Cytochrome P450nor: A nitric oxide reductase - Structure, spectra, and mechanism

Abstract

The present study characterizes key proposed intermediates and reaction pathway steps in the enzymatic cycle of cytochrome P450nor, a nitric oxide reductase. This unusual P450, with reductase rather than monoxygenase function, catalyzes the conversion of •NO to N2O, an important detoxification step in the global nitrogen cycle. Density functional theoretical (DFT) and semiempirical INDO/ROHF/CI quantum chemical characterization of the putative species in the enzymatic cycle have been made and direct comparisons made with published experimental spectral and structural data. The computed INDO/ROHF-CIS spectra of the NO bound heme species indicate a previously reported experimental transient electronic spectrum to be due the NO bound reduced ferrous heine species. The identity of the active heme species in the P450nor mechanism is unknown and may be either a nitrogen (Fe=N) N-ferryl analogue of the oxyferryl/compound I P450 species or the NO bound reduced ferrous heme species. Formation of products from either requires the addition of two protons and a second •NO to the NO bound reduced ferrous heme form to form N2O, water, and resting state heme products. The mechanism of proton transfer to the NO bound reduced ferrous NO species is assessed and contrasted with that discovered for the reduced oxyferrous heme species of cytochrome P450 monoxygenases. DFT reaction pathways studies are reported for the addition of •NO to the N-ferryl species and indicate it to be a competent active species forming N2O via an exothermic, barrierless process. © 2002 Wiley Periodicals, Inc. Int. J. Quantum Chem.