Question at position 40 Which of the following acts as a serine protease that, with cofactors, cleaves C3b and C4b?Factor HMCP (CD46)ProperdinDAF (CD55)Factor ISingle choice
A
Factor H
B
MCP (CD46)
C
Properdin
D
DAF (CD55)
E
Factor I

Question

Question at position 40 Which of the following acts as a serine protease that, with cofactors, cleaves C3b and C4b?Factor HMCP (CD46)ProperdinDAF (CD55)Factor I

BlackTom AI · Accepted Answer

Factor I is the serine protease that, together with specific cofactors, proteolytically inactivates C3b and C4b.

1) Relevant knowledge points and concepts
- The complement system has regulators that prevent uncontrolled activation. C3b and C4b must be inactivated to stop complement amplification and protect host cells.
- Factor I is a circulating serine protease produced by the liver that cleaves C3b and C4b, but it requires cofactors (e.g., Factor H, MCP/CD46, CR1/CD35) to present and orient the substrates.
- Other regulators: Factor H and MCP act as cofactors for Factor I; DAF (CD55) accelerates decay of C3/C5 convertases (not a protease); Properdin stabilizes the alternative pathway C3 convertase (not an inhibitor or protease).

2) Complete reasoning process step by step
- Identify the enzyme activity asked for: a serine protease that cleaves C3b and C4b.
- Recognize which complement proteins have protease activity: Factor I is a known serine protease in plasma.
- Note cofactors are required: the question includes cofactors, consistent with Factor I’s requirement for cofactors such as Factor H, MCP (CD46) or CR1.
- Therefore the molecule described must be Factor I.

3) Intermediate mechanistic steps (how cleavage occurs)
- Cofactor (e.g., Factor H or MCP) binds C3b or C4b on a surface.
- Factor I binds the cofactor–substrate complex.
- Factor I’s serine active site cleaves C3b into iC3b and smaller fragments (further to C3dg/C3c) or cleaves C4b similarly, rendering them inactive for formation of convertases.

4) Why the correct answer is correct
- Only Factor I among the options is a serine protease with the documented function of cleaving C3b and C4b in a cofactor-dependent manner.

5) Why the other options are incorrect
- Factor H: a regulatory cofactor that binds C3b and promotes Factor I activity, but it is not a protease.
- MCP (CD46): a membrane cofactor that helps present C3b/C4b to Factor I, not a protease.
- Properdin: a positive regulator that stabilizes the alternative pathway C3 convertase (C3bBb), not a protease or inactivator.
- DAF (CD55): accelerates decay (dissociation) of C3/C5 convertases, but does not cleave C3b/C4b.

6) Conclusion
This explanation confirms that Factor I is the serine protease which, with cofactors, cleaves C3b and C4b. The correct answer matches the provided answer: Factor I.

Question at position 40 Which of the following acts as a serine protease that, with cofactors, cleaves C3b and C4b?Factor HMCP (CD46)ProperdinDAF (CD55)Factor ISingle choice

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