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Multi‐frequency EPR and Mössbauer spectroscopic studies on freeze‐quenched reaction intermediates of nitric oxide synthase
Author(s) -
Jung C.,
Lendzian F.,
Schünemann V.,
Richter M.,
Böttger L. H.,
Trautwein A. X.,
Contzen J.,
Galander M.,
Ghosh D. K.,
Barra A.L.
Publication year - 2005
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1694
Subject(s) - chemistry , nitric oxide synthase , electron paramagnetic resonance , ferric , heme , porphyrin , mössbauer spectroscopy , photochemistry , nitric oxide , reaction intermediate , nuclear magnetic resonance , stereochemistry , medicinal chemistry , catalysis , inorganic chemistry , crystallography , enzyme , organic chemistry , physics
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 57 Fe‐labeled unreacted nNOSox shows a ferric low‐spin heme‐iron (δ = 0.38 mms −1 , Δ E Q = 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 Δ E Q = 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.