AbstractThe copper‐ and heme‐containing nitrite reductases (NiRs) are key enzymes in denitrification. Their subunits contain two distinct redox‐active metal centers, an electron‐accepting site and a nitrite‐reducing site, to carry out the single‐electron reduction of nitrite to nitic oxide. Catalytic cycles of both enzyme families employ intramolecular electron transfer that can be rate‐determining for their activity. Herein, we report results comparing these two enzyme families in order to resolve the different mechanisms controlling intramolecular electron transfer in these proteins.