Can Reduction of NO to N2O in Cytochrome c Dependent Nitric Oxide Reductase Proceed through a Trans-Mechanism?

As part of microbial denitrification, NO is reduced to N2O in the membrane bound enzyme nitric oxide reductase, NOR. The N-N coupling occurs in the diiron binuclear active site, BNC, and different mechanisms for this reaction step have been suggested. Computational studies have supported a so-called cis:b3-mechanism, in which the hyponitrite product of the reductive N-N bond formation coordinates with one nitrogen to the heme iron and with both oxygens to the non-heme iron in the BNC. In contrast, experimental results have been interpreted to support a so-called trans-mechanism, in which the hyponitrite intermediate coordinates with one nitrogen atom to each of the two iron ions. Hybrid density functional theory is used here to perform an extensive search for possible intermediates of the NO reduction in the cNOR enzyme. It is found that hyponitrite structures coordinating with their negatively charged oxygens to the positively charged iron ions are the most stable ones. The hyponitrite intermediate involved in the suggested trans-mechanism, which only coordinates with the nitrogens to the iron ions, is found to be prohibitively high in energy, leading to a too slow reaction, which should rule out this mechanism. Furthermore, intermediates binding one NO molecule to each iron ion in the BNC, which have been suggested to initiate the trans-mechanism, are found to be too high in energy to be observable, indicating that the experimentally observed electron paramagnetic resonance signals, taken to support such an iron-nitrosyl dimer intermediate, should be reinterpreted.