Serpins are a
superfamily of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s with similar structures that were first identified for their
protease inhibition activity and are found in all
kingdoms of life.
The acronym serpin was originally coined because the first serpins to be identified act on chymotrypsin-like
serine protease
Serine proteases (or serine endopeptidases) are enzymes that cleave peptide bonds in proteins. Serine serves as the nucleophilic amino acid at the (enzyme's) active site.
They are found ubiquitously in both eukaryotes and prokaryotes. Seri ...
s (serine protease inhibitors).
They are notable for their unusual mechanism of action, in which they
irreversibly inhibit their target
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 ...
by undergoing a large
conformational change to disrupt the target's
active site.
This contrasts with the more common
competitive
Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
mechanism for protease inhibitors that bind to and block access to the protease active site.
Protease inhibition by serpins controls an array of biological processes, including
coagulation
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanis ...
and
inflammation
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
, and consequently these proteins are the target of
medical research.
Their unique conformational change also makes them of interest to the
structural biology
Structural biology is a field that is many centuries old which, and as defined by the Journal of Structural Biology, deals with structural analysis of living material (formed, composed of, and/or maintained and refined by living cells) at every le ...
and
protein folding
Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reproduc ...
research communities.
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
In medicine, proteinopathy (; 'pref''. protein -pathy 'suff''. disease proteinopathies ''pl''.; proteinopathic ''adj''), or proteopathy, protein conformational disorder, or protein misfolding disease refers to a class of diseases in which certa ...
and the formation of inactive long-chain
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s.
Serpin
polymerisation
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many for ...
not only reduces the amount of active inhibitor, but also leads to accumulation of the polymers, causing
cell death
Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as d ...
and
organ failure
Organ dysfunction is a condition where an organ does not perform its expected function. Organ failure is organ dysfunction to such a degree that normal homeostasis cannot be maintained without external clinical intervention.
It is not a diagnosis ...
.
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
Thyroxine-binding globulin (TBG) is a globulin protein that in humans is encoded by the ''SERPINA7'' gene. TBG binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) respon ...
,
cortisol-binding globulin) and
molecular chaperoning (
HSP47
Heat shock protein 47, also known as SERPINH1 is a serpin which serves as a human chaperone protein for collagen.
Function
This protein is a member of the serpin superfamily of serine proteinase inhibitors. Its expression is induced by heat sho ...
).
The term ''serpin'' is used to describe these members as well, despite their non-inhibitory function, since they are evolutionarily related.
History
Protease inhibitory activity in blood plasma was first reported in the late 1800s, but it was not until the 1950s that the serpins
antithrombin
Antithrombin (AT) is a small glycoprotein that inactivates several enzymes of the coagulation system. It is a 432-amino-acid protein produced by the liver. It contains three disulfide bonds and a total of four possible glycosylation sites. α-A ...
and
alpha 1-antitrypsin were isolated, with the subsequent recognition of their close family homology in 1979. 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,'' but was subsequently succinctly renamed as the Serpins. The initial characterisation of the new family centred on
alpha1-antitrypsin, a serpin present in high concentration in blood plasma, the common
genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
of which was shown to cause a predisposition to the lung disease
emphysema and to liver
cirrhosis
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, and end-stage liver disease, is the impaired liver function caused by the formation of scar tissue known as fibrosis due to damage caused by liver disease. Damage causes tissue rep ...
. The identification of the S and Z mutations 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, centred on a methionine in alpha1-antitrypsin as an inhibitor of tissue elastase and on arginine in antithrombin as an inhibitor of thrombin.
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.
This active-centre specificity of inhibition was also evident in the many other families of protease inhibitors
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.
This together with the subsequent solving of the structure of native (uncleaved) ovalbumin
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.
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.
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,
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
in allowing the subtle modulation of inhibitory activity, as notably seen at tissue level 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
kingdom
Kingdom commonly refers to:
* A monarchy ruled by a king or queen
* Kingdom (biology), a category in biological taxonomy
Kingdom may also refer to:
Arts and media Television
* ''Kingdom'' (British TV series), a 2007 British television drama s ...
s of life—
animals
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
,
plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
, and
archaea—and some
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
.
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. In the 2000s, a systematic nomenclature was introduced in order to categorise members of the serpin superfamily based on their evolutionary relationships.
Serpins are therefore the largest and most diverse superfamily of protease inhibitors.
