Core hinge of human immunoglobulin G3 as a system of four independent co-operative blocks.

On the heat absorption curves of human immunoglobulin G3 (hIgG3) Kuc melting the scanning calorimetry method reveals a high-temperature (high-T(m)) peak of high intensity that is absent at the curves of other hIgG subclasses and IgG of other species. An analogous peak is observed also at the curves of melting of hIgG3 fragments containing the hinge segments. The high-T(m) peak is accompanied by characteristic changes in circular dichroism (CD) spectra at 220-230 nm. This allows relating the peak to the melting of a poly-L-proline conformation of an extremely long hIgG3 core hinge. The comparison of deltaH(cal) and deltaH(eff) testifies that the core hinge can be considered as a system of four independent co-operative blocks connected by flexible sites. These sites may provide additional flexibility to the hIgG3 molecule and also permit a transition of the rod-like shape of the hinge to compact globule-like conformation.