C3-convertase

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Classical-complement-pathway C3/C5 convertase
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Surface rendering of C3 convertase (C3bBb) stabilized by SCIN
Identifiers
EC no. 3.4.21.43
CAS no. 56626-15-4
Alt. namesC42 , C4bC2b (formerly C4bC2a), C3bBb, complement C.hivin.4.hivin2, complement C3 convertase
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The classical and alternative complement pathways. Complement pathway.svg
The classical and alternative complement pathways.
Complement-pathways. Complement-pathways.png
Complement-pathways.

C3 convertase (C4bC2b, formerly C4b2a) belongs to family of serine proteases and is necessary in innate immunity as a part of the complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 convertase formation and cell lysis.

Contents

C3 convertase can be used to refer to the form produced in the alternative pathway (C3bBb) or the classical and lectin pathways (C4bC2b, formerly C4b2a). Once formed, both C3 convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase").

The smaller fragment called C3a serves to increase vascular permeability and promote extravasation of phagocytes, while the larger C3b fragment can be used as an opsonin or bind to either type of C3 convertase to form the trimolecular C5 convertase to activate C5 for the membrane attack complex.

Formation

C3 convertase formation can occur in three different pathways: the classical, lectin, and alternative pathways.

Alternative pathway

Cleavage of complement C3 by a free floating convertase, thrombin, plasmin or even a bacterial enzyme leads to formation of C3a and C3b fragments. C3b, the larger fragment, becomes covalently attached to the microbial surface or to the antibody molecules through the thioester domain at the site of complement activation. After cleavage and binding to cell surface, the C3b fragment is ready to bind a plasma protein called Factor B. The Factor B (a zymogen) is cleaved by a plasma serine protease Factor D releasing a small fragment called Ba and generating a larger fragment called Bb that remains attached to C3b. Also Mg2+ ions are necessary for forming a functional C3 convertase. Thus, the alternative C3 convertase (C3bBb) is formed and is able to cleave C3 via its dimeric Bb subunit. [1] [2]

Since C3 convertases cleave C3 to produce C3b which can then form an additional C3 convertase through the alternative pathway, this is a potential mechanism of signal amplification in the complement cascade resulting in the deposition of large numbers of C3b molecules on the surface of activating particles, enabling opsonisation and acute local inflammation. [3]

Classical and lectin pathways

The C3 convertase formed in the classical or lectin pathways is formed of C4b and C2b instead (NB: C2b, the larger fragment of C2 cleavage, was formerly known as C2a). The cleavage of C4 and C2 is mediated by serine proteases. In the classical pathway, this is by sequential proteolytic activation of proteins within the C1 complex (C1q, C1r, C1s) in response to binding to CRP or immunoglobulin, and in the lectin pathway it is driven by mannose binding lectin and its associated serine proteases (MASPs, particularly MASP2 but also MASP1).

C4 is homologous to C3 in that it contains an internal thioester bond that ends up on C4b. Thus it can form covalent amide or ester linkages with the plasma membrane of the pathogen and any associated antibodies, where it then behaves as an opsonin. The larger C2b produced by C2 hydrolysis attaches to the C4b to form the classical C3 convertase, C4b2b (formerly called C4b2a). [4]

The smaller fragments of proteolysis, C4a and C2a are released. C4a is an anaphylatoxin. [1]

Regulation

Nevertheless, this positive feedback mechanism can be regulated by binding of the control protein, nonproteolytic glycoprotein β1H (factor H), to C3b, which prevents association of factor B, and facilitates the decay-dissociation of Bb in the C3bBb complex, in addition to enhancing proteolytic inactivation of C3b by C3b inactivator (C3bINA – endopeptidase).

Membrane-associated sialic acid promotes high-affinity binding of β1H to C3b without influencing the affinity of B for C3b.

Decay-accelerating factor (DAF) is another negative regulator of C3 convertase. It is a membrane protein and regulates also C5 convertase of the classical and alternative pathway. DAF protects host cells from damage by autologous complement. DAF acts on C2b and Bb and dissociates them rapidly from C4b and C3b – thereby preventing the assembly of the C3 convertase. [7]

C4 binding protein (C4BP) interferes with the assembly of the membrane-bound C3 convertase of the classical pathway. C4BP is a cofactor for the enzyme C3bINA. C4b-binding protein inhibits the haemolytic function of cell-bound C4b. C4b-binding protein and C3b inactivator control the C3 convertase of the classical pathway in a similar way to that described for β1H and C3b inactivator in the alternative pathway. [8]

C3b has different binding site for C3bINA, β1H, factor B and properdin. Binding β1H to C3b increases C3bINA binding, while factor B binding prevents C3bINA binding and is competitive with β1H binding. [9]

Regulation of the amplification phase of the alternative pathway is exerted by multiple mechanisms:

Location on chromosome

The genes encoding C2, C4 and factor B are located on chromosome 6 between the B locus of class I products and the D locus of class II products in the MHC.

