A thermodynamic study of cooperative structures in rabbit immunoglobulin G.

The intramolecular melting of rabbit immunoglobulin G and its proteolytic fragments has been studied by scanning microcalorimetry in the acidic pH region. By thermodynamic analysis of the heat capacity function the following conclusions have been made. 1. The Fab and Fc fragments in IgG molecules are thermodynamically practically independent subunits in the pH region studied. 2. The Fab fragment exhibits three cooperative transitions at melting. Two of these transitions are explained assuming that equivalent domains in this fragment are paired, forming two cooperative blocks, while the third transition seems to correspond to the disruption of contacts between these cooperative blocks. 3. Fc and pFc' fragments exhibit four and two cooperative transitions, respectively. These transitions can be explained only by assuming that each of the structural domains of these fragments consists of two subparts and that the pairs of equivalent subparts are merged into the cooperative blocks thus forming four cooperative blocks in the Fc fragment.