THE KINETICS OF CONFORMATIONAL-CHANGES OF ALPHA(2)-MACROGLOBULIN DETERMINED BY TIME-RESOLVED X-RAY SOLUTION SCATTERING

The rate of gross conformational change of alpha(2)-macrogrobulin (alpha(2)M) during its proteinase trapping was directly determined for the first time using time-resolved X-ray solution scattering. Decrease of radius of gyration was observed under pseudo-first-order conditions with excess proteinases, which exhibited a monophasic timecourse. The rate constants were 0.5 +/- 0.1 s(-1) and 0.8 +/- 0.2 s(-1) for the reaction with chymotrypsin and trypsin, respectively. There was no concentration dependence of the observed rate constants. Therefore, the rate-limiting step of the gross conformational change was not the bimolecular encounter reaction between alpha(2)M and proteinases, which requires a new proposal of pre-trapping of proteinases before the gross conformational change.