Related Research Articles

<span class="mw-page-title-main">Complement system</span> Part of the immune system that enhances the ability of antibodies and phagocytic cells

The complement system, also known as complement cascade, is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. It is part of the innate immune system, which is not adaptable and does not change during an individual's lifetime. The complement system can, however, be recruited and brought into action by antibodies generated by the adaptive immune system.

<span class="mw-page-title-main">Classical complement pathway</span> Aspect of the immune system

The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM.

<span class="mw-page-title-main">Alternative complement pathway</span> Type of cascade reaction of the complement system

The alternative pathway is a type of cascade reaction of the complement system and is a component of the innate immune system, a natural defense against infections.

Opsonins are extracellular proteins that, when bound to substances or cells, induce phagocytes to phagocytose the substances or cells with the opsonins bound. Thus, opsonins act as tags to label things in the body that should be phagocytosed by phagocytes. Different types of things ("targets") can be tagged by opsonins for phagocytosis, including: pathogens, cancer cells, aged cells, dead or dying cells, excess synapses, or protein aggregates. Opsonins help clear pathogens, as well as dead, dying and diseased cells.

<span class="mw-page-title-main">Complement component 3</span> Protein-coding gene in the species Homo sapiens

Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.

<span class="mw-page-title-main">C5-convertase</span> Serine protease that plays key role in innate immunity.

C5 convertase is an enzyme belonging to a family of serine proteases that play key role in the innate immunity. It participates in the complement system ending with cell death.

<span class="mw-page-title-main">Properdin</span> Protein-coding gene in the species Homo sapiens

Properdin is protein that in humans is encoded by the CFP gene.

<span class="mw-page-title-main">Lectin pathway</span>

The lectin pathway or lectin complement pathway is a type of cascade reaction in the complement system, similar in structure to the classical complement pathway, in that, after activation, it proceeds through the action of C4 and C2 to produce activated complement proteins further down the cascade. In contrast to the classical complement pathway, the lectin pathway does not recognize an antibody bound to its target. The lectin pathway starts with mannose-binding lectin (MBL) or ficolin binding to certain sugars.

Alternative-complement-pathway C3/C5 convertase is an enzyme. This enzyme catalyses the following chemical reaction

<span class="mw-page-title-main">Complement component 2</span> Protein-coding gene in the species Homo sapiens

Complement C2 is a protein that in humans is encoded by the C2 gene. The protein encoded by this gene is part of the classical pathway of the complement system, acting as a multi-domain serine protease. Deficiency of C2 has been associated with certain autoimmune diseases.

<span class="mw-page-title-main">MASP1 (protein)</span> Protein-coding gene in the species Homo sapiens

Mannan-binding lectin serine protease 1 also known as mannose-associated serine protease 1 (MASP-1) is an enzyme that in humans is encoded by the MASP1 gene.

<span class="mw-page-title-main">Factor D</span> Class of enzymes

Factor D is a protein which in humans is encoded by the CFD gene. Factor D is involved in the alternative complement pathway of the complement system where it cleaves factor B.

C4b-binding protein (C4BP) is a protein complex involved in the complement system where it acts as inhibitor. C4BP has an octopus-like structure with a central stalk and seven branching alpha-chains. The main form of C4BP in human blood is composed of 7 identical alpha-chains and one unique beta-chain, which in turn binds anticoagulant, vitamin K-dependent protein S.

<span class="mw-page-title-main">Decay-accelerating factor</span> Mammalian protein found in Homo sapiens

Complement decay-accelerating factor, also known as CD55 or DAF, is a protein that, in humans, is encoded by the CD55 gene.

Complement control protein are proteins that interact with components of the complement system.

Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with the other numerous components. Furthermore, it is a crucial factor in connecting the recognition pathways of the overall system instigated by antibody-antigen (Ab-Ag) complexes to the other effector proteins of the innate immune response. For example, the severity of a dysfunctional complement system can lead to fatal diseases and infections. Complex variations of it can also lead to schizophrenia. The C4 protein was thought to derive from a simple two-locus allelic model, which however has been replaced by a much more sophisticated multimodular RCCX gene complex model which contain long and short forms of the C4A or C4B genes usually in tandem RCCX cassettes with copy number variation, that somewhat parallels variation in the levels of their respective proteins within a population along with CYP21 in some cases depending on the number of cassettes and whether it contains the functional gene instead of pseudogenes or fragments. Originally defined in the context of the Chido/Rodgers blood group system, the C4A-C4B genetic model is under investigation for its possible role in schizophrenia risk and development.

<span class="mw-page-title-main">C3b</span>

C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apoptotic cells for phagocytosis. Additionally, C3b plays a role in forming a C3 convertase when bound to Factor B, or a C5 convertase when bound to C4b and C2b or when an additional C3b molecule binds to the C3bBb complex.

<span class="mw-page-title-main">C3a (complement)</span>

C3a is one of the proteins formed by the cleavage of complement component 3; the other is C3b. C3a is a 77 residue anaphylatoxin that binds to the C3a receptor (C3aR), a class A G protein-coupled receptor. It plays a large role in the immune response.

Complement 3 deficiency is a genetic condition affecting complement component 3 (C3). People can suffer from either primary or secondary C3 deficiency. Primary C3 deficiency refers to an inherited autosomal-recessive disorder that involves mutations in the gene for C3. Secondary C3 deficiency results from a lack of factor I or factor H, two proteins that are key for the regulation of C3. Both primary and secondary C3 deficiency are characterized by low levels or absence of C3.

The C1 complex is a protein complex involved in the complement system. It is the first component of the classical complement pathway and is composed of the subcomponents C1q, C1r and C1s.

References

  1. 1 2 Abbas AK, Lichtman AH, Pillai S (2010). Cellular and Molecular Immunology (6th ed.). Elsevier. ISBN   978-1-4160-3123-9.
  2. ↑ Smith C, Vogel C-W, Müller-Eberhard H (1984). "MHC Class III Products: An Electron Microscopic Study of the C3 Convertases of Human Complement". J Exp Med. 159 (1): R324–329. doi:10.1084/jem.159.1.324. PMC   2187187 . PMID   6559206.
  3. ↑ Pangburn, M K; Schreiber, R D; Müller-Eberhard, H J (1 October 1983). "C3b deposition during activation of the alternative complement pathway and the effect of deposition on the activating surface". The Journal of Immunology. 131 (4): 1930–1935. PMID   6225800.
  4. ↑ Kozlov, L. V; Shibanova, E. D; Zinchenko, A. A (1987). "Formation of classical C3 convertase during the alternative pathway of human complement activation". Biokhimiia (Moscow, Russia). 52 (4): 660–6. PMID   3647798.
  5. ↑ Hourcade D, Holers VM, Atkinson JP (1989). "The regulators of complement activation (RCA) gene cluster". Adv Immunol. Advances in Immunology. 45: R381–416. doi:10.1016/s0065-2776(08)60697-5. ISBN   9780120224456. PMID   2665442.
  6. ↑ Hourcade D (2006). "The Role of Properdin in the Assembly of the Alternative Pathway C3 Convertases of Complement". J Biol Chem. 281 (4): R2128–2132. doi: 10.1074/jbc.m508928200 . PMID   16301317.
  7. ↑ Fujita T; et al. (1987). "The Mechanism of Action of Decay-Accelerating Factor (DAF)". J Exp Med. 166 (5): R1221–1228. doi:10.1084/jem.166.5.1221. PMC   2189641 . PMID   2445886.
  8. ↑ Gigli I, Fujita T, Nussenzweig V (1979). "Modulation of the Classical Pathway C3 Convertase by Plasma Proteins C4 Binding Protein and C3b Inactivator". Proc Natl Acad Sci USA. 76 (12): R6596–6600. Bibcode:1979PNAS...76.6596G. doi: 10.1073/pnas.76.12.6596 . PMC   411913 . PMID   293746.
  9. ↑ Pangburn M, Müller-Eberhard H (1978). "Complement C3 Convertase: Cell surface restriction of β1H control and generation of restriction on neuroaminidase-treated cells". Proc Natl Acad Sci USA. 75 (5): R2416–2420. Bibcode:1978PNAS...75.2416P. doi: 10.1073/pnas.75.5.2416 . PMC   392564 . PMID   276881.
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