By combining spectroscopic, biochemical and genetic techniques the study of structural and functional aspects of the denitrifying enzymes nitrous oxide (N2O) reductase and nitrogen monoxide (NO) reductase has recently made substantial progress. This has revealed unexpected evolutionary relationships between aerobic respiration and denitrification enzymes. N2O reductase shares with cytochrome c oxidase the mixed-valent, binuclear CuA center as a structurally novel metal site. The cytochrome b subunit of NO reductase shows a striking conservation with the heme-binding transmembrane segments of the subunit I of cytochrome c oxidase. Its metal centers consist of an isolated heme b, and probably a heme b and nonheme Fe binuclear site. N2O reductase and NO reductase may be ancestors of energy conserving enzymes of the heme-Cu oxidase superfamily.