Understanding the catalytic centre, Cu-z, of nitrous oxide reductase by spectroscopy

Some bacteria use oxidised forms of nitrogen as electron acceptors to survive under anaerobic conditions by denitrification. The last step of denitrification, the two-electron reduction of nitrous oxide to dinitrogen, is catalysed by the multi-copper enzyme nitrous oxide reductase. Each subunit of a functional homodimer has a binuclear copper centre, CUA, as an electron transfer group, and a novel tetranuclear copper centre, Cuz, as the catalytic site ~. Different spectroscopic methods (UV/Vis, MCD, EPR, RR) and inorganic sulfur determinations showed that Cuz has a central bridging sulfur atom as ligand 2 which was confirmed by the X-ray crystal structure 3. Cuz is therefore the fncst example of a copper-sulfide centre in biology. The spectra are analysed to understand the electronic structure of Cuz. Of five theoretically possible redox-states of Cuz only two, interconverted by one electron, have been detected by spectroscopy. In another enzyme form, isolated when the enzyme is incubated with oxygen, Cuz is fixed in one redox-state although the enzyme shows normal steady-state activity. These results are interpreted in terms of a catalytic mechanism. [Support is from the UK BBSRC]