Structure of alpha 2-macroglobulin in solution and its interaction with proteases: an X-ray scattering study using the contrast variation method.

X-Ray scattering study of alpha 2-macroglobulin in solvents of variable electron densities (sucrose in water) shows that alpha 2-macroglobulin obeys the invariant volume hypothesis; thus, the structure of the particle is independent of the sucrose concentration of the solution. The particle structure is quantitatively described by a set of parameters such as the gyration radius, R = 8.0 nm, the volume, V = 1200 nm3, and the maximum distance within the particle, Dmax = 25 nm. The contrast dependence of the gyration radius indicates that in alpha 2-macroglobulin the regions of higher electron density are located closer to the center (core) than the regions of lower electron density. The core, which may be the place occupied by the carbohydrate, has a maximum dimension of 16 nm and it can be described as a flat cylinder. X-Ray scattering titrations indicate that alpha 2-macroglobulin forms a 1:2 complex as the main product with both trypsin and chymotrypsin simultaneously as the particle contracts. The formation of a ternary 1:1:1 complex with trypsin and chymotrypsin and the absence of higher complexes indicate that the sites for these proteases are closely related. This is further substantiated by the p(r) functions which are virtually identical for the 1:1:1 and 1:2 complexes.