Activity
![Serpin and protease](https://upload.wikimedia.org/wikipedia/commons/0/0f/Serpin_and_protease.png)
Most serpins are
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 ...
inhibitors, targeting extracellular,
chymotrypsin-like
serine protease
Serine proteases (or serine endopeptidases) are enzymes that cleave peptide bonds in proteins. Serine serves as the nucleophilic amino acid at the (enzyme's) active site.
They are found ubiquitously in both eukaryotes and prokaryotes. Seri ...
s. These proteases possess a
nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
serine residue in a
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 ...
in their
active site. Examples include
thrombin
Thrombin (, ''fibrinogenase'', ''thrombase'', ''thrombofort'', ''topical'', ''thrombin-C'', ''tropostasin'', ''activated blood-coagulation factor II'', ''blood-coagulation factor IIa'', ''factor IIa'', ''E thrombin'', ''beta-thrombin'', ''gamma- ...
,
trypsin
Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting these long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the d ...
, and
human neutrophil elastase
Neutrophil elastase (, ''leukocyte elastase'', ''ELANE'', ''ELA2'', ''elastase 2'', ''neutrophil'', ''elaszym'', ''serine elastase'', subtype ''human leukocyte elastase (HLE)'') is a serine proteinase in the same family as chymotrypsin and has bro ...
. Serpins act as
irreversible,
suicide inhibitors by trapping an intermediate of the protease's catalytic mechanism.
Some serpins inhibit other protease classes, typically
cysteine protease
Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad.
Discovered by Gopal Ch ...
s, 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. Nonetheless, the enzymatic chemistry is similar, and the mechanism of inhibition by serpins is the same for both classes of protease. 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 protease
Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad.
Discovered by Gopal Ch ...
s.
Biological function and localization
Protease inhibition
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
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanis ...
(antithrombin), the
inflammatory and
immune responses
An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...
(antitrypsin,
antichymotrypsin, and
C1-inhibitor
C1-inhibitor (C1-inh, C1 esterase inhibitor) is a protease inhibitor belonging to the serpin superfamily. Its main function is the inhibition of the complement system to prevent spontaneous activation but also as the major regulator of the cont ...
) and tissue remodelling
(PAI-1).
By inhibiting
signalling cascade
A biochemical cascade, also known as a signaling cascade or signaling pathway, is a series of chemical reactions that occur within a biological cell when initiated by a stimulus. This stimulus, known as a first messenger, acts on a receptor that ...
proteases, they can also affect
development
Development or developing may refer to:
Arts
*Development hell, when a project is stuck in development
*Filmmaking, development phase, including finance and budgeting
*Development (music), the process thematic material is reshaped
* Photograph ...
.
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. For example, one of the best-characterised human intracellular serpins is
Serpin B9, which inhibits the
cytotoxic granule protease
granzyme B
Granzyme B (GrB) is one of the serine protease granzymes most commonly found in the granules of natural killer cells (NK cells) and cytotoxic T cells. It is secreted by these cells along with the pore forming protein perforin to mediate apoptosi ...
. In doing so, Serpin B9 may protect against inadvertent release of granzyme B and premature or unwanted activation of
cell death
Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as d ...
pathways.
Some
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
es use serpins to disrupt protease functions in their host. The
cowpox
Cowpox is an infectious disease caused by the ''cowpox virus'' (CPXV). It presents with large blisters in the skin, a fever and swollen glands, historically typically following contact with an infected cow, though in the last several decades more ...
viral serpin CrmA (cytokine response modifier A) is used in order to avoid inflammatory and
apoptotic
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes incl ...
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. In
eukaryotes, a plant serpin inhibits both
metacaspase Metacaspases are members of the C14 class of cysteine proteases and thus related to caspases, orthocaspases and paracaspases. The metacaspases are arginine/lysine-specific, in contrast to caspases, which are aspartate-specific.
Structure and Phy ...
s
and a papain-like cysteine protease.
Non-inhibitory roles
Non-inhibitory extracellular serpins also perform a wide array of important roles.
Thyroxine-binding globulin
Thyroxine-binding globulin (TBG) is a globulin protein that in humans is encoded by the ''SERPINA7'' gene. TBG binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) respon ...
and
transcortin
Transcortin, also known as corticosteroid-binding globulin (CBG) or serpin A6, is a protein produced in the liver in animals. In humans it is encoded by the SERPINA6 gene. It is an alpha-globulin.
