Which molecule forms the C3 convertase of the Lectin/Classic pathway?

Which molecule forms the C3 convertase of the Lectin/Classic pathway?

• A. C3bBb
• B. C4b2a3b
• C. C4b2b
• D. C3bBbC3b

Answer: (C)

In the lectin and classical complement pathways, the C3 convertase is the enzyme that cleaves C3 into C3a and C3b. This convertase is formed when C4b binds to the surface and then associates with C2a, creating the complex C4b2a. That C3 convertase then acts on C3. If a C3b molecule joins this complex, you obtain the C5 convertase, described as C4b2a3b. By contrast, the alternative pathway uses a different C3 convertase, C3bBb, with C3bBbC3b acting as the C5 convertase. The key point: the C3 convertase of the lectin/classic pathway is C4b2a, not C4b2b, which would involve the wrong component (C2b) and does not form the active C3 convertase.

In the lectin and classical complement pathways, the C3 convertase is the enzyme that cleaves C3 into C3a and C3b. This convertase is formed when C4b binds to the surface and then associates with C2a, creating the complex C4b2a. That C3 convertase then acts on C3. If a C3b molecule joins this complex, you obtain the C5 convertase, described as C4b2a3b. By contrast, the alternative pathway uses a different C3 convertase, C3bBb, with C3bBbC3b acting as the C5 convertase. The key point: the C3 convertase of the lectin/classic pathway is C4b2a, not C4b2b, which would involve the wrong component (C2b) and does not form the active C3 convertase.