Determination of the active form of the tetranuclear copper sulfur cluster in nitrous oxide reductase

N2OR has been found to have two structural forms of its tetranuclear copper active site, the 4CuS Cu-Z* form and the 4Cu2S Cu-Z form. EPR, resonance Raman, and MCD spectroscopies have been used to determine the redox states of these sites under different reductant conditions, showing that the Cu-Z* site accesses the 1-hole and fully reduced redox states, while the Cu-Z site accesses the 2-hole and 1-hole redox states. Single-turnover reactions of N2OR for Cu-Z and Cu-Z* poised in these redox states and steady-state turnover assays with different proportions of Cu-Z and Cu-Z* show that only fully reduced Cu-Z* is catalytically competent in rapid turnover with N2O.