Function
This gene encodes an alpha-globulin p ...
transport the hormones
thyroxine
File:Thyroid_system.svg, upright=1.5, The thyroid system of the thyroid hormones T3 and T4
rect 376 268 820 433 Thyroid-stimulating hormone
rect 411 200 849 266 Thyrotropin-releasing hormone
rect 297 168 502 200 Hypothalamus
rect 66 216 386 25 ...
and
cortisol, respectively.
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.
Heat shock serpin 47 is a
chaperone, essential for proper
folding
Fold, folding or foldable may refer to:
Arts, entertainment, and media
* ''Fold'' (album), the debut release by Australian rock band Epicure
* Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot
*Abov ...
of
collagen. It acts by stabilising collagen's
triple helix
In the fields of geometry and biochemistry, a triple helix (plural triple helices) is a set of three congruent geometrical helices with the same axis, differing by a translation along the axis. This means that each of the helices keeps the same ...
whilst it is being processed in the
endoplasmic reticulum.
Some serpins are both protease inhibitors and perform additional roles. For example, the nuclear cysteine protease inhibitor
MENT
Ment or MENT may refer
People
* Eva Ment (1606–1652), Dutch governor's wife
Other uses
* Master of Enterprise
* MENT BC, a Greek basketball club
* Myeloid and erythroid nuclear termination stage specific protein a member of the serpin
...
, in
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s also acts as a
chromatin remodelling
Chromatin remodeling is the dynamic modification of chromatin architecture to allow access of condensed genomic DNA to the regulatory transcription machinery proteins, and thereby control gene expression. Such remodeling is principally carried out ...
molecule in a bird's
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s.
Structure
All serpins share a common
structure (or fold), despite their varied functions. All typically have three
β-sheets
The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...
(named A, B and C) and eight or nine
α-helices
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
(named hA–hI).
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
In chemistry, a dynamic equilibrium exists once a reversible reaction occurs. Substances transition between the reactants and products at equal rates, meaning there is no net change. Reactants and products are formed at such a rate that the co ...
between these two states.
The RCL also only makes temporary interactions with the rest of the structure, and is therefore highly flexible and exposed to the solvent.
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
Structural biology is a field that is many centuries old which, and as defined by the Journal of Structural Biology, deals with structural analysis of living material (formed, composed of, and/or maintained and refined by living cells) at every le ...
has therefore played a central role in the understanding of serpin function and biology.
Conformational change and inhibitory mechanism
Inhibitory serpins do not inhibit their target proteases by the typical
competitive
Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
(''lock-and-key'') mechanism used by most small
protease inhibitors
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 replicat ...
(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).
Serine and
cysteine protease
Cysteine proteases, also known as thiol proteases, are hydrolase enzymes that degrade proteins. These proteases share a common catalytic mechanism that involves a nucleophilic cysteine thiol in a catalytic triad or dyad.
Discovered by Gopal Ch ...
s catalyse peptide bond cleavage by a two-step process. Initially, the catalytic residue of the active site
triad performs a
nucleophilic
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
attack on the peptide bond of the substrate. This releases the new
N-terminus and forms a covalent
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
-bond between the enzyme and the substrate.
This covalent complex between enzyme and substrate is called an
acyl-enzyme intermediate. For standard
substrates, the ester bond is
hydrolysed
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysis ...
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.
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.
Serpins are classed as
irreversible inhibitors and as
suicide inhibitors since each serpin protein permanently inactivates a single protease, and can only function once.
Allosteric activation
![Serpin activation by heparin (unannotated)](https://upload.wikimedia.org/wikipedia/commons/6/65/Serpin_activation_by_heparin_%28unannotated%29.png)
The
conformational mobility of serpins provides a key advantage over static lock-and-key protease inhibitors.
In particular, the function of inhibitory serpins can be
regulated
Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
by
allosteric interactions with specific
cofactors. The
X-ray crystal structures of
antithrombin
Antithrombin (AT) is a small glycoprotein that inactivates several enzymes of the coagulation system. It is a 432-amino-acid protein produced by the liver. It contains three disulfide bonds and a total of four possible glycosylation sites. α-A ...
,
heparin cofactor II
Heparin cofactor II (HCII), a protein encoded by the SERPIND1 gene, is a coagulation factor that inhibits IIa, and is a cofactor for heparin and dermatan sulfate ("minor antithrombin").
