Terminal Sequence
The terminal sequence of complement activation involves C5b, C6, C7 C8 and C9. These components interact to form a structure known as the MAC (membrane attack complex) A large transmembrane channel is created that disrupts the membrane and enables ions and small molecules to diffuse freely. C5 is cleaved by the C5 convertase of either the Classical or Alternative Pathways. The C5 is cleaved into C5a and C5b. The C5a fragment diffuses away, and the C5b fragment binds to the surface of the target cell. This component is very labile and is inactivated within two minutes unless C6 binds and stabilizes its activity. Once C7 binds, the complex undergoes a hydrophilic-amphiphilic transition that exposes hydrophobic regions, which allow the complex to be inserted into the phospholipid bilayer. If the reaction occurs on an immune complex or other noncellular surface, the complex cannot be anchored and it is released causing "innocent bystander lysis". In a number of diseases in which immune complexes are produced, tissue damage results from such innocent-bystander lysis. Binding of C8 induces a structural change in C8 exposing a hydrophobic region, which interacts with the plasma membrane. A small pore ( 10Å ) is formed. The final step in the sequence is the polymerization of C9 ( a perforin-like molecule). As many as 10-16 C9s can be polymerized by a single C5bC6C7C8 complex. Once bound, C9 undergoes a hydrophilic-amphiphilic transition, so that it too can be inserted into the plasma membrane. The resulting hole is 70-100 Å in diameter. Even nucleated cells cannot maintain osmotic stability and are lysed by an influx of water and loss of electrolytes. Alternative Pathway of Complement
Once called the Properdin Pathway (misnomer because properdin plays a relatively small role). An alternative pathway for generating bound C5b, which is required for the formation of the MAC. Terminal reaction C5b-C9 is the same for both pathways. The alternative pathway is regarded as the more primitive pathway and is really part of our innate immune response. Four serum proteins are involved: C3 Factor B Factor D Properdin The Alternative Pathway is activated by various cell surface consitutents foreign to the host and includes: LPS (gram - cell wall) Techoic Acid (gram + cell wall) Zymosan (component of fungal cell wall) Some viruses and some virally infected cells Also several non-pathogenic substances The pathway is initiated due to a spontaneous "tick-over" of C3. Serum C3 is somewhat unstable (unstable thioester bond) and you get spontaneous cleavage to C3a and C3b. C3b binds to pathogen surface, viral particles and even the host's own cells. Most vertebrate cells have high levels of sialic acid on their surface which contributes to the rapid inactivation of bound C3b. Factors listed above tend to stabilize bound C3b. [N-acyl derivatives of neuramic acid ar generally called sialic acids. N-acetylneuramic acid is very often the terminal monosaccharide of glycoproteins. In the pathway, Factor B binds to bound C3b. C3b + Factor B------------> C3b.B (this bond is magnesium dependent) Once Factor B binds, there is a conformational change which exposes a site on Factor B where Factor D can cleave. _____ C3b.B-------Factor D---------> C3bBb + Ba (which diffuses away from the site) Properdin can bind to this complex and stabilize it. In the absence of properdin there is a half-life of < 5 minutes. With properdin, the half-life is extended to 30 minutes. ______ C3bPBb is analagous to C4bC2b of the classical pathway. [functions as a C3 convertase] In other words, unhydrolyzed C3 is activated to generate more C3b. _____ C3bPBb C3--------------------------------------------> C3b + C3a The initial steps in the pathway are repeated and amplified. > 2 X 106 molecules of C3b can be deposited in less than 5 minutes. In addition, the C5 convertase of the alternative pathway is formed. __________ C3bPBbC3b C5-----------------------------------------> C5b + C5a