Mechanisms for enzymatic reduction of nitric oxide to nitrous oxide - A comparison between nitric oxide reductase and cytochrome c oxidase.

Cytochrome c oxidases (CcO) reduce O to HO in the respiratory chain of mitochondria and many aerobic bacteria. In addition, some species of CcO can also reduce NO to NO and water while others cannot. Here, the mechanism for NO-reduction in CcO is investigated using quantum mechanical calculations. Comparison is made to the corresponding reaction in a "true" cytochrome c-dependent NO reductase (cNOR). The calculations show that in cNOR, where the reduction potentials are low, the toxic NO molecules are rapidly reduced, while the higher reduction potentials in CcO lead to a slower or even impossible reaction, consistent with experimental observations. In both enzymes the reaction is initiated by addition of two NO molecules to the reduced active site, forming a hyponitrite intermediate. In cNOR, NO can then be formed using only the active-site electrons. In contrast, in CcO, one proton-coupled reduction step most likely has to occur before NO can be formed, and furthermore, proton transfer is most likely rate-limiting. This can explain why different CcO species with the same heme a-Cu active site differ with respect to NO reduction efficiency, since they have a varying number and/or properties of proton channels. Finally, the calculations also indicate that a conserved active site valine plays a role in reducing the rate of NO reduction in CcO.