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

The radius of gyration (R-g) of bovine trypsinogen and beta -trypsin was me asured 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 acqui sition by EDXD; (2) at pH 4 the active enzyme has one class of chloride bin ding 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 doe s 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 cha rge are small in absolute magnitude and produce no gradient across the acti ve site.