title: Kinetic unfolding mechanism of the inducible nitric oxide synthase oxygenase domain determined by time-resolved electrospray mass spectrometry.
abstract: The inducible nitric oxide synthase core oxygen domain (iNOS(COD)) is a homodimeric protein complex of ca. 100 kDa. In this work, the subunit disassembly and unfolding of the protein following a pH jump from 7.5 to 2.8 were monitored by on-line rapid mixing in conjunction with electrospray (ESI) time-of-flight mass spectrometry. Various protein species become populated during the denaturation process. These can be distinguished by their ligand binding behavior, and by the different charge states that they produce during ESI. Detailed intensity-time profiles were obtained for all of these species, and the kinetics were subjected to a global analysis which allows a model of the denaturation process to be developed. The data are described well by three relaxation times (tau(1) = 0.36 s, tau(2) = 0.62 s, and tau(3) = 3.3 s), each of which has a characteristic amplitude spectrum. The initial step of the reaction is the disruption of the iNOS(COD) dimer, to generate heme-bound monomeric species in various degrees of unfolding. This first step is accompanied by the loss of two tetrahydrobiopterin cofactors. Subsequent heme loss generates monomeric apoproteins exhibiting various degrees of unfolding. In addition, the formation of proteins that are bound to two heme groups is observed. A subpopulation of holo monomers undergoes substantial unfolding while retaining contact with the heme cofactor. Together with previous studies, the results of this work suggest that the occurrence of complex reaction mechanisms involving several short-lived intermediates is a common feature for the denaturation of large multiprotein complexes.
authors.affiliation: Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
authors.forename: Derek J
authors.initials: DJ
authors.lastname: Wilson
authors.forename: Steven P
authors.initials: SP
authors.lastname: Rafferty
authors.forename: Lars
authors.initials: L
authors.lastname: Konermann
mesh_terms: D000818:Animals; D019281:Dimerization; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D051379:Mice; D008956:Models, Chemical; D019001:Nitric Oxide Synthase / Q000737:chemistry* / Q000378:metabolism; D052247:Nitric Oxide Synthase Type II; D010105:Oxygenases / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods; D013997:