Small‐Angle X‐Ray Scattering Studies of Tryptophan Synthase from Escherichia coli and Its α and β2 Subunits

Abstract

The .alpha. and .beta.2 subunits of E. coli tryptophan synthase were investigated by small-angle X-ray scattering. The molecular parameters are the following: radius of gyration, .alpha., 1.95 nm, .beta.2, 3.01 nm; maximum particle diameter, .alpha., 5.8 nm, .beta.2, 10.5 nm; and hydrated volume, .alpha., 60 nm3, .beta.2 160 nm3. The shape of the .alpha. subunit can be described by a circular cylinder, slightly tapered at 1 end. An elongated elliptical cylinder with its cross section larger in the middle than at the ends was a model equivalent in scattering to the .beta.2 subunit. The .alpha.2.beta.2 enzyme complex had a radius of gyration of 4.01 nm, a maximum length of 13.5 nm and a hydrated volume of 270 nm3. No satisfactory fit of the scattering data was obtainable by mere apposition of the models of the .alpha. and .beta.2 subunits. Two cylinders overlapping laterally fit the experimental data considerably better, suggesting changes in the conformation of the subunits on forming the .alpha.2.beta.2 complex.