><ref>Template:Cite journal</ref> To treat severe antitrypsin deficiency-related disease, lung and liver transplantation has proven effective.<ref name="Janciauskiene_2011"/><ref name="Fregonese_2008">Template:Cite journal</ref> In animal models, gene targeting in induced pluripotent stem cells has been successfully used to correct an antitrypsin polymerisation defect and to restore the ability of the mammalian liver to secrete active antitrypsin.<ref name="Yusa_2011">Template:Cite journal</ref> Small molecules have also been developed that block antitrypsin polymerisation in vitro.<ref name="Mallya_2007">Template:Cite journal</ref><ref name="Gosai_2010">Template:Cite journal</ref>
EvolutionEdit
Serpins are the most widely distributed and largest superfamily of protease inhibitors.<ref name="Silverman_2001" /><ref name="Rawlings_2004"/> They were initially believed to be restricted to eukaryote organisms, but have since been found in bacteria, archaea and some viruses.<ref name="Irving_2000"/><ref name="Irving_2002">Template:Cite journal</ref><ref>Template:Cite journal</ref> It remains unclear whether prokaryote genes are the descendants of an ancestral prokaryotic serpin or the product of horizontal gene transfer from eukaryotes. Most intracellular serpins belong to a single phylogenetic clade, whether they come from plants or animals, indicating that the intracellular and extracellular serpins may have diverged before the plants and animals.<ref>Template:Cite journal</ref> Exceptions include the intracellular heat shock serpin HSP47, which is a chaperone essential for proper folding of collagen, and cycles between the cis-Golgi and the endoplasmic reticulum.<ref name="Mala_2010" />
Protease-inhibition is thought to be the ancestral function, with non-inhibitory members the results of evolutionary neofunctionalisation of the structure. The S to R conformational change has also been adapted by some binding serpins to regulate affinity for their targets.<ref name="Zhou_2006"/>
DistributionEdit
AnimalEdit
HumanEdit
The human genome encodes 16 serpin clades, termed Template:Proper name through Template:Proper name, including 29 inhibitory and 7 non-inhibitory serpin proteins.<ref name="Law_2006" /><ref name="Heit_2013" /> The human serpin naming system is based upon a phylogenetic analysis of approximately 500 serpins from 2001, with proteins named Template:Proper name, where X is the clade of the protein and Y the number of the protein within that clade.<ref name="Silverman_2001" /><ref name="Irving_2000">Template:Cite journal</ref><ref name="Heit_2013" /> The functions of human serpins have been determined by a combination of biochemical studies, human genetic disorders, and knockout mouse models.<ref name="Heit_2013" />
| Gene name | Common Name | Localisation | Function / Activity<ref name="Law_2006">Template:Cite journal</ref><ref name="Heit_2013">Template:Cite journal</ref> | Effect of deficiency<ref name="Law_2006"/><ref name="Heit_2013"/> | Human disease | Chromosomal location | Protein structure |
|---|---|---|---|---|---|---|---|
| SERPINA1 | α1-antitrypsin | Extracellular | Inhibitor of human neutrophil elastase.<ref name="Stoller_2005">Template:Cite journal</ref> The C-terminal fragment of cleaved SERPINA1 may inhibit HIV-1 infection.<ref>Template:Cite journal</ref> | Deficiency results in emphysema, polymerisation results in cirrhosis (serpinopathy).<ref name="Janciauskiene_2011"/><ref>Template:Cite journal</ref> | 14q32.1 | Template:PDB2, Template:PDB2, Template:PDB2 | |
| SERPINA2 | Antitrypsin-related protein | Extracellular | Possible pseudogene.<ref>Template:Cite journal</ref> | 14q32.1 | |||
