Structural and Mechanistic Insights into the Copper-Modulated Unfolding Pathways of Azurin

Abstract

Characterization of energy landscape of metalloproteins is challenging because of the subtle changes imparted by metal binding on the stability and conformational flexibility of proteins. Here we present copper-binding induced changes in the mechanical unfolding pathways of azurin, a β-barrel protein with type-I copper center, through a joint experimental-computational study. Single-molecule force spectroscopy experiments reveal an intermediate along the unfolding pathway of apo-azurin in half of population. Steered molecular dynamics simulations attribute the native and intermediate unfolding transition states (TSs) to the rupture of interactions between the pairs of β-strands, 2B-8 and 4-7, respectively. We show that the copper-binding does not change the first TS of azurin and its mechanical properties, but influences the second TS. The rupture of 4-7 β-strand pair is delayed in holo-azurin because of constraints imposed by the copper coordination sphere and occurs after the second TS. Our experimental and computational approach extracts unprecedented details along the unfolding landscape for apo- and holo- forms of azurin.