Conformational study of proteins by SAXS and EDXD: the case of trypsin and trypsinogen

The radius of gyration (R-g) of bovine trypsinogen and beta -trypsin was measured by an energy-dispersive X-ray technique (EDXD) and by small-angle X-ray scattering (SAXS), under different solvent conditions. Both techniques gave superimposable results. The experimental evidence demonstrated that: (1) no structural modifications and/or damage occurred during the data acquisition by EDXD; (2) at pH 4 the active enzyme has one class of chloride binding sites in common with the zymogen, whereas the latter protease shows an additional class able to reverse the effects on R-g induced by chloride at low concentration; and (3) the PH profile of the R-g of both proteases does not resemble at all the pH effect on beta -trypsin activity, a result in line with the finding that the electrical potentials induced by surface charge are small in absolute magnitude and produce no gradient across the active site.