Substrate binding to a nitrite reductase induces a spin transition.

The multiheme enzyme nitrite reductase catalyzes a 6-electron reduction of nitrite to ammonia. The reaction is initiated by substrate binding to the free axial position of the high spin penta-coordinated heme active site. The spin configuration of the resulting complex is crucial for discrimination between the heterolytic vs homolytic character of the cleavage of the N-O bond and, therefore, subsequent steps of the catalytic cycle. Here, we report the first experimental evidence, based on resonance Raman spectroscopy, that nitrite binding to the enzyme from D. vulgaris induces a transition from the high spin to the low spin configuration in the catalytic heme, thereby favoring the heterolytic route.