Study of the Zn-containing DD-carboxypeptidase of Streptomyces albus G by small-angle X-ray scattering in solution.

Study of the Zn2+-containing D-alanyl-D-alanine-cleaving carboxypeptidase of Streptomyces albus G by small-angle X-ray scattering in solution yielded the following molecular parameters: radius of gyration R = 1.82 +/- 0.05 nm; largest diameter D = 5.9 +/- 0.2 nm; relative molecular mass Mr = 17000 +/- 2000; volume V approximately equal to 35 +/- 2 nm3; degree of hydration: 0.25 +/- 0.02 g water/g protein. By reference to theoretical scattering curves of rigid triaxial homogeneous bodies, a model which fits all experimental data is an elliptical cylinder. Such a model is compatible with that observed in the crystal structure. At those high concentrations necessary to form inactive enzyme-ligand associations the non-competitive beta-lactam inhibitors, cephalothin and cephalosporin C, drastically altered the scattering behaviour of the protein.