Copper‐Containing Nitrite Reductase Employing Proton‐Coupled Spin‐Exchanged Electron‐Transfer and Multiproton Synchronized Transfer to Reduce Nitrite

The possible catalytic mechanisms of the reduction of nitrite by copper-containing nitrite reductases (CuNiR) is examined using the M06 function according to two-copper models, which include type one copper (T1Cu) and type two copper (T2Cu) sites. Our examinations confirm that the protonations of two residues, His255 and Asp98 near the T2Cu site, can modulate the redox states of T1Cu and T2Cu but also cannot directly cause electron transfer from T1Cu to T2Cu. The electron hole remains at the T2Cu site when only one residue, His255 or Asp98, is protonated. However, the hole resides at the T1Cu site when both His255 and Asp98 are protonated. Then, the first protonation of nitrite take places via an indirect proton transfer from the protonated His255 through the bridging H₂O and Asp98 with three protons moving together, which can not cause the cleavage of the HO-NO bond. After that, the substrate requires getting another proton from reprotonated His255 via the bridging H₂O. The reprotonation of nitrite induces the generation of nitric oxide (NO) and H₂O at the T2Cu site via a special double-proton-coupled spin-exchanged electron transfer mechanism with indirect proton transfer from His255 to the substrate, a beta electron of T2Cu(I) shift to the NO cation and the remained alpha electron changing spin direction at the same time. These results may provide useful information to better understand the detailed proton/electron transfer reactions for the catalytic processes of CuNiR.