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Damage-free structures of the resting state of one of the most studied copper nitrite reductases were obtained independently by X-ray free-electron laser (XFEL) and neutron crystallography, representing the first direct comparison of neutron and XFEL structural data for any protein. In addition, damage-free structures of the reduced and nitrite-bound forms have been obtained to high resolution from cryogenically maintained crystals by XFEL crystallography., Copper-containing nitrite reductases (CuNiRs) that convert NO2 − to NO via a CuCAT–His–Cys–CuET proton-coupled redox system are of central importance in nitrogen-based energy metabolism. These metalloenzymes, like all redox enzymes, are very susceptible to radiation damage from the intense synchrotron-radiation X-rays that are used to obtain structures at high resolution. Understanding the chemistry that underpins the enzyme mechanisms in these systems requires resolutions of better than 2 Å. Here, for the first time, the damage-free structure of the resting state of one of the most studied CuNiRs was obtained by combining X-ray free-electron laser (XFEL) and neutron crystallography. This represents the first direct comparison of neutron and XFEL structural data for any protein. In addition, damage-free structures of the reduced and nitrite-bound forms have been obtained to high resolution from cryogenically maintained crystals by XFEL crystallography. It is demonstrated that AspCAT and HisCAT are deprotonated in the resting state of CuNiRs at pH values close to the optimum for activity. A bridging neutral water (D2O) is positioned with one deuteron directed towards AspCAT Oδ1 and one towards HisCAT N∊2. The catalytic T2Cu-ligated water (W1) can clearly be modelled as a neutral D2O molecule as opposed to D3O+ or OD−, which have previously been suggested as possible alternatives. The bridging water restricts the movement of the unprotonated AspCAT and is too distant to form a hydrogen bond to the O atom of the bound nitrite that interacts with AspCAT. Upon the binding of NO2 − a proton is transferred from the bridging water to the Oδ2 atom of AspCAT, prompting electron transfer from T1Cu to T2Cu and reducing the catalytic redox centre. This triggers the transfer of a proton from AspCAT to the bound nitrite, enabling the reaction to proceed.

Nitric oxide (NO) reductase from the fungus Fusarium oxysporum is a P450-type enzyme (P450nor) that catalyzes the reduction of NO to nitrous oxide (N(2)O) in the global nitrogen cycle. In this enzymatic reaction, the heme-bound NO is activated by the direct hydride transfer from NADH to generate a short-lived intermediate (I), a key state to promote N–N bond formation and N–O bond cleavage. This study applied time-resolved (TR) techniques in conjunction with photolabile-caged NO to gain direct experimental results for the characterization of the coordination and electronic structures of I. TR freeze-trap crystallography using an X-ray free electron laser (XFEL) reveals highly bent Fe–NO coordination in I, with an elongated Fe–NO bond length (Fe–NO = 1.91 Å, Fe–N–O = 138°) in the absence of NAD(+). TR-infrared (IR) spectroscopy detects the formation of I with an N–O stretching frequency of 1,290 cm(−1) upon hydride transfer from NADH to the Fe(3+)–NO enzyme via the dissociation of NAD(+) from a transient state, with an N–O stretching of 1,330 cm(−1) and a lifetime of ca. 16 ms. Quantum mechanics/molecular mechanics calculations, based on these crystallographic and IR spectroscopic results, demonstrate that the electronic structure of I is characterized by a singly protonated Fe(3+)–NHO(•−) radical. The current findings provide conclusive evidence for the N(2)O generation mechanism via a radical–radical coupling of the heme nitroxyl complex with the second NO molecule.