Human myeloma immunoglobulins of the fourth subclass (IgG4 MAM) contain a fraction with different properties of CH2 domains.

A long-lived metastable minor fraction has been detected and characterized in myeloma protein IgG4 MAM by hydro- and thermodynamic methods. The sedimentation constants of the minor and the major protein fractions are different. The stability of the two CH2 domains in the minor fraction varies. The unique characteristics of these IgG4 MAM conformers arise from the fact that on exchange of the heavy chains between IgG4 molecules, in some of them only one noncanonical bond Cys226-Cys229 is formed in the central part of the "hinge region" instead of two canonical interchain disulfide bonds Cys226-Cys226 and Cys229-Cys229. This leads to asymmetric structure of the IgG4 MAM molecules.