| SERPINA3 | α1-antichymotrypsin | Extracellular | Inhibitor of cathepsin G.<ref name="Kalsheker_1996">Template:Cite journal</ref> Additional roles in chromatin condensation in hepatic cells.<ref name="Santamaria_2013">Template:Cite journal</ref> | Mis-regulation results in Alzheimer's disease (serpinopathy).<ref>Template:Cite journal</ref> | 14q32.1 | Template:PDB2, Template:PDB2 | |
| SERPINA4 | Kallistatin | Extracellular | Inhibitor of kallikrein, regulator of vascular function.<ref name="Chao_1997">Template:Cite journal</ref><ref>Template:Cite journal</ref> | Depletion in hypertensive rats exacerbates renal and cardiovascular injury.<ref name="Liu_2012">Template:Cite journal</ref> | 14q32.1 | ||
| SERPINA5 | Protein C inhibitor | Extracellular | Inhibitor of active protein C.<ref name="Geiger_2007">Template:Cite journal</ref> Intracellular role in preventing phagocytosis of bacteria.<ref name="Baumgärtner_2007">Template:Cite journal</ref> | Knockout in male mice causes infertility.<ref>Template:Cite journal</ref> Accumulation occurs in chronic active plaques in multiple sclerosis.<ref name="Han_2008">Template:Cite journal</ref> | 14q32.1 | Template:PDB2, Template:PDB2 | |
| SERPINA6 | Transcortin | Extracellular | Non-inhibitory. Cortisol binding.<ref name="cbg"/> | Deficiency associated with chronic fatigue.<ref>Template:Cite journal</ref> | 14q32.1 | Template:PDB2, Template:PDB2, Template:PDB2 | |
| SERPINA7 | Thyroxine-binding globulin | Extracellular | Non-inhibitory. Thyroxine binding.<ref name="Zhou_2006"/> | Deficiency causes hypothyroidism.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | Xq22.2 | Template:PDB2, Template:PDB2, Template:PDB2 | |
| SERPINA8 | Angiotensinogen | Extracellular | Non-inhibitory, cleavage by renin results in release of angiotensin I.<ref name="Kumar_2007">Template:Cite journal</ref> | Knockout in mice causes hypotension.<ref name="Tanimoto_1994">Template:Cite journal</ref> | Variants linked to hypertension.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | 1q42-q43 | Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2 |
| SERPINA9 | Centerin / GCET1 | Extracellular | Inhibitory, maintenance of naive B cells.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | Strongly expressed in most B-cell lymphomas.<ref name="Paterson_2008">Template:Cite journal</ref><ref>Template:Cite journal</ref> | 14q32.1 | ||
| SERPINA10 | Protein Z-related protease inhibitor | Extracellular | Binds protein Z and inactivates factor Xa and factor XIa.<ref name="Han_2000">Template:Cite journal</ref> | 14q32.1 | Template:PDB2, Template:PDB2 | ||
| SERPINA11 | – | Probably extracellular | Unknown | 14q32.13 | |||
| SERPINA12 | Vaspin | Extracellular | Inhibitor of Kallikrein-7. Insulin-sensitizing adipocytokine.<ref>Template:Cite journal</ref> | High plasma levels associated with type II diabetes.<ref>Template:Cite journal</ref> | 14q32.1 | Template:PDB2 | |
| SERPINA13 | – | Probably extracellular | Unknown | 14q32 | |||
| SERPINB1 | Monocyte neutrophil elastase inhibitor | Intracellular | Inhibitor of neutrophil elastase.<ref>Template:Cite journal</ref> | Knockout in mice causes neutrophil survival defect and immune deficiency.<ref>Template:Cite journal</ref> | 6p25 | Template:PDB2 | |
| SERPINB2 | Plasminogen activator inhibitor-2 | Intracellular/extracellular | Inhibitor of extracellular uPA. Intracellular function unclear, but may protect against viral infection.<ref name="Antalis_1998">Template:Cite journal</ref> | Deficiency in mice reduces immune response to nematode infection.<ref name="Zhao_2013">Template:Cite journal</ref> Knockout in mice causes no obvious phenotype.<ref>Template:Cite journal</ref> | 18q21.3 | Template:PDB2 | |
