A 302-amino-acid fragment in the alpha chain (672-1663) of C3b. It is generated when C3b is inactivated (iC3b) and its alpha chain is cleaved by complement factor i into C3c, and C3dg (955-1303) in the presence complement factor h. serum proteases further degrade C3dg into C3d (1002-1303) and C3g (955-1001).