Factor D cleaves factor B when the latter is complexed with factor C3b, activating the C3bbb complex, which then becomes the C3 convertase of the alternate pathway. Its function is homologous to that of C1s in the classical pathway. Selective cleavage of Arg-|-Lys bond in complement factor B when in complex with complement subcomponent C3b or with cobra venom factor. N-glycosylated. Belongs to the peptidase S1 family.