| SERPINB3 | Squamous cell carcinoma antigen-1 (SCCA-1) | Intracellular | Inhibitor of papain-like cysteine proteases<ref name="Schick_1998"/> and cathepsins K, L and S.<ref>Template:Cite journal</ref><ref name="CT">Template:Cite journal</ref> | Knockout in mice of Serpinb3a (the murine homolog of both human SERPINB3 and SERPINB4) have reduced mucus production in a murine model of asthma.<ref name="Sivaprasad_2011" /> | 18q21.3 | Template:PDB2 | |
| SERPINB4 | Squamous cell carcinoma antigen-2 (SCCA-2) | Intracellular | Inhibitor of chymotrypsin-like serine proteases, cathepsin G and chymase.<ref name="CT" /><ref>Template:Cite journal</ref> | Knockout in mice of Serpinb3a (the murine homolog of both human SERPINB3 and SERPINB4) have reduced mucus production in a murine model of asthma.<ref name="Sivaprasad_2011">Template:Cite journal</ref> | 18q21.3 | ||
| SERPINB5 | Maspin | Intracellular | Non-inhibitory, function unclear<ref name="Teoh_2010">Template:Cite journal</ref><ref name="maspin">Template:Cite journal</ref><ref name="Teoh_2014">Template:Cite journal</ref> (see also maspin) | Knockout in mice originally reported as lethal,<ref name="Gao_2004">Template:Cite journal</ref> but subsequently shown to have no obvious phenotype.<ref name="Teoh_2014"/> Expression may be a prognostic indicator that reflects expression of a neighbouring tumour suppressor gene (the phosphatase PHLPP1).<ref name="Teoh_2014" /> | 18q21.3 | Template:PDB2 | |
| SERPINB6 | PI-6 | Intracellular | Inhibitor of cathepsin G.<ref name="Scott_1999">Template:Cite journal</ref> | Knockout in mice causes hearing loss<ref>Template:Cite journal</ref> and mild neutropenia.<ref>Template:Cite journal</ref> | Deficiency associated with hearing loss.<ref>Template:Cite journal</ref> | 6p25 | |
| SERPINB7 | Megsin | Intracellular | Involved in megakaryocyte maturation.<ref>Template:Cite journal</ref> | Over-expression in mice causes kidney disease.<ref name="Miyata_2007" /> Knockout in mice does not cause histological abnormalities.<ref name="Miyata_2007">Template:Cite journal</ref> | Mutations associated with Nagashima-type Palmoplantar Keratosis.<ref>Template:Cite journal</ref> | 18q21.3 | |
| SERPINB8 | PI-8 | Intracellular | Possible inhibitor of furin.<ref>Template:Cite journal</ref> | 18q21.3 | |||
| SERPINB9 | PI-9 | Intracellular | Inhibitor of the cytotoxic granule protease granzyme B.<ref name="Sun_1996">Template:Cite journal</ref> | Knockout in mice causes immune dysfunction.<ref>Template:Cite journal</ref><ref name="Rizzitelli_2012">Template:Cite journal</ref> | 6p25 | ||
| SERPINB10 | Bomapin | Intracellular | Unknown<ref>Template:Cite journal</ref> | Knockout in mice causes no obvious phenotype (C57/BL6; lab strain BC069938). | 18q21.3 | ||
| SERPINB11 | Intracellular | Unknown<ref name="serpinb112">Template:Cite journal</ref> | Murine Serpinb11 is an active inhibitor whereas the human orthalogue is inactive.<ref name="serpinb112"/> Deficiency in ponies is associated with hoof wall separation disease.<ref name="Finno_2015">Template:Cite journal</ref> | 18q21.3 | |||
| SERPINB12 | Yukopin | Intracellular | Unknown<ref>Template:Cite journal</ref> | 18q21.3 | |||
| SERPINB13 | Hurpin/Headpin | Intracellular | Inhibitor of papain-like cysteine proteases.<ref>Template:Cite journal</ref> | 18q21.3 | |||
