Factor H acts by competing with which of the following for the same binding site?

Factor H acts by competing with which of the following for the same binding site?

• A. Factor D
• B. Factor I
• C. Factor B
• D. C3b

Answer: (C)

Factor H regulates the alternative pathway by binding to C3b on host surfaces and blocking another protein from attaching there. For the amplification loop to proceed, Factor B must bind to C3b to form the C3bB complex, which Factor D then cleaves to create the active C3 convertase C3bBb. When Factor H occupies the binding site on C3b, Factor B cannot bind, so the C3 convertase isn’t formed and further amplification is halted. Factor I can then inactivate C3b with Factor H as a required cofactor, but that’s a separate step from the initial competition for binding. Therefore, Factor B is the molecule that competes with Factor H for the same binding site.

Factor H regulates the alternative pathway by binding to C3b on host surfaces and blocking another protein from attaching there. For the amplification loop to proceed, Factor B must bind to C3b to form the C3bB complex, which Factor D then cleaves to create the active C3 convertase C3bBb. When Factor H occupies the binding site on C3b, Factor B cannot bind, so the C3 convertase isn’t formed and further amplification is halted. Factor I can then inactivate C3b with Factor H as a required cofactor, but that’s a separate step from the initial competition for binding. Therefore, Factor B is the molecule that competes with Factor H for the same binding site.