The possible catalytic mechanism of the reduction of nitrite by copper-containing nitrite reductases (CuNiRs) is examined by using the M06 function according to two copper models, which include type-one copper (T1Cu) and type-two copper (T2Cu) sites. Examinations confirm that the protonation of two residues, His255 and Asp98, near the T2Cu site, can modulate the redox states of T1Cu and T2Cu, but 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 takes place through indirect proton transfer from protonated His255 through the bridging H O and Asp98 with three protons moving together, which cannot cause the cleavage of the HO-NO bond. Subsequently, the substrate is required to obtain another proton from reprotonated His255 through the bridging H O. The reprotonation of nitrite induces the generation of nitric oxide (NO) and H O at the T2Cu site through a special double-proton-coupled spin-exchanged electron-transfer mechanism with indirect proton transfer from His255 to the substrate, a beta-electron of T2Cu shift to the NO cation, and the remaining alpha-electron changing spin direction at the same time. These results may provide useful information to better understand detailed proton-/electron-transfer reactions for the catalytic processes of CuNiR.

Single-atom catalysts with iron atomically dispersed on nitrogen-doped carbon matrix (FeN 4 C) is attractive for electrochemically converting carbon dioxide (CO 2 ) to carbon monoxide (CO) or formic acid (HCOOH) due to its unique properties and activity. However, the influence of the oxidation state with the different spins of Fe ion in FeN 4 C on the electrochemical mechanisms of conversion of CO 2 to CO/ HCOOH remains unclear. Herein, we got insight into the influence on the CO 2 reduction reaction (CO 2 RR) catalyzed by single-Fe coordinated by four pyridinic N ligands (Fe II/III N 4 C) using density functional theory calculations. Our calculations revealed that the two-electron CO 2 RR takes place via two sequential proton-coupled electron-transfer reactions with proton transfer from a N or C site on the FeN 4 C surface to CO 2 and at the same time electron transfer from FeN 4 C to CO 2 . The potential energy analyses uncovered that the energy barrier (0.72/0.52 eV) of the rate-limiting step for the CO/HCOOH formation catalyzed by the middle spin Fe II N 4 C M is lowest among the two oxidation states with the different spins. In addition, the overpotential (−0.03/0.29 V) of the *CO/*HCOOH formation catalyzed by Fe II N 4 C M is also significantly lower than these of the other cases. Both confirm that the Fe 2+ ion with the middle spin in FeN 4 C is most favorable for the conversion of CO 2 to CO/HCOOH. These findings thus provide valuable information to develop new strategies for designing more efficient Fe single-atom catalysts.

The peroxide-dependent coproheme decarboxylase ChdC from Geobacillus stearothermophilus catalyzes two key steps in the synthesis of heme b, i.e., two sequential oxidative decarboxylations of coproporphyrinogen III (coproheme III) at propionate groups P2 and P4. In the binding site of coproheme III, P2 and P4 are anchored by different residues (Tyr144, Arg217, and Ser222 for P2 and Tyr113, Lys148, and Trp156 for P4); however, strong experimental evidence supports that the generated Tyr144 radical acts as an unique intermediary for hydrogen atom transfer (HAT) from both reactive propionates. So far, the reaction details are still unclear. Herein, we carried out quantum mechanics/molecular mechanics calculations to explore the decarboxylation mechanism of coproheme III. In our calculations, the coproheme Cpd I, Fe­(IV) = O coupled to a porphyrin radical cation (por•+) with four propionate groups, was used as a reactant model. Our calculations reveal that Tyr144 is directly involved in the decarboxylation of propionate group P2. First, the proton-coupled electron transfer (PCET) occurs from Tyr144 to P2, generating a Tyr144 radical, which then abstracts a hydrogen atom from the Cβ of P2. The β-H extraction was calculated to be the rate-limiting step of decarboxylation. It is the porphyrin radical cation (por•+) that makes the PCET from Tyr144 to P2 to be quite easy to initiate the decarboxylation. Finally, the electron transfers from the Cβ• through the porphyrin to the iron center, leading to the decarboxylation of P2. Importantly, the decarboxylation of P4 mediated by Lys148 was calculated to be very difficult, which suggests that after the P2 decarboxylation, the generated harderoheme III intermediate should rebind or rotate in the active site so that the propionate P4 occupies the binding site of P2, and Tyr144 again mediates the decarboxylation of P4. Thus, our calculations support the fact that Tyr144 is responsible for the decarboxylation of both P2 and P4.

Proton-coupled electron transfer (PCET) reaction plays an important role in promoting the progresses of many biological and chemical reactions. Usually the rate of PCET reaction increases with the increase of...

Glucose oxidase (GOx) can catalyze the oxidation of β-D-glucose in mild conditions to directly convert biological energy into electrical energy, which has great potential applications in the fields of enzyme...