Determinants of the Lead(II) Affinity in pbrR Protein: A Computational Study

Investigation of the diverse evolutionary developed mechanisms enabling bacteria to maintain homeostasis and to be resistant to lead is crucial for the discovery of novel strategies for isolation of this highly toxic metal and its subsequent elimination from contaminated environments. The metalloregulatory protein pbrR and its homologues that were identified in the Cupriavidus metallidurans CH34 chromosome are the only characterized natural metalloproteins that have a special affinity toward Pb(II) and that bind it with at least a 1000-fold selectivity over other heavy metals. The X-ray structures of apo and Pb(II)-bound pbrR have been recently reported. In the present study, the binding of Pb(II) at pbrR was investigated by means of multiscale computational modeling. Molecular dynamics simulations substantiated how conformations amenable for the Pb(II) complexation through the tris-cysteine motif are formed from the antiparallel coiled-coil packing interaction of two dimerization helices of two pbrR monomers, and the phase space of apo pbrR has been extensively sampled. Hybrid quantum mechanics/molecular mechanics (QM/MM) calculations on metal-bound structures of pbrR also allowed us to determine the most probable protonation state for the lead binding motif and evaluate the structural features mostly affecting the Pb(II) coordination in this protein. In agreement with available experimental data, we found that pbrR may control its Pb(II) affinity, probably, by conformational changes that affect the distance between Cys78' and Cys122 and their protonation states, thus being able to switch on the Pb(II) sequestration/ release-prone states in response to external stimuli. The protein structure enveloping the metal binding motif favors the thiol- thiolate-thiolate protonation state of Pb(II)-pbrR, thus probably enhancing the binding selectivity for Pb(II), compared to other metal ions.