The product encoded by this gene is a serine protease inhibit ...
,
MENT
Ment or MENT may refer
People
* Eva Ment (1606–1652), Dutch governor's wife
Other uses
* Master of Enterprise
* MENT BC, a Greek basketball club
* Myeloid and erythroid nuclear termination stage specific protein a member of the serpin
...
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 beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a g ...
. The partially inserted conformation is important because co-factors are able to conformationally switch certain partially inserted serpins into a fully expelled form. 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
Thrombin (, ''fibrinogenase'', ''thrombase'', ''thrombofort'', ''topical'', ''thrombin-C'', ''tropostasin'', ''activated blood-coagulation factor II'', ''blood-coagulation factor IIa'', ''factor IIa'', ''E thrombin'', ''beta-thrombin'', ''gamma- ...
and
factor Xa
Factor X, also known by the eponym Stuart–Prower factor, is an enzyme () of the coagulation cascade. It is a serine endopeptidase (protease group S1, PA clan). Factor X is synthesized in the liver and requires vitamin K for its synthesis.
Fa ...
. 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
Fondaparinux (trade name Arixtra) is an anticoagulant medication chemically related to low molecular weight heparins. It is marketed by GlaxoSmithKline. A generic version developed by Alchemia is marketed within the US by Dr. Reddy's Laboratorie ...
, a synthetic form of Heparin pentasaccharide used as an
anti-clotting drug.
Latent conformation
![Serpin latent state (unannotated)](https://upload.wikimedia.org/wikipedia/commons/a/ac/Serpin_latent_state_%28unannotated%29.png)
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.
Regulation of the latency transition can act as a control mechanism in some serpins, such as
PAI-1
Plasminogen activator inhibitor-1 (PAI-1) also known as endothelial plasminogen activator inhibitor or serpin E1 is a protein that in humans is encoded by the ''SERPINE1'' gene. Elevated PAI-1 is a risk factor for thrombosis and atherosclerosis
P ...
. 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
Vitronectin (VTN or VN) is a glycoprotein of the hemopexin family which is abundantly found in serum, the extracellular matrix and bone. In humans it is encoded by the ''VTN'' gene.
Vitronectin binds to integrin alpha-V beta-3 and thus promotes ...
.
Similarly, antithrombin can also spontaneously convert to the latent state, as an additional modulation mechanism to its allosteric activation by heparin. Finally, the N-terminus of , a serpin from ''
Thermoanaerobacter tengcongensis
''Thermoanaerobacter'' is a genus in the phylum Bacillota (Bacteria). Members of this genus are thermophilic and anaerobic, several of them were previously described as ''Clostridium'' species and members of the now obsolete genera ''Acetogenium ...
'', 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.
Conformational change in non-inhibitory functions
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
Thyroxine-binding globulin (TBG) is a globulin protein that in humans is encoded by the ''SERPINA7'' gene. TBG binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) respon ...
has high affinity for thyroxine, whereas the cleaved (R) form has low affinity. Similarly,
transcortin
Transcortin, also known as corticosteroid-binding globulin (CBG) or serpin A6, is a protein produced in the liver in animals. In humans it is encoded by the SERPINA6 gene. It is an alpha-globulin.
Function
This gene encodes an alpha-globulin p ...
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.
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.
The S to R transition is therefore used to alert cells to the presence of protease activity.
This differs from the usual mechanism whereby serpins affect signalling simply by inhibiting proteases involved in a signalling cascade.
Degradation
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.
Similarly, the ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
'' serpin, necrotic, is degraded in the
lysosome
A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane pr ...
after being trafficked into the cell by the Lipophorin Receptor-1 (homologous to the mammalian
LDL receptor
The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids (after removal of 21-amino acid signal peptide) that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor ...
family).
Disease and serpinopathies
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.
The disorder
alpha-1 antitrypsin deficiency
Alpha-1 antitrypsin deficiency (A1AD or AATD) is a genetic disorder that may result in lung disease or liver disease. Onset of lung problems is typically between 20 and 50 years of age. This may result in shortness of breath, wheezing, or an inc ...
is one of the most common
hereditary disease
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
s.
Inactivity or absence
![Serpin delta conformation](https://upload.wikimedia.org/wikipedia/commons/4/4f/Serpin_delta_conformation.png)
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.
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.
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'', 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.
