The structural basis of the low reorganization energy of cupredoxins has long been debated. These proteins reconcile a conformationally heterogeneous and exposed metal-chelating site with the highly rigid copper center required for efficient electron transfer. Here we combine single-molecule mechanical unfolding experiments with statistical analysis and computer simulations to show that the metal-binding region of apo-azurin is mechanically flexible and that high mechanical stability is imparted by copper binding. The unfolding pathway of the metal site depends on the pulling residue and suggests that partial unfolding of the metal binding site could be facilitated by the physical interaction with certain regions of the redox protein.

We are grateful to A. Donaire and I. Díez-Pérez for discussions, and to the Catalan government (grant 2014SGR-1251), the Spanish government (grant CTQ2013-43892R) and the European Research Council (PELE ERC-2009-Adg 25027) for financial support.

Peer Reviewed