Structure
Cell-bound C3 and C5 convertase differ in their C3b requirement. C3-convertase (C3bBb) need only one molecule of C3b to form, whereas two or more C3b are required for generation of C5 convertase (C3bBb). It means, when C3b is randomly distributed on the surface of a cell, only C3 convertase activity appears after addition of Factors B and D. However, when C3b is distributed in clusters, C3 and C5 convertase activity is generated upon addition of Factors B and D. [3]
The classical pathway C5 convertase is composed of the fragments of complement proteins, C4b, C2a produced by cleavage mediated by C1 complex , and C3b produced by cleavage mediated by the classical pathway C3 convertase (C4bC2a). The formation of the alternative pathway C5 convertase (C3bBbC3b) starts by spontaneous cleavage of C3 protein exposing previously hidden thioester bond. In the presence of pathogen the fragment C3b binds to microbial cell-surface through the newly showed thioester bond. On the other hand, if the infection does not occur, C3b interacts with molecules of water, therefore the protein becomes inactive. However, when C3b undergoes its post-cleavage conformational change, a binding site for a plasma protein called Factor B is also exposed. Factor B then binds to C3b and is cleaved by a plasma serine protease Factor D . The C3bBb complex (= alternative pathway C3 convertase) remains attached to the cell-surface. This complex might interact with another C3b and thus form the alternative pathway C5 convertase. [4] CVFBb is a noncovalent association product of CVF3 and the complement fragment Bb. The catalytic subunits of these multimolecular proteases are C2b and Bb. These subunits belong to atypical serine proteases. [5] [6] CVFBb does not require C3 for cleavage of C5, whereas C4b2boxy need native C3 for cleavage of C5 protein. The modified C5 convertase, C4b2boxy3b, contains C2b that is derived from C2 oxidized by iodine.
Function
The target of C5 convertase is complement protein C5. C5 is a two-chain (α, β) plasma glycoprotein (Mr = 196,000). C5 and C3 have similar structure. However, C5 does not appear to contain the internal thiol ester group reported for C3 and C4. C5 has relatively few disulfide bonds. There are three disulfide bonds in C5a, the α-chain has 15 half-Cystines, and the β-chain has only 6 half-Cystines. This comparatively low level of stabilizing disulfide bridges may provide a partial explanation for the irreversible conformational change imparted on C5 after cleavage to C5a and C5b. In addition, the relatively low number of disulfide bonds could account for instability of C5 when exposed to chaotropic agents such as potassium thiocyanate. [2] Electron micrographs of negatively stained C5 indicate that the protein is irregular in shape and contains several lobes. [7]
First of all, C5 has to bind to C3b fragment. The capacity to bind C3b is a stable feature of component C5, as C5b also has this binding capacity. The C5 convertase selectively cleaves an Arginyl-Leucine peptide bond at position 74-75 in the α-chain (Mr = 116,000) of C5. Research has shown that during the classical pathway of the complement system, an inactive A6 allotype of c4 completely stalls the molecules' ability to act as a c5 binding subunit1. [8] This defect in C4A6 activity happens during the C5 binding step to the 4b and c3b complex. [9] α´-chain (Mr, = 105,000) and the activation peptide, C5a, is formed, while β-chain (Mr = 80,000) remains unchanged. [2]
The complement component C5 can be also activated by fluid phase C5 convertase. C5 is activated by CVFBb in the presence of complement component C6 and the C5b6 complex is formed. However, when C6 is added after C5 has been converted to C5b, the C5b6 complex fails to form. Therefore, the activation of C5 results in a transient binding site for C6. Hydrophobic sites are probably exposed upon C5 activation because C5b undergoes aggregation when C5 is converted to C5b in the absence of C6. Interactions between C5 and C6 or C5 and membranes are noncovalent. (In contrast, it is the labile thiol ester that permits covalent attachment between C3 and nucleophilic acceptors.) The proteolytic cleavage of C5 is the only known enzymatic event in assembly of the cytolytic membrane attack complex of complement. [7]
Once bound, C5 is exceptionally efficient in producing hemolysis, requiring less than seven specifically bound molecules per cell for the production of a hemolytic lesion. The extent of formation of the C5 intermediate complex is primarily dependent on the number of molecules of C4, C2 and C3 present on the cells employed for its generation. In these respects, the mode of action of C5 is completely analogous to that of the other components of complement. The C5 step differs, however, in other aspects. The binding of C5 is influenced by C6 and C7, components which are thought to act subsequent to it in the complement sequence. In addition, the hemolytic activity of the isolated C5 intermediate complex is exceedingly labile, having an average half-life at 30 °C of only 9 rain. This characteristic distinguishes the C5 step, along with the C2 step, as potentially rate-limiting in the complement reaction. However, unlike C2, C5 remains firmly cell-bound during the decay process and apparently undergoes an alteration in situ which renders it hemolyticly unreactive. Finally, C5 is unique in that it readily adsorbs in native form to unsensitized erythrocytes. This nonspecifically bound C5 remains firmly attached, although it may be specifically utilized as a source of C5 by an ongoing complement reaction. [1]
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.
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.
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.
C3 convertase 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.
The membrane attack complex (MAC) or terminal complement complex (TCC) is a complex of proteins typically formed on the surface of pathogen cell membranes as a result of the activation of the host's complement system, and as such is an effector of the immune system. Antibody-mediated complement activation leads to MAC deposition on the surface of infected cells. Assembly of the MAC leads to pores that disrupt the cell membrane of target cells, leading to cell lysis and death.
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.
C5a is a protein fragment released from cleavage of complement component C5 by protease C5-convertase into C5a and C5b fragments. C5b is important in late events of the complement cascade, an orderly series of reactions which coordinates several basic defense mechanisms, including formation of the membrane attack complex (MAC), one of the most basic weapons of the innate immune system, formed as an automatic response to intrusions from foreign particles and microbial invaders. It essentially pokes microscopic pinholes in these foreign objects, causing loss of water and sometimes death. C5a, the other cleavage product of C5, acts as a highly inflammatory peptide, encouraging complement activation, formation of the MAC, attraction of innate immune cells, and histamine release involved in allergic responses. The origin of C5 is in the hepatocyte, but its synthesis can also be found in macrophages, where it may cause local increase of C5a. C5a is a chemotactic agent and an anaphylatoxin; it is essential in the innate immunity but it is also linked with the adaptive immunity. The increased production of C5a is connected with a number of inflammatory diseases.
Properdin is a protein that in humans is encoded by the CFP gene.
The lectin pathway or MBL 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
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.
Complement component 5 is a protein that in humans is encoded by the C5 gene.
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.
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.
Complement factor B is a protein that in humans is encoded by the CFB 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.
Complement factor I, also known as C3b/C4b inactivator, is a protein that in humans is encoded by the CFI gene. Complement factor I is a protein of the complement system, first isolated in 1966 in guinea pig serum, that regulates complement activation by cleaving cell-bound or fluid phase C3b and C4b. It is a soluble glycoprotein that circulates in human blood at an average concentration of 35 μg/mL.
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.
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.
