The complement system is an important part of the innate immune response and plays a major role in the killing and clearance of invading pathogens, including bacteria, viruses, fungi, and parasites. The system is composed of over 30 proteins, most of which are found in serum, while the remaining proteins are membrane-bound proteins and receptors. On activation, complement proteins that participate in the activation of either the classical or alternative pathways of complement.
interact in a highly specific enzymatic cascade, and generate proteolytic fragments that mediate the numerous biological functions of the complement system. In this video, we will focus on the activation and biological functions of complement. Activation of the classical pathway The classical pathway is typically activated by antigen-antibody complexes. Although there are many other activators of this pathway, Only two isotypes of antibody, IgM and as shown here IgG, will activate the classical pathway. While one pentameric IgM antibody will activate the classical pathway, a minimum of two IgG antibodies in close proximity are required.
In either case, antibody must be bound to antigen in order to activate the classical pathway. The classical pathway is activated when C1, the first protein in the pathway, binds to the FC portion of the antibody. C1 is a large macromolecule composed of a C1q molecule and two molecules each of C1r and C1s.
It's the globular head portion of the C1q that binds to the FC binding site on the antibody. This binding causes a conformational change in C1q and subsequent autocatalytic conversion of C1r to an active serine protease. Activated C1R cleaves C1S to an active serine protease.
Activation of C1 by binding antibody is also termed complement fixation. The next step in the activation of the classical pathway is the cleavage of C4 by the activated C1 molecule. C4 is cleaved by the activated C1S molecule in C1 into two proteolytic fragments, a small peptide called C4A and a larger fragment, C4B.
C4b attaches covalently to the antigen antibody complex and has a binding site for C2, the next protein cleaved in the classical pathway. C2 is also cleaved into two fragments, and the resulting C2a fragment binds to C4b. This bimolecular complex of C4b2a is the C3 convertase of the classical pathway and serves to cleave C3 through the enzymatic activity in the C2a portion of the complex.
Classical Pathway C3 Convertase The C3 molecule is cleaved by the C3 convertase into C3a and C3b. The C3a fragment has potent biological activities, which will be discussed shortly. The C3b fragment can covalently attach to the C3 convertase, resulting in a trimolecular complex consisting of C4b, 2a, 3b. This multi-molecular complex is called the C5 convertase of the classical pathway and is specific for cleaving C5. Classical Pathway C5 Convertase The C5 molecule is cleaved by the C5 convertase into C5a and C5b, both of which mediate important host defense functions of the complement system.
Both C3a and C5a are potent chemoattractants. C3a attracts mast cells to sites of complement activation, and binding of C3a and C5a to these cells, as well as basophils, induces degranulation and release of histamine and other vasoactive amines. C5a is a chemoattractant for macrophages and neutrophils, and binding of C5a to these cells primes them for mediating their host defense functions. The terminal complement pathway or formation of the membrane attack complex.
One of the best known host defense functions of the complement system is its ability to detect and detect the host's attack. ability to lyse many bacteria, envelope viruses, and nucleated cells. Lysis of cells by the complement system is mediated by a large macromolecular structure called the membrane attack complex or MAC.
The MAC is formed through what is called the terminal complement pathway which starts with the generation of C5B. C5B associates with C6, C7, and C8, forming a large multi-molecular complex that associates with and begins to disrupt the cell membrane. Subsequently, C9 binds to C8, and this is followed by the binding of many additional C9 molecules, leading to the formation of a large pore in the cell membrane.
The pore in the membrane is large enough to allow water, ions, and small molecules to enter the cell, leading to lysis. Activation of the Alternative Pathway The Alternative Pathway of complement is activated by LPS and other lipopolysaccharides found on the surface of invading pathogens. The Alternative Pathway is always active at a very low level.
This activation is due to the spontaneous hydrolysis of an internal thiol-ester bond in C3. Hydrolyzed C3, called C3 water, has a binding site for factor B, another protein in the alternative pathway. Factor B binds to C3 water and becomes a substrate for cleavage by factor D, denoted here as FD.
Factor D cleaves factor B into two fragments, BA and BB. The enzymatically active BB fragment remains associated with C3 water, and this bimolecular complex is the alternative pathway C3 initiation convertase. Alternative Pathway C3 Initiation Convertase This initiation convertase cleaves C3 into two fragments, C3A and C3B.
The newly generated C3B can covalently attach to nearby surfaces, just as C4B does in the classical pathway. In the absence of an appropriate activator for the alternative pathway, Complement regulatory proteins block further activation of the pathway. However, C3b, on interacting with an activating surface such as a bacterial cell, will bind factor B, which will be cleaved by factor D, resulting in the formation of the alternative pathway C3 convertase of C3b, Bb. Alternative pathway C3 convertase. The C3 convertase of the alternative pathway cleaves C3 to C3a and C3b.
Some of the C3b generated by this cleavage does not interact with the activating surface and remains soluble. Some of the C3b generated will covalently interact with the activating surface and start the formation of additional alternative pathway convertases, thus rapidly amplifying activation of complement through this pathway. The surface of an activator such as a bacterial cell, can rapidly be covered with many thousands of C3b molecules in a few minutes as a result of this amplifying effect.
And, as with the classical pathway, some of the C3b generated will interact with the C3 convertase itself, resulting in the formation of a trimolecular complex of two C3b molecules and Bb. This multi-molecular complex is called the C5 convertase of the alternative pathway. and is specific for cleaving C5.
The C3a and C5a fragments generated by activation of the alternative pathway mediate the same functions of chemoattracting and activating granulocytes and phagocytic cells. C3a attracts mast cells to sites of complement activation, and binding of C3a and C5a to these cells, as well as basophils, induces degranulation and release of histamine and other vasoactive amines. C5a is a chemoattractant for macrophages and neutrophils, and binding of C5a to these cells primes them for mediating their host defense functions. Activation of the alternative pathway results in the formation of the membrane attack complex, leading to the lysis of the invading pathogen. Lysis of cells by the complement system is mediated by a large macromolecular structure called the membrane attack complex, or MAC.
The MAC is formed through what is called the terminal complement pathway, which starts with the generation of C5b. C5b associates with C6, C7, and C8, forming a large multi-molecular complex that associates with and begins to disrupt the cell membrane. Subsequently, C9 binds to C8, and this is followed by the binding of many additional C9 molecules, leading to the formation of a large pore in the cell membrane.
The pore in the membrane is large enough to allow water, ions, and small molecules to enter the cell. Leading to lysis.