The alternatively folded state of the antibody C(H)3 domain

The C(H)3 domain of antibodies is characterized by two antiparallel beta -s heets forming a disulfide-linked sandwich-like structure. At acidic pH valu es and low ionic strength, C(H)3 becomes completely unfolded. The addition of salt transforms the acid-unfolded protein into an alternatively folded s tate exhibiting a characteristic secondary structure. The transition from n ative to alternatively folded C(H)3 is a fast reaction. Interestingly, this reaction involves the formation of a defined oligomer consisting of 12-14 subunits. Association is completely reversible and the native dimer is quan titatively reformed at neutral pH. This alternatively folded protein is rem arkably stable against thermal and chemical denaturation and the unfolding transitions are highly cooperative. With a t(m) of 80 degreesC, the stabili ty of the alternatively folded state is comparable to that of the native st ate of C(H)3. The defined oligomeric structure of C(H)3 at pH 2 seems to be a prerequisite for the cooperative unfolding transitions. (C) 2001 Academi c Press.