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.
The non-inhibitory proteins related to serpins can also cause diseases when mutated. For example, mutations in SERPINF1 cause
osteogenesis imperfecta
Osteogenesis imperfecta (; OI), colloquially known as brittle bone disease, is a group of genetic disorders that all result in bones that break easily. The range of symptoms—on the skeleton as well as on the body's other organs—may b ...
type VI in humans.
In the absence of a required serpin, the protease that it normally would regulate is over-active, leading to pathologies.
Consequently, simple deficiency of a serpin (e.g. a
null mutation
A null allele is a nonfunctional allele (a variant of a gene) caused by a genetic mutation. Such mutations can cause a complete lack of production of the associated gene product or a product that does not function properly; in either case, the all ...
) can result in disease.
Gene knockouts, particularly in
mice, are used experimentally to determine the normal functions of serpins by the effect of their absence.
Specificity change
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
Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, vag ...
disorder.
Polymerisation and aggregation
The majority of serpin diseases are due to
protein aggregation
In molecular biology, protein aggregation is a phenomenon in which intrinsically-disordered or mis-folded proteins aggregate (i.e., accumulate and clump together) either intra- or extracellularly. Protein aggregates have been implicated in a wi ...
and are termed "serpinopathies".
Serpins are vulnerable to disease-causing mutations that promote formation of misfolded polymers due to their inherently unstable structures.
Well-characterised serpinopathies include
α1-antitrypsin deficiency (alpha-1), which may cause familial
emphysema, and sometimes
liver cirrhosis
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, and end-stage liver disease, is the impaired liver function caused by the formation of scar tissue known as fibrosis due to damage caused by liver disease. Damage causes tissue repai ...
, certain familial forms of
thrombosis
Thrombosis (from Ancient Greek "clotting") is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel (a vein or an artery) is injured, the body uses platelets (t ...
related to
antithrombin deficiency, types 1 and 2
hereditary angioedema
Hereditary angioedema (HAE) is a disorder that results in recurrent attacks of severe swelling. The swelling most commonly affects the arms, legs, face, intestinal tract, and airway. If the intestinal tract is affected, abdominal pain and vomitin ...
(HAE) related to deficiency of
C1-inhibitor
C1-inhibitor (C1-inh, C1 esterase inhibitor) is a protease inhibitor belonging to the serpin superfamily. Its main function is the inhibition of the complement system to prevent spontaneous activation but also as the major regulator of the cont ...
, and
familial encephalopathy with neuroserpin inclusion bodies
Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is a progressive disorder of the nervous system that is characterized by a loss of intellectual functioning (dementia) and seizures. At first, affected individuals may have difficult ...
(FENIB; a rare type of
dementia
Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
caused by neuroserpin polymerisation).
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.
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
A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass.
These cells are involved in:
* Protein synthesis
* Protein storage
* Transformation of carbohydrates
* Synthesis of cholesterol, ...
, sometimes resulting in liver damage and
cirrhosis
Cirrhosis, also known as liver cirrhosis or hepatic cirrhosis, and end-stage liver disease, is the impaired liver function caused by the formation of scar tissue known as fibrosis due to damage caused by liver disease. Damage causes tissue rep ...
. Within the cell, serpin polymers are slowly removed via degradation in the endoplasmic reticulum.
However, the details of how serpin polymers cause cell death remains to be fully understood.
Physiological serpin polymers are thought to form via
domain swapping events, where a segment of one serpin protein inserts into another.
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.
Both
dimer
Dimer may refer to:
* Dimer (chemistry), a chemical structure formed from two similar sub-units
** Protein dimer, a protein quaternary structure
** d-dimer
* Dimer model, an item in statistical mechanics, based on ''domino tiling''
* Julius 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.
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).
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).
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.
Therapeutic strategies
Several therapeutic approaches are in use or under investigation to treat the most common serpinopathy: antitrypsin deficiency.
Antitrypsin augmentation therapy is approved for severe antitrypsin deficiency-related emphysema.
In this therapy, antitrypsin is purified from the plasma of blood donors and administered intravenously (first marketed as
Prolastin
Alpha-1 antitrypsin or α1-antitrypsin (A1AT, α1AT, A1A, or AAT) is a protein belonging to the serpin superfamily. It is encoded in humans by the ''SERPINA1'' gene. A protease inhibitor, it is also known as alpha1–proteinase inhibitor (A1PI) ...
