Multi‐frequency EPR and Mössbauer spectroscopic studies on freeze‐quenched reaction intermediates of nitric oxide synthase

It is believed by analogy to chloroperoxidase (CPO) from Caldariomyces fumago that the electronic structure of the intermediate iron‐oxo species in the catalytic cycle of nitric oxide synthase (NOS) corresponds to an iron(IV) porphyrin‐π ‐cation radical. Such species can also be produced by the reaction of ferric NOS with external oxidants within the shunt pathway. We present multi‐frequency EPR (9.6, 94, 285 GHz) and Mössbauer spectroscopic studies on freeze‐quenched intermediates of the oxygenase domain of nitric oxide synthase which has reacted with peroxy acetic acid within 8–200 ms. The intermediates of the oxygenase domain of both the cytokine inducible NOS (iNOSox) and the neuronal NOS (nNOSox) show an organic radical signal in the 9.6‐GHz spectrum overlapping with the spectrum of an unknown species with g‐values of 2.24, 2.23 and 1.96. Using 94‐ and 285‐GHz EPR the organic radical signal is assigned to a tyrosine radical on the basis of g‐values (i.e. Tyr*562 in nNOSox and Tyr*341 in iNOSox). Mössbauer spectroscopy of 57Fe‐labeled unreacted nNOSox shows a ferric low‐spin heme‐iron (δ = 0.38 mms−1, ΔEQ = 2.58 mms−1). The reaction of nNOSox with peroxy acetic acid for 8 ms leads to the disappearance of the magnetic background characteristic for native nNOSox and a new species with δ = 0.27 mms−1 and ΔEQ = 2.41 mms−1 is detected at 4.2 K which does not resemble the parameters typical for a Fe(IV) center. It is proposed that this intermediate species corresponds to a ferric low‐spin species which magnetically couples to an amino acid radical (presumably Trp*409). Copyright © 2005 John Wiley & Sons, Ltd.