C3 convertase activation

Fodor WL, Rollins SA, Bianco-Caron S, Rother RP, Guilmette ER, Burton WV, Albrecht JC, Fleckenstein B, Squinto SP (1995) The complement control protein homolog of herpesvirus saimiri regulates serum complement by inhibiting C3 convertase activity. J Virol 69 3889-3892... 

Several protease inhibitors from plasma, tissue, and plant sources did not inhibit the esterase and C3 convertase activity (Cooper, 1975b). Recently, Medicus et al. (1976a) proposed that C2a is a serine esterase. 

Proteins B, D and P also amplify the effects ofthe classical pathway in that some of the 3b generated by this pathway interacts with these proteins to form additional C3 convertase that supplements that provided by C4b.2a. Likewise, enhanced cleavage of C5 occurs due to the dual activity of C4.2a.3b and C3b.Bb.C3b complexes. 

Figure 1.15. Complement activation via the alternative pathway. Always present in serum are trace amounts of C3b, which may attach to recognition sites on, for example, yeast cell walls. C3b may combine with serum factor B to form C3bB, and this complex is acted upon by factor D to form C3bBb. This latter complex is a C3 convertase and may act upon C3 to form more C3b to amplify the process. The C3bBb-coated particles may activate other complement components (C4-C9) or be recognised by complement receptors on neutrophils.

Any one of these pathways, or all three, result in proteolytic cleavage of a protein known as C3 convertase to produce the active form, C3b. The latter is involved in different mechanisms that kill bacteria (Figure 17.7). 

Figure 17.24 Activation of C3 convertase and effects of activation. There are three pathways that activate complement and four mechanisms that facilitate the killing of pathogens (see text). C3 is the convertase enzyme.

The classic pathway is triggered by the formation of factor Cl at IgG or IgM on the surface of microorganisms (left). Cl is an 18-part molecular complex with three different components (Clq, Clr, and Cls). Clq is shaped like a bunch of tulips, the flowers of which bind to the Fc region of antibodies (left). This activates Clr, a serine proteinase that initiates the cascade of the classic pathway. First, C4 is proteolytically activated into C4b, which in turn cleaves C2 into C2a and C2b. C4B and C2a together form C3 convertase [1], which finally catalyzes the cleavage of C3 into C3a and C3b. Small amounts of C3b also arise from non-enzymatic hydrolysis of C3. 

The anticoagulant activity of dextran derivatives were assessed by measuring the thrombin clotting time (ThNIH units) of freshly prepared platelets from plasma in the presence of the CMD, CMDB, CMDBSSu and CMDSu polymers and of human thrombin (1094 NIH units mL 1). The anticomplementary activity was expressed as the amount of polymer that inhibits 50% formation of the alternative and classical pathway C3 convertase [220,290,291]. 

There is an alternative pathway for the activation of the membrane-attack complex that can act quickly after infection, not needing to wait for the production of specific antibodies. In the alternative pathway a small amount of C3b, which apparently is produced continuously in low amounts, binds with a protein called factor B. C3b,B can then be cut by another protein, factor D, to give C3b,Bb. This can now act as a C3 convertase. When more C3b is made, a second molecule of C3b can attach to yield (C3b)2Bb. Remarkably, this is now a C5 convertase, which produces C5b, which then goes on to start the formation of the membrane-attack complex in the way described above for the first pathway. 

Figure 15.1 Complement activation pathways. The classical, lectin and alternative pathways converge into a final common pathway when C3 convertase (C3 con) cleaves C3 into C3a and C3b. Ab = antibody, Ag = antigen, Cl-INH = Cl inhibitor, MAC = membrane attack complex, MASP = MBL-associated serine protease, MBL = mannose-binding lectin, P = properdin. Overbar indicates activation.

Fig. 8.3 A highly schematized overview of the activation cascade for the alternative complement pathway on a microbial membrane surface. In the presence of a microbial membrane the C3b formed by C3 tickover deposits on the microbial membrane (step A). C3a diffuses away leading to leucocyte activation. The deposited C3b leads to the generation of a stabilized C3 convertase (step B) which, through a positive feedback loop, leads to the amplified cleavage of more C3. Some C3b associates with the C3 convertase to generate a C5 convertase (step C) which will eventually lead to the generation of an...

This newly generated stable C3 convertase enzymically cleaves C3 to generate further C3b and C3a molecules, leading to leucocyte activation (by... 

C3a) and greater deposition of C3b on the microbial membrane and hence further generation of C3 convertase molecules. In effect the microbial membrane has activated a positive feedback loop with cleavage of C3 to generate high amounts of C3b and C3a molecules. 

Differences between host cell membranes and microbial cell membranes mean that the cascade is only activated in the presence of microorganisms, so C3 tickover cannot give rise to full activation of the alternative pathway in the absence of microbial membrane. Stable deposition of a functional C3 convertase only occurs on the microbial cell surface. The differences that exist include, for example ... 

P (properdin) can also bind to the C3 convertase. Its role is to stabilize the complex and hence it is considered a cofactor-activator in the alternative pathway. These components plus additional C3b molecules form the C5 convertase, an enzyme complex that peoteolytically converts C5 to C5a and CSb. Properdin stabilizes C3b and Bb in the complex and protects these proteins from proteolytic inactivation by factor I. Factor H competes for Bb in the C5 convertases, the same as it does in the C3 convertase. The alternative pathway has also been called the properdin pathway because of properdin s participation in alternative pathway C3 and C5 convertases. C5b is a component in the terminal complex of the complement activation process, the MAC. The MAC is composed of a self-assembled, noncovalent complex of C5b, C6, C7, C8, and C9. Together these components produce a pore-like structure that makes the membrane of the cell to which it is attached permeable and causes cell death. Under the electron microscope the MAC appears like an impact crater similar to those observed on the surface of the moon. C5a is also an anaphylatoxin like C3a, but it is more potent. C5a is also a chemokine and attracts phagocytic cells to the site of complement activation. 

The next step in the sequence of classical pathway activation is cleavage of components C4 and C2. Two products these cleavages, C4b and C2a, form the classical pathway C3 convertase. Proteolysis of C3 leads to a second convertase. The second convertase is the C5 convertase and is composed of C4b, C2a, and C3b. This convertase cleaves C5 to form C5b and C5a. 

Activation of C2 and C4 produces the C3 convertase and, via the same reactions sequences of the classical pathway, leads to formation of the MAC. 

Some of the best known anti-inflammatory triterpenoids have been shown to have inhibitory activity on the complement cascade. A mixture of the aforementioned boswellic acids reduced in a dose-dependent manner the classic pathway activity by as much as 77%, and C3-convertase by 72% [118]. Oleanolic acid showed 85% and 71% inhibition, respectively, in the same tests, at a single dose of 100 jxg/ml [119], and P-glycyrrhetinic acid inhibited the classic human pathway with an IC50 of 35 xM. The component affected was C2. The a form of glycyrrhetinic acid was fairly inactive [120]. None of these three triterpenoids appreciably inhibited the alternative complement pathway. 

Linhardt, R. J., Rice, K. G., Kim, Y. S., Engelken, J. D., Weiler, J. M. (1988). Homogeneous, structurally defined heparin-oligosaccharides with low anticoagulant activity inhibit the generation of the amphfication pathway C3 convertase in vitro. Journal of Bio-logical Chemistry, 263(26), 13090—13096. 

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