Ab initio and constrained modeling of phosphorylase

The joint use of small-angle X-ray scattering (SAXS) and hydrodynamic data permits biologically useful reconstructions of protein structures to be determined. Low-resolution shapes of proteins can be obtained by SAXS-based modeling approaches, among them the ab initio approaches being the most recent and challenging ones. The programs DAMMIN and GASBOR have been applied to C. callunae starch phosphorylase in a case study, to test in a systematic manner the principles governing the evaluation strategies of the approaches applied. Therefore, emphasis was laid on the elaboration of modeling aspects rather than on biological details. Optimum results concerning the predictions of particle shapes and molecule properties have been obtained by utilizing tight constraints for modeling, such as symmetry and anisometry information. The use of pure ab initio conditions yields rather moderate shape and parameter predictions. Application of erroneous constraints generally leads to unrealistic particle shapes, although the parameter predictions may be satisfactory. The usage of the program DAMMIN turned out to be superior to application of the program GASBOR, whether the latter approach was used in the reciprocal- or real-space version. For hydrodynamic modeling, a modified version of the program HYDRO was adopted. By recourse to known crystallographic 3D structures for phosphorylases from other sources, SAXS profiles of anhydrous proteins can be modeled. Procedures for the addition of individual water molecules to anhydrous protein envelopes based on the atomic coordinates yield biologically relevant models for hydrated phosphorylases. This requires the usage of advanced surface calculation programs such as SIMS and of appropriate hydration algorithms such as those implemented in our programs HYDCRYST and HYDMODEL. The resulting SAXS profiles and structural and hydrodynamic parameters of the hydrated proteins can be compared with the data obtained by solution scattering.