H factor
Factor H was discovered by Nilson et al. (1965) and named it β1H according to the electrophoretic site, while Whaley and Ruddy named it C3b inactivator acceleration factor. It has been determined that it is a single-chain glycoprotein composed of 1213 amino acids, with a molecular weight of 155 kDa, and has both a long rod-shaped portion and a spherical region. A recent examination with a transmission electron microscope revealed that the image of the H factor is a long, pliable molecule. The elongated molecule is 49.5nn long and the cross-sectional diameter is 3.4mm, but most of it is not linear but folded. Therefore, the actual length of the molecule is only half of the stretched type, but its conformation is diverse. The analysis by garden dichroism shows that factor H has neither α helix nor β sheet, and maintains its functionally active conformation through disulfide bonds. The function of factor H includes the following aspects: (1) It is the auxiliary factor of factor I, which can increase the sensitivity of C4b to factor I. In the absence of factor H, the binding of factor I to C3b is filamentous; while in the presence of factor H, the binding of factor I to C3b becomes curved and the binding affinity with C3b is enhanced (compared to the absence of factor H 15 times higher). The mechanism by which factor H strengthens factor I may be that after factor H binds to C3b, it causes some conformational changes in C3b, increasing the affinity for factor I binding. The active site where factor H binds to C3b exists in its N-terminal 35 kDa part. (2) Accelerate the change of C3 convertase: Factor H can expel factor B or Bb that has been combined with C3b from C3 enzyme, and make it lose its enzyme activity. (3) Prevent the formation of initial and amplified C3 convertase in alternative pathways. It has been confirmed that factor H and factor B have the same binding site on C3b, so factor H can compete with factor B or Bb for binding to C3b. In the presence of factor H, factor B is not easy to combine with C3 (H2C) and C3b, so it is not easy to form C3 (H2C) Bb or C 3bBb. However, the factor H has a different effect on C3b on the solid phase and in the liquid phase. Cleavage in the liquid phase or in combination with the non-activator solid phase. For C3b immobilized on the surface of an activator (such as yeast polysaccharides, etc.), factor H has an affinity for C3b that is comparable to factor B. As a result of the competition between the two, some C3 convertases can be formed to ensure the activation of alternative pathways. Studies have reported that the chemical components that can enhance the affinity of C3b for factor H on cell membranes are sialic acid and heparin aminopolysaccharides. Due to the lack of sialic acid on the surface of most bacteria, these bacteria can activate alternative pathways after they invade the body, which helps to control the infection at an early stage. (4) Factor H has a certain effect on C 3bBbP or C 3bBbNeF that has been combined with P factor or nephritis factor (NeF), but its efficacy is much worse than CR1. Factor H also has an inhibitory effect on the activity of C5 convertase (c 3bnBb or C 3bBb3b) and can compete with C5 to bind C3b so that C5 cannot be cleaved. In addition, it has been discovered in recent years that factor H can also induce monocytes to secrete IL-1 and participate in the regulation of immune responses.
The gene encoding factor H is located in the long arm region 32 of human chromosome 1 and has polymorphism. It has been found to have 5 variants, namely FH1-5. The nucleotide sequence of factor H has been identified, and the primary structure of all its amino acids has been deduced, containing 20 SCRs to bind to C3b. Factor H has homology with MCP, CR1, CR2, DAF, and C4bp, and is a member of the complement activation regulator (RCA) gene family.
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