Nitrous oxide reductase (N2OR) and cytochrome c oxidase (CcO) catalyze the reductions of N2O to N2 during microbial denitrification and of O2 to H2O during cellular respiration, respectively. 1, 2 These disparate metalloenzymes have in common an unusual copper site, CuA, that functions analogously to the ubiquitous type 1 monocopper centers3 to transfer electrons during enzyme turnover. Controversy surrounding the structure of CuA appears ...