Explain how the alternative C3 convertase on pathogen cell surfaces is (A) formed and (B) stabilized
What will be an ideal response?
A. Spontaneous hydrolysis of C3 without cleavage exposes its highly reactive thioester bond, forming iC3. Factor B binds to iC3, is cleaved by factor D, and consequently releases a small fragment called Ba. The larger fragment, Bb, remains associated with iC3 to form iC3Bb, a soluble C3 convertase, which cleaves C3 into C3a and C3b. The reactive thioester bond of C3b is attacked by R–OH and R–NH2 groups on the surface of the pathogen, where it becomes anchored and binds to factor B. Factor D then cleaves factor B, releasing fragment Ba and forming C3bBb on the pathogen surface.
B. Factor P (properdin) binds to C3 convertase (C3bBb) bound to the pathogen surface, and inhibits the proteolytic degradation of C3bBb. This stabilizes the C3 convertase and enhances the rate of C3b deposition on the pathogen surface.
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