Key Intermediate Species Reveal the Copper(II)-Exchange Pathway in Biorelevant ATCUN/NTS Complexes

The amino-terminal copper and nickel/N-terminal site (ATCUN/NTS) present in proteins and bioactive peptides exhibits high affinity towards Cu-II ions and have been implicated in human copper physiology. Little is known, however, about the rate and exact mechanism of formation of such complexes. We used the stopped-flow and microsecond freeze-hyperquenching (MHQ) techniques supported by steady-state spectroscopic and electrochemical data to demonstrate the formation of partially coordinated intermediate Cu-II complexes formed by glycyl-glycyl-histidine (GGH) peptide, the simplest ATCUN/NTS model. One of these novel intermediates, characterized by two-nitrogen coordination, t(1/2)approximate to 100 ms at pH 6.0 and the ability to maintain the Cu-II/Cu-I redox pair is the best candidate for the long-sought reactive species in extracellular copper transport.