The solution shapes of IgG3 immunoglobulin and its Fch and Fc fragments. A small-angle X-ray scattering study.

A homogeneous IgG3 protein and its corresponding Fch and Fc fragments have been studied in solution using the small-angle X-ray scattering method. The Fch and Fc fragments were produced by short digestion of IgG3 with papain and trypsin. The results indicate that the overall shape of the IgG3 molecule in solution can best be described as an elliptical cylinder with a total length of 29 nm and with a cross-section having the semiaxes 3.8 and 0.9 nm. Thus, the overall shape of IgG3 is considerably different from the Y-shape normally adopted for the IgG1 molecule. The analysis of the data obtained for the Fch and Fc fragments also yields elliptical cylinders with almost the same dimensions of the cross-section but with shorter total lengths, 11.4 and 6.7 nm, respectively. The molecular weights of the IgG3 protein and the Fch and Fc fragments were determined to be 1.8 x 10(5), 0.61 x 10(5), and 0.50 x 10(5), respectively.