| SERPINC1 | Antithrombin | Extracellular | Inhibitor of coagulation, specifically factor X, factor IX and thrombin.<ref name="Huntington_2006">Template:Cite journal</ref> | Knockouts in mice are lethal.<ref>Template:Cite journal</ref> | Deficiency results in thrombosis and other clotting disorders (serpinopathy).<ref name="Patnaik MM 2516">Template:Cite journal</ref><ref>Template:Cite journal</ref> | 1q23-q21 | Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2 |
| SERPIND1 | Heparin cofactor II | Extracellular | Inhibitor of thrombin.<ref>Template:Cite journal</ref> | Knockouts in mice are lethal.<ref>Template:Cite journal</ref> | 22q11 | Template:PDB2, Template:PDB2 | |
| SERPINE1 | Plasminogen activator inhibitor 1 | Extracellular | Inhibitor of thrombin, uPA and TPa.<ref name="Cale_2007">Template:Cite journal</ref> | 7q21.3-q22 | Template:PDB2, Template:PDB2 | ||
| SERPINE2 | Glia derived nexin / Protease nexin I | Extracellular | Inhibitor of uPA and tPA.<ref name="Lino_2007">Template:Cite journal</ref> | Abnormal expression leads to male infertility.<ref>Template:Cite journal</ref> Knockout in mice causes epilepsy.<ref name="Lüthi_1997">Template:Cite journal</ref> | 2q33-q35 | Template:PDB2 | |
| SERPINF1 | Pigment epithelium derived factor | Extracellular | Non-inhibitory, potent anti-angiogenic molecule.<ref name="pedf2">Template:Cite journal</ref> PEDF has been reported to bind the glycosaminoglycan hyaluronan.<ref name="Becerra_2008">Template:Cite journal</ref> | Knockout in mice affects the vasculature and mass of the pancreas and the prostate.<ref name="pedf2"/> Promotes Notch–dependent renewal of adult periventricular neural stem cells.<ref name="PEDF2">Template:Cite journal</ref> Mutations in humans cause osteogenesis imperfecta type VI.<ref name="Homan_2011">Template:Cite journal</ref> | 17p13.3 | Template:PDB2 | |
| SERPINF2 | α2-antiplasmin | Extracellular | Inhibitor of plasmin, inhibitor of fibrinolysis.<ref name="Wiman_1979">Template:Cite journal</ref> | Knockouts in mice show increased mice show increased fibrinolysis but no bleeding disorder.<ref>Template:Cite journal</ref> | Deficiency causes a rare bleeding disorder.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | 17pter-p12 | Template:PDB2 |
| SERPING1 | Complement 1-inhibitor | Extracellular | Inhibitor of C1 esterase.<ref name="Beinrohr_2007">Template:Cite journal</ref> | Several polymorphisms associated with macular degeneration<ref>Template:Cite journal</ref> and hereditary angeoedema.<ref>Template:Cite journal</ref> | 11q11-q13.1 | Template:PDB2 | |
| SERPINH1 | 47 kDa Heat shock protein (HSP47) | Intracellular | Non-inhibitory, molecular chaperone in collagen folding.<ref name="Mala_2010"/> | Knockouts in mice are lethal.<ref>Template:Cite journal</ref> | Mutation in humans causes severe osteogenesis imperfecta.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | 11p15 | Template:PDB2 |
| SERPINI1 | Neuroserpin | Extracellular | Inhibitor of tPA, uPA and plasmin.<ref name="Osterwalder_1998">Template:Cite journal</ref> | Mutation causes FENIB dementia (serpinopathy).<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> | 3q26 | Template:PDB2, Template:PDB2, Template:PDB2, Template:PDB2 | |
| SERPINI2 | Pancpin | Extracellular | Unknown<ref name="pancipin">Template:Cite journal</ref> | Deficiency in mice causes pancreatic insufficiency via acinar cell loss.<ref name="Loftus_2005">Template:Cite journal</ref> | 3q26 |
Specialised mammalian serpinsEdit
