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 N2 O, 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 heme 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 N2 O, 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 N2 O via an exothermic, barrierless process.
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