).
To treat severe antitrypsin deficiency-related disease, lung and liver
transplantation has proven effective.
In animal models, gene targeting in
induced pluripotent stem cell
Induced pluripotent stem cells (also known as iPS cells or iPSCs) are a type of pluripotent stem cell that can be generated directly from a somatic cell. The iPSC technology was pioneered by Shinya Yamanaka's lab in Kyoto, Japan, who showed in ...
s has been successfully used to correct an antitrypsin polymerisation defect and to restore the ability of the mammalian liver to secrete active antitrypsin.
Small molecules have also been developed that block antitrypsin polymerisation ''in vitro''.
Evolution
Serpins are the most widely distributed and largest superfamily of protease inhibitors.
They were initially believed to be restricted to
eukaryote organisms, but have since been found in
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
,
archaea and some
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
.
It remains unclear whether prokaryote genes are the descendants of an ancestral prokaryotic serpin or the product of
horizontal gene transfer
Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
from eukaryotes. Most intracellular serpins belong to a single
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
clade, whether they come from plants or animals, indicating that the intracellular and extracellular serpins may have diverged before the plants and animals. 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.
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.
Distribution
Animal
Human
The human genome encodes 16 serpin clades, termed through , including 29 inhibitory and 7 non-inhibitory serpin proteins.
The human serpin naming system is based upon a
phylogenetic
In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
analysis of approximately 500 serpins from 2001, with proteins named , where X is the clade of the protein and Y the number of the protein within that clade.
The functions of human serpins have been determined by a combination of
biochemical studies, human
genetic disorder
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
s, and
knockout mouse models.
Specialised mammalian serpins
Many
mammalian serpins have been identified that share no obvious orthology with a human serpin counterpart. Examples include numerous
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
serpins (particularly some of the
murine
The Old World rats and mice, part of the subfamily Murinae in the family Muridae, comprise at least 519 species. Members of this subfamily are called murines. In terms of species richness, this subfamily is larger than all mammal families ex ...
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
Laurasiatheria ("laurasian beasts") is a superorder of placental mammals that groups together true insectivores ( eulipotyphlans), bats ( chiropterans), carnivorans, pangolins ( pholidotes), even-toed ungulates ( artiodactyls), odd-toed ungulat ...
clade under the influence of
progesterone or
estrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...
.
They are probably not functional proteinase inhibitors and may function during pregnancy to inhibit maternal immune responses against the
conceptus
A conceptus (from Latin: ''concipere'' to conceive) is an embryo and its appendages (adnexa), the associated membranes, placenta, and umbilical cord; the products of conception or, more broadly, "the product of conception at any point between fert ...
or to participate in transplacental transport.
Insect
The ''
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...
'' 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.
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''
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s. 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).
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.
In ''Tenebrio molitor'' (a large beetle), a protein (SPN93) comprising two discrete tandem serpin domains functions to regulate the toll proteolytic cascade.
Nematode
The genome of the
nematode worm ''
C. elegans
''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...
'' contains 9 serpins, all of which lack signal sequences and so are likely intracellular.
However, only 5 of these serpins appear to function as protease inhibitors.
One, SRP-6, performs a protective function and guards against stress-induced
calpain
A calpain (; , ) is a protein belonging to the family of calcium-dependent, non-lysosomal cysteine proteases ( proteolytic enzymes) expressed ubiquitously in mammals and many other organisms. Calpains constitute the C2 family of protease clan C ...
-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.
Plant
Plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
serpins were amongst the first members of the superfamily that were identified. 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. 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
Prolamins are a group of plant storage proteins having a high proline amino acid content. They are found in plants, mainly in the seeds of cereal grains such as wheat (gliadin), barley (hordein), rye ( secalin), corn (zein), sorghum ( kafirin) ...
storage proteins of the endosperm. 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)
and cucumber plants. 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.
Alternative roles and protease targets for plant serpins have been proposed. The ''Arabidopsis'' serpin, AtSerpin1 (At1g47710; ), mediates set-point control over programmed cell death by targeting the 'Responsive to Desiccation-21' (RD21) papain-like cysteine protease.
AtSerpin1 also inhibits
metacaspase Metacaspases are members of the C14 class of cysteine proteases and thus related to caspases, orthocaspases and paracaspases. The metacaspases are arginine/lysine-specific, in contrast to caspases, which are aspartate-specific.
