Small-angle X-ray scattering study of the interaction between human alpha 2-macroglobulin and trypsin.

The interaction between human alpha 2-macroglobulin and trypsin yields a complex of two trypsin molecules and one alpha 2-macroglobulin molecule as the main product. This is indicated from small-angle X-ray titration measurements in 0.02 M Tris/HCl buffer of pH 7.40 containing 0.2 M NaCl and 5 mM EDTA. The radius of gyration for the complex was determined to be 7.1 nm. The complex formation coincides with a decrease in the first side maximum of the scattering curve for alpha 2-macroglobulin. A comparison of the experimental data with those calculated from various models indicates that the data are consistent with the scattering from a hollow cylinder containing two sites, each of which is partly filled with one trypsin molecule.