Small-angle scattering study of α₁ inhibitor III from rat blood plasma

The α₁ proteinase inhibitor III from rat blood plasma, homologous to the α₂-macroglobulin family of proteins, has been studied in solution using small-angle scattering of X-rays and of neutrons: the radius of gyration, R_g, was found to be 4.5 nm, and the largest distance within the molecule, D_max = 14 nm. When the inhibitor reacts with chymotrypsin or methylamine, the resulting derivatives yield slightly higher R_g-values, 4.7 and 4.85 nm, respectively. The data of the native protein are consistent with a model, the projection of which resembles the letter V and which is formed by the two identical halves of an elliptic cylinder with semi-axes of 2.1 and 5.5 nm and a length of 11 nm. This elliptic cylinder model also explained the scattering from the monomeric complement proteins C3 and C4, as well as that from the monomers of the dimeric and tetrameric α2-macroglobulin family of proteins (Österberg, R., et al. (1991), Biochemistry 30, 7873–7878). Due to the conformational change occurring when the thiol ester bond is split, the cleft in the V-form seems to be closed; and as a result, the models of the chymotrypsin and methylamine derivatives are more compact than that of the native protein.