Pseudomonas stutzeri N sub 2 O reductase contains Cu sub A -type sites

N{sub 2}O reductase (N{sub 2}O {yields} N{sub 2}) is the terminal enzyme in the energy-conserving denitrification pathway of soil and marine denitrifying bacteria. The protein is composed of two identical subunits and contains eight copper ions per enzyme molecule. The magnetic circular dichroism spectrum of resting (oxidized) N{sub 2}O reductase is strikingly similar to the magnetic circular dichroism spectrum of the Cu{sub A} site in mammalian cytochrome c oxidase and is unlike the magnetic circular dichroism spectra of all other biological copper chromophores obtained to data. Sulfur (or chlorine) scatterers are required to fit the copper extended x-ray absorption fine structure data of both the oxidized and reduced forms of N{sub 2}O reductase. Satisfactory fits require a Cu-N or Cu-O interaction at 2.0 {angstrom}, a Cu-(S, Cl) interaction at 2.3 {angstrom} and an additional Cu-(S, Cl) interaction at {approx} 2.6 {angstrom} (oxidized) or {approx} 2.7 {angstrom} (reduced). Comparison of the N{sub 2}O reductase sequence, determined by translating the structural NosZ gene, with cytochrome c oxidase subunit II sequences from several sources indicates that a Gly-Xaa-Xaa-Xaa-Xaa-Xaa-Cys-Ser-Xaa-Xaa-Cys-Xaa-Xaa-Xaa-Xaa-Xaa-His stretch if high conserved. This sequence contains three of the probable ligands in a Cu{sub A}-type site Collectively these data establish that Pseudomonas stutzeri N{sub 2}O reductase contains Cu{sub A}-type sites.