Structure and Phy ...
-like proteases ''in vitro''.
Two other ''Arabidopsis'' serpins, AtSRP2 (At2g14540) and AtSRP3 (At1g64030) appear to be involved in responses to DNA damage.
Fungal
A single
fungal
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
serpin has been characterized to date: from ''
Piromyces''
spp.
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate se ...
strain E2. ''Piromyces'' is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
of anaerobic fungi found in the gut of ruminants and is important for digesting plant material. is predicted to be inhibitory and contains two N-terminal
dockerin
Dockerin is a protein domain found in the cellulosome cellular structure of anaerobic bacteria. It is found on many endoglucanase enzymes. The dockerin's binding partner is the cohesin domain, located on the scaffoldin protein. This interaction ...
domains in addition to its serpin domain. Dockerins are commonly found in proteins that localise to the fungal
cellulosome
Cellulosomes are multi-enzyme extracellular complexes. Cellulosomes are associated with the cell surface and mediate cell attachment to insoluble substrates and degrade them to soluble products which are then absorbed. Cellulosome complexes are in ...
, a large extracellular multiprotein complex that breaks down cellulose.
It is therefore suggested that may protect the cellulosome against plant proteases. Certain bacterial serpins similarly localize to the cellulosome.
Prokaryotic
Predicted serpin genes are sporadically distributed in
prokaryote
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
s. ''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
extremophile
An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
s. Accordingly, and in contrast to mammalian serpins, these molecules possess elevated resistance to heat denaturation.
The precise role of most bacterial serpins remains obscure, although ''
Clostridium thermocellum
''Acetivibrio thermocellus'' is an anaerobic, thermophilic bacterium. ''A. thermocellusm'' has garnered research interest due to its cellulolytic and ethanologenic abilities, being capable of directly converting a cellulosic substrate into etha ...
'' serpin localises to the
cellulosome
Cellulosomes are multi-enzyme extracellular complexes. Cellulosomes are associated with the cell surface and mediate cell attachment to insoluble substrates and degrade them to soluble products which are then absorbed. Cellulosome complexes are in ...
. It is suggested that the role of cellulosome-associated serpins may be to prevent unwanted protease activity against the cellulosome.
Viral
Serpins are also expressed by
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsk ...
es as a way to evade the host's immune defense.
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.
Serp1 suppresses the TLR-mediated innate immune response and allows indefinite cardiac
allograft
Allotransplant (''allo-'' meaning "other" in Greek) is the transplantation of cells, tissues, or organs to a recipient from a genetically non-identical donor of the same species. The transplant is called an allograft, allogeneic transplant, o ...
survival in rats.
Crma and Serp2 are both cross-class inhibitors and target both serine (granzyme B; albeit weakly) and cysteine proteases (caspase 1 and caspase 8).
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.
References
External links
*
Merops protease inhibitor claudication (Family I4)*
James Whisstock laboratoryat
Monash University
Monash University () is a public research university based in Melbourne, Victoria, Australia. Named for prominent World War I general Sir John Monash, it was founded in 1958 and is the second oldest university in the state. The university h ...
Jim Huntington laboratory at
University of Cambridge
The University of Cambridge is a public collegiate research university in Cambridge, England. Founded in 1209 and granted a royal charter by Henry III in 1231, Cambridge is the world's third oldest surviving university and one of its most pr ...
Frank Church laboratoryat
University of North Carolina at Chapel Hill
A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States ...
Paul Declerck laboratoryat
Katholieke Universiteit LeuvenTom Roberts laboratoryat
University of Sydney
The University of Sydney (USYD), also known as Sydney University, or informally Sydney Uni, is a public research university located in Sydney, Australia. Founded in 1850, it is the oldest university in Australia and is one of the country's ...
Robert Fluhr laboratoryat
Weizmann Institute of Science
The Weizmann Institute of Science ( he, מכון ויצמן למדע ''Machon Vaitzman LeMada'') is a public research university in Rehovot, Israel, established in 1934, 14 years before the State of Israel. It differs from other Israeli unive ...
Peter Gettins laboratoryat
University of Illinois at Chicago
The University of Illinois Chicago (UIC) is a public research university in Chicago, Illinois. Its campus is in the Near West Side community area, adjacent to the Chicago Loop. The second campus established under the University of Illinois ...
*
{{Serpins
*
Protein superfamilies