C4b-binding protein alpha chain (C4BPA)

Controls the classical pathway of complement activation. It binds as a cofactor to C3b/C4b inactivator (C3bINA), which then hydrolyzes the complement fragment C4b.

It also hastens the degradation of the C4bC2a complex (C3 convertase) by dissociating the complement fragment C2a. Alpha string binds C4b.