Hydrolytic Activation of the Fifth Component of Complement (C5) by C3 Convertases Depends on its Binding to Surface-fixed C3b; Specific Interference with Binding, by Properdin, Factor B, or Propamidine, Leads to Inhibition of C5 Cleavage and Utilization

The C3 convertases of the classical and alternative pathways, and C3bBb (P) are capable of cleaving C5 only in the presence of C3b fixed to a solid surface (S-C3b) immediately subsequent to its release at that surface (e.g., on red cells, agarose, zymosan). The fact that fluid-phase and to a lesser degree soluble , attack C5 in the presence of S-C3b in EDTA medium, indicates that S-C3b interacts with and modulates the substrate, C5, rather than the convertase. In fact, reversible binding occurs between S-C3b and C5. Soluble C3b shows no indication of C5 binding, and does not support C5 cleavage by or C3b Bb. Cobra venom factor (CVF) which supports a B-dependent C5-cleaving enzyme, CVFBb, combines with C5 even in solution. While CVFBb cleaves C5 efficiently only when substrate (C5) and enzyme (Bb fragment) are bound to the same CVF molecule (free CVF added to a mixture of CVFBb and C5 is inhibitory), S-C3b has to be free of other ligands in order to function as a C5-binding site.