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Reactivity of the heme–dioxygen complex of the inducible nitric oxide synthase in the presence of alternative substrates
Author(s) -
LefèvreGroboillot David,
Boucher JeanLuc,
Mansuy Daniel,
Stuehr Dennis J.
Publication year - 2006
Publication title -
the febs journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.981
H-Index - 204
eISSN - 1742-4658
pISSN - 1742-464X
DOI - 10.1111/j.1742-4658.2005.05056.x
Subject(s) - pterin , tetrahydrobiopterin , chemistry , heme , reactivity (psychology) , nitric oxide synthase , kinetics , stereochemistry , cofactor , nitric oxide , medicinal chemistry , enzyme , organic chemistry , medicine , alternative medicine , physics , pathology , quantum mechanics
Single turnover reactions of the inducible nitric oxide synthase oxygenase domain (iNOSoxy) in the presence of several non α‐amino acid N ‐hydroxyguanidines and guanidines were studied by stopped‐flow visible spectroscopy, and compared with reactions using the native substrates l ‐arginine ( l ‐arg) or N ω ‐hydroxy‐ l ‐arginine (NOHA). In experiments containing dihydrobiopterin, a catalytically incompetent pterin, and each of the studied substrates, l ‐arg, butylguanidine (BuGua), para ‐fluorophenylguanidine (FPhGua), NOHA, N ‐butyl‐ and N ‐( para ‐fluorophenyl)‐ N ′‐hydroxyguanidines (BuNOHG and FPhNOHG), the formation of a iron(II) heme–dioxygen intermediate (Fe II O 2 ) was always observed. The Fe II O 2 species then decayed to iron(III) iNOSoxy at rates that were dependent on the nature of the substrate. Identical reactions containing the catalytically competent cofactor tetrahydrobiopterin (BH 4 ), iNOSoxy and the three N ‐hydroxyguanidines, all exhibited an initial formation of an Fe II O 2 species that was successively converted to an Fe III NO complex and eventually to high‐spin iron(III) iNOSoxy. The formation and decay kinetics of the Fe III NO complex did not vary greatly as a function of the N ‐hydroxyguanidine structure, but the formation of Fe III NO was substoichiometric in the cases of BuNOHG and FPhNOHG. Reactions between BH 4 ‐containing iNOSoxy and BuGua exhibited kinetics similar to those of the corresponding reaction with l ‐arginine, with formation of an Fe II O 2 intermediate that was directly converted to high‐spin iron(III) iNOSoxy. In contrast, no Fe II O 2 intermediate was observed in the reaction of BH 4 ‐containing iNOSoxy and FPhGua. Multi‐turnover reaction of iNOS with FPhGua did not lead to formation of NO or to hydroxylation of the substrate, contrary to reactions with BuGua or l ‐arg. Our results reveal how different structural and chemical properties of NOS substrate analogues can impact on the kinetics and reactivity of the Fe II O 2 intermediate, and support an important role for substrate p K a during NOS oxygen activation.