Many mammalian serpins have been identified that share no obvious orthology with a human serpin counterpart. Examples include numerous rodent serpins (particularly some of the murine intracellular serpins) as well as the uterine serpins. The term uterine serpin refers to members of the serpin A clade that are encoded by the SERPINA14 gene. Uterine serpins are produced by the endometrium of a restricted group of mammals in the Laurasiatheria clade under the influence of progesterone or estrogen.<ref name="Padua_2010">Template:Cite journal</ref> They are probably not functional proteinase inhibitors and may function during pregnancy to inhibit maternal immune responses against the conceptus or to participate in transplacental transport.<ref name="Padua_Hansen_2010">Template:Cite journal</ref>
InsectEdit
The Drosophila melanogaster genome contains 29 serpin encoding genes. Amino acid sequence analysis has placed 14 of these serpins in serpin clade Q and three in serpin clade K with the remaining twelve classified as orphan serpins not belonging to any clade.<ref name="Reichhart_2005" >Template:Cite journal</ref> The clade classification system is difficult to use for Drosophila serpins and instead a nomenclature system has been adopted that is based on the position of serpin genes on the Drosophila chromosomes. Thirteen of the Drosophila serpins occur as isolated genes in the genome (including Serpin-27A, see below), with the remaining 16 organised into five gene clusters that occur at chromosome positions 28D (2 serpins), 42D (5 serpins), 43A (4 serpins), 77B (3 serpins) and 88E (2 serpins).<ref name="Reichhart_2005"/><ref name="Tang_2008">Template:Cite journal</ref><ref name="Scherfer_2008">Template:Cite journal</ref>
Studies on Drosophila serpins reveal that Serpin-27A inhibits the Easter protease (the final protease in the Nudel, Gastrulation Defective, Snake and Easter proteolytic cascade) and thus controls dorsoventral patterning. Easter functions to cleave Spätzle (a chemokine-type ligand), which results in toll-mediated signaling. As well as its central role in embryonic patterning, toll signaling is also important for the innate immune response in insects. Accordingly, serpin-27A also functions to control the insect immune response.<ref name="Hashimoto 945–950"/><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> In Tenebrio molitor (a large beetle), a protein (SPN93) comprising two discrete tandem serpin domains functions to regulate the toll proteolytic cascade.<ref name="Jiang_2011">Template:Cite journal</ref>
Serpins have been found in tick saliva, suppressing T lymphocyte production and inhibiting expression of TNF-α, IFN-γ, and IL-6.<ref>Template:Cite journal</ref>
NematodeEdit
The genome of the nematode worm C. elegans contains 9 serpins, all of which lack signal sequences and so are likely intracellular.<ref name="elegans1">Template:Cite journal</ref> However, only 5 of these serpins appear to function as protease inhibitors.<ref name="elegans1"/> One, SRP-6, performs a protective function and guards against stress-induced calpain-associated lysosomal disruption. Further, SRP-6 inhibits lysosomal cysteine proteases released after lysosomal rupture. Accordingly, worms lacking SRP-6 are sensitive to stress. Most notably, SRP-6 knockout worms die when placed in water (the hypo-osmotic stress lethal phenotype or Osl). It has therefore been suggested that lysosomes play a general and controllable role in determining cell fate.<ref>Template:Cite journal</ref>
PlantEdit
Plant serpins were amongst the first members of the superfamily that were identified.<ref>Template:Cite journal</ref> The serpin barley protein Z is highly abundant in barley grain, and one of the major protein components in beer. The genome of the model plant, Arabidopsis thaliana contain 18 serpin-like genes, although only 8 of these are full-length serpin sequences.
