Which of the following is referred to as C3 convertase?

Which of the following is referred to as C3 convertase?

• A. C1qrs
• B. C3bD
• C. C3bBb
• D. C4b5a

Answer: (C)

C3 convertase is the enzyme that cleaves C3 into C3a and C3b, driving the downstream complement responses. There are two forms: in the classical and lectin pathways the C3 convertase is C4b2a, while in the alternative pathway the C3 convertase is C3bBb. The C3bBb complex forms when C3b on a surface binds factor B, which is then cleaved by factor D to Bb; this complex is stabilized by properdin and then cleaves fresh C3 molecules. Among the options, the C3bBb complex is the one that specifically functions as the C3 convertase, making it the best answer. The initiating C1 complex (C1qrs) starts the classical pathway, and while C5 convertases exist (such as C4b2a3b in classical or C3bBb3b in the alternative), C3 convertase is defined by its ability to cleave C3, which is the role of C3bBb here.

C3 convertase is the enzyme that cleaves C3 into C3a and C3b, driving the downstream complement responses. There are two forms: in the classical and lectin pathways the C3 convertase is C4b2a, while in the alternative pathway the C3 convertase is C3bBb. The C3bBb complex forms when C3b on a surface binds factor B, which is then cleaved by factor D to Bb; this complex is stabilized by properdin and then cleaves fresh C3 molecules. Among the options, the C3bBb complex is the one that specifically functions as the C3 convertase, making it the best answer. The initiating C1 complex (C1qrs) starts the classical pathway, and while C5 convertases exist (such as C4b2a3b in classical or C3bBb3b in the alternative), C3 convertase is defined by its ability to cleave C3, which is the role of C3bBb here.