Kinetics of Copper Incorporation into a Biosynthetic Purple Cu_A Azurin: Characterization of Red, Blue, and a New Intermediate Species

Abstract

Evolutionary links between type 1 blue copper (T1 Cu), type 2 red copper (T2 Cu), and purple Cu_A cupredoxins have been proposed, but the structural features and mechanism responsible for such links as well as for assembly of Cu_A sites in vivo are poorly understood, even though recent evidence demonstrated that the Cu(II) oxidation state plays an important role in this process. In this study, we examined the kinetics of Cu(II) incorporation into the Cu_A site of a biosynthetic Cu_A model, Cu_A azurin (Cu_AAz) and found that both T1 Cu and T2 Cu intermediates form on the path to final Cu_A reconstitution in a pH-dependent manner, with slower kinetics and greater accumulation of the intermediates as the pH is raised from 5.0 to 7.0. While these results are similar to those observed previously in the native Cu_A center of nitrous oxide reductase, the faster kinetics of copper incorporation into Cu_AAz allowed us to use lower copper equivalents to reveal a new pathway of copper incorporation, including a novel intermediate that has not been reported in cupredoxins before, with intense electronic absorption maxima at ∼410 and 760 nm. We discovered that this new intermediate underwent reduction to Cu(I), and proposed that it is a Cu(II)–dithiolate species. Oxygen-dependence studies demonstrated that the T1 Cu species only formed in the presence of molecular oxygen, suggesting the T1 Cu intermediate is a one-electron oxidation product of a Cu(I) species. By studying Cu_AAz variants where the Cys and His ligands are mutated, we have identified the T2 Cu intermediate as a capture complex with Cys116 and the T1 Cu intermediate as a complex with Cys112 and His120. These results led to a unified mechanism of copper incorporation and new insights regarding the evolutionary link between all cupredoxin sites as well as the in vivo assembly of Cu_A centers