Plant serpins are potent inhibitors of mammalian chymotrypsin-like serine proteases in vitro, the best-studied example being barley serpin Zx (BSZx), which is able to inhibit trypsin and chymotrypsin as well as several blood coagulation factors.<ref>Template:Cite journal</ref> However, close relatives of chymotrypsin-like serine proteases are absent in plants. The RCL of several serpins from wheat grain and rye contain poly-Q repeat sequences similar to those present in the prolamin storage proteins of the endosperm.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> It has therefore been suggested that plant serpins may function to inhibit proteases from insects or microbes that would otherwise digest grain storage proteins. In support of this hypothesis, specific plant serpins have been identified in the phloem sap of pumpkin (CmPS-1)<ref name="plant">Template:Cite journal</ref> and cucumber plants.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Although an inverse correlation between up-regulation of CmPS-1 expression and aphid survival was observed, in vitro feeding experiments revealed that recombinant CmPS-1 did not appear to affect insect survival.<ref name="plant"/>
Alternative roles and protease targets for plant serpins have been proposed. The Arabidopsis serpin, AtSerpin1 (At1g47710; Template:PDB2), mediates set-point control over programmed cell death by targeting the 'Responsive to Desiccation-21' (RD21) papain-like cysteine protease.<ref name="Lampl_2010"/><ref>Template:Cite journal</ref> AtSerpin1 also inhibits metacaspase-like proteases in vitro.<ref name="Vercammen_2006"/> Two other Arabidopsis serpins, AtSRP2 (At2g14540) and AtSRP3 (At1g64030) appear to be involved in responses to DNA damage.<ref>Template:Cite journal</ref>
FungalEdit
A single fungal serpin has been characterized to date: Template:Proper name from Piromyces spp. strain E2. Piromyces is a genus of anaerobic fungi found in the gut of ruminants and is important for digesting plant material. Template:Proper name is predicted to be inhibitory and contains two N-terminal dockerin domains in addition to its serpin domain. Dockerins are commonly found in proteins that localise to the fungal cellulosome, a large extracellular multiprotein complex that breaks down cellulose.<ref name="Steenbakkers_2008"/> It is therefore suggested that Template:Proper name may protect the cellulosome against plant proteases. Certain bacterial serpins similarly localize to the cellulosome.<ref name="thermo2"/>
ProkaryoticEdit
Predicted serpin genes are sporadically distributed in prokaryotes. In vitro studies on some of these molecules have revealed that they are able to inhibit proteases, and it is suggested that they function as inhibitors in vivo. Several prokaryote serpins are found in extremophiles. Accordingly, and in contrast to mammalian serpins, these molecules possess elevated resistance to heat denaturation.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The precise role of most bacterial serpins remains obscure, although Clostridium thermocellum serpin localises to the cellulosome. It is suggested that the role of cellulosome-associated serpins may be to prevent unwanted protease activity against the cellulosome.<ref name="thermo2">Template:Cite journal</ref>
ViralEdit
Serpins are also expressed by viruses as a way to evade the host's immune defense.<ref name="Turner_2002">Template:Cite journal</ref> In particular, serpins expressed by pox viruses, including cow pox (vaccinia) and rabbit pox (myxoma), are of interest because of their potential use as novel therapeutics for immune and inflammatory disorders as well as transplant therapy.<ref name="Richardson_2006">Template:Cite journal</ref><ref name="Jiang_2007">Template:Cite journal</ref> Serp1 suppresses the TLR-mediated innate immune response and allows indefinite cardiac allograft survival in rats.<ref name="Richardson_2006"/><ref name="Dai_2003">Template:Cite journal</ref> Crma and Serp2 are both cross-class inhibitors and target both serine (granzyme B; albeit weakly) and cysteine proteases (caspase 1 and caspase 8).<ref name="Turner_1999">Template:Cite journal</ref><ref name="Munuswamy-Ramanujam_2006">Template:Cite journal</ref> In comparison to their mammalian counterparts, viral serpins contain significant deletions of elements of secondary structure. Specifically, crmA lacks the D-helix as well as significant portions of the A- and E-helices.<ref name="Renatus_2000">Template:Cite journal</ref>
ReferencesEdit
Template:Portal Template:Reflist
External linksEdit
- Template:PDB Molecule of the Month
- Merops protease inhibitor claudication (Family I4) Template:Webarchive
- Template:MeshName
- James Whisstock laboratory at Monash University
- Jim Huntington laboratory Template:Webarchive at University of Cambridge
- Frank Church laboratory at University of North Carolina at Chapel Hill
- Paul Declerck laboratory at Katholieke Universiteit Leuven
- Tom Roberts laboratory at University of Sydney
- Robert Fluhr laboratory at Weizmann Institute of Science
- Peter Gettins laboratory at University of Illinois at Chicago
- Template:PDBe-KB2