Abstract

Transient changes in protein conformation dependent on environmental conditions appear to play a role in modulating metal binding and release mechanisms in proteins. In this context conformational multiplicities were investigated using two proteins; a Cd-binding type I metallothionein isolated from durum wheat (dMT) and the periplasmic ferric binding protein from H. influenzae (FbpA). We used synchrotron X-ray small angle scattering (SAXS) combined with further biophysical analyses, and molecular dynamics calculations to gain insights into the solution structures of structures of dMT and FbpA in holo and apo forms. Effects of environmental conditions such as pH and ionic strength on the protein structures were also studied. SAXS results show that dMT, which has no stable secondary structure displays a flexible extended structure in the apo form and folds into a more rigid structure in the presence of metals. Its folding process, the final structure and metal-binding are pH dependent. FbpA, on the other hand, despite having stable globular structures in both apo and holo forms, undergoes subtle conformational differences depending on the metal content and the environmental conditions. In low ionic strength buffers the structure of FbpA can be well represented by the available crystal structure, whereas at high ionic strength populations with multiple conformations coexist. Implications of these results will be discussed in the framework of mechanisms for metal binding in proteins.  

This work was partly supported by Turkish Atomic Energy Authority (TAEK) and TUBITAK.