Factor B is a zymogen that carries the catalytic site of the complement alternative pathway C3 convertase. During convertase assembly, factor B associates with C3b and Mg 2+ forming a pro-convertase C3bB(Mg 2+) that is cleaved at a single factor B site by factor D. In free factor B, a pair of salt bridges binds the Arg 234 side chain to Glu 446 and to Glu 207, forming a double latch structure that sequesters the scissile bond (between Arg 234 and Lys 235) and minimizes its unproductive cleavage. It is unknown how the double latch is released in the pro-convertase. Here, we introduce single amino acid substitutions into factor B that preclude one or both of the Arg 234 salt bridges, and we examine their impact on several different pro-convertase complexes. Our results indicate that loss of the Arg 234-Glu 446 salt bridge partially stabilizes C3bB(Mg 2+). Loss of the Arg 234-Glu 207 salt bridge has lesser effects. We propose that when factor B first associates with C3b, it bears two intact Arg 234 salt bridges. The complex rapidly dissociates unless the Arg 234-Glu 446 salt bridge is released whereupon conformational changes occur that activate the metal ion-dependent adhesion site and partially stabilize the complex. The remaining salt bridge is then released, exposing the scissile bond and permitting factor D cleavage.