Complement is a powerful host defense system that contributes to both innate and acquired immunity. There are three pathways of complement activation, the classical pathway, lectin pathway, and alternative pathway. Each generates a C3 convertase, a serine protease that cleaves the central complement protein, C3. C3 convertase can be used to refer to the form produced in the alternative pathway (C3bBb) or the classical and lectin pathways (C4bC2a). Once formed, both C3 convertases will catalyze the proteolytic cleavage of C3 into C3a and C3b (hence the name "C3-convertase"). Nearly all the biological consequences of complement are dependent on the resulting cleavage products.
C3b,Bb is the C3 convertase (EC 188.8.131.52) of the alternative pathway of complement. C3 enzyme is responsible for amplification of pathway activation and for deposition on target cells of C3b and the membrane attack complex. It is controlled by the serum proteins Factor H, Factor I (EC 184.108.40.206), and properdin. The formation of C3 convertase requires C3b, Factor B, Factor D (EC 220.127.116.11), and Mg2+. After formation of the reversible, bimolecular complex C3b,B(Mg), Factor D cleaves Factor B, releases the activation fragment Ba, and generates the active enzyme C3b,Bb, which has a calculated molecular weight of 239,000 (C3b, M, = 176,000; Bb, M, = 63,000). C3 enzyme is a serine protease whose catalytic site resides in the Bb subunit. It's inherently labile, and the spontaneous dissociation of its subunits results in irreversible loss of enzymatic activity. Due to this lability, previous attempts to directly demonstrate this enzyme by ultracentrifugation, electrophoresis, or molecular sieve chromatography have failed. Since the spontaneous decay dissociation of C3b,Bb is not accelerated by EDTA, it was uncertain whether the metal ion required for enzyme formation remained associated with the active C3 convertase. It has been found that Ni2+ can replace Mg in C3b,Bb formation and thathe enzyme formed with Ni (CSb,Bb(Ni)) is 6 to 10 times more stable than that formed with Mg (C3b,Bb(Mg)). Utilizing Ni and its radioisotope 63Ni, it was possible in the present study to demonstrate the active C3b,Bb complex by ultracentrifugation and to detect the metal ion in the complex.
Regulation of the amplification phase of the alternative pathway is exerted by multiple mechanisms: (i) intrinsic decay of C3 convertase, (ii) stabilization of this enzyme by properdin, (iii) disassembly of this enzyme by the serum glycoprotein ,β1H, (iv) inactivation of C3b by the CMb inactivator (C3bINA) and β1H, and (v) protection of C3 convertase from the action of these control proteins afforded by the surface properties of certain cells and other activators of the alternative pathway.
The classical complement pathway is initiated by the interaction of antigen-antibody complexes with the first component of complement, C1. The bound C1 develops proteolytic activity (C1) that is responsible for the activation of C4 and C2 and the formation of an enzymatic active complex (C4b2a or C42), the C3 convertase, which mediates the cleavage of C3 into C3a and C3b. The C42 enzyme is labile due to the decay of C2 (C2a), but it can be regenerated on the residual C4 (C4b) by the uptake of additional C2 and its cleavage by C1.
The orderly progression of the classical pathway is modulated, in part, by this instability of the C42 enzyme. Researchers found a previously unrecognized control of the classical C3 convertase by a normal serum protein, C4 binding protein (C4-bp). This protein serves as a cofactor for the enzyme C3b inactivator (C3bINA/CFI) in the further degradation of fluid-phase C4b and, to a lesser extent, C3b. The experiments described here demonstrate that C4-bp acts as a modulator of the C42 enzyme, accelerating the decay of C2a. In addition, they show that C4-bp participates, in conjunction with C3bINA, in the inactivation of the hemolytically active C4b bound to the erythrocyte membrane, thereby preventing the regeneration of the classical pathway convertase.
The classical and lectin pathways of complement are major recognition systems of innate immunity that are found in mammals and other animal species. By means of several multimolecular proteases, each comprising a recognition protein and a protease component, pathogens and other targets are detected and thereby trigger proteolytic reactions. The classical pathway is initiated by C1, a complex of the recognition molecule C1q and two associated enzymes, C1r and C1s, whereas the lectin pathway is initiated by carbohydrate recognition molecules (mannan-binding lectin (MBL) or one of the three ficolins or collectin-LK) in complex with another set of enzymes, MBL-associated serine proteases (MASPs). Both pathways converge to the formation of C3 convertase, a complex protease that cleaves C3, the central component of the complement system. Proteolytic cleavage of C3 generates a series of fragments and elicits various effector mechanisms, including inflammation and phagocytosis. These mechanisms contribute to the elimination of pathogenic microorganisms and altered host cells from blood and tissues and modulate the adaptive immune response.
1. Hourcade D E. (2006). The role of properdin in the assembly of the alternative pathway C3 convertases of complement. Journal of Biological Chemistry, 281(4), 2128-2132.
2. Fishelson Z, et al. (1983). C3 convertase of the alternative complement pathway. Demonstration of an active, stable C3b, Bb (Ni) complex. Journal of Biological Chemistry, 258(12), 7411-7415.