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Old 03-10-2012
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Immunology What is C3 Convertase?

Several different sources have C3 convertase as C4b2b (HY, goljan) while others have it as C4b2a (FA 2011, corrected it on their website from 2b to 2a)

Does anyone know which one is correct.
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Old 03-10-2012
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Default C4b2a

C4b and C2a form the classical complement C3 convertase.

It should be in your immunology book from school or Harrison's if you still have it around.
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Old 03-10-2012
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Look through my immunobiology book by Janeways and it say its C4b2b.

Oh well not gonna stress over it since there are too many conflicting sources.
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Old 05-08-2012
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I was just wondering about this. FA 2012 errata corrected it to C4b2b ("In the image for the lectin and classical pathways, the C3 convertase is C4b2b (not C4b2a)."

Any further clarification on this or not so much?
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Old 05-09-2012
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Complement components are numbered in the order in which they were discovered. During activation, some complement components are split into two parts. The larger part of the molecule called "b" while the smaller fragment called "a" may diffuse away. In most cases it is the "b" fragment binds to the surface of the cell to be lysed (the fragments of C2 are an exception to this rule: C2a binds to the membrane while C2b is freed into serum or tissue spaces).

Activated C1s enzymatically cleaves C4 into C4a and C4b. C4b binds to the membrane while C4a remains a biologically active peptide at the reaction site. C4b binds C2, which becomes susceptible to C1s and is cleaved into C2a and C2b. C2a remains complexed with C4b whereas C2b is released. C4b2a complex is known as C3 convertase.

But Now things are different. C2a is released into tissue spaces and C2b remains associated with C4b. forming new C4b2b complex..

Hope it helps.
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Old 06-03-2012
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@Alisom: Thanks.

But if C4b2a is C3 convertase, isnt it this complex which combines with c3b to form c5 convertase?

Why is there even a C4b2b complex? And if C2a is released into tissue space instead of C2b, how will c3 convertase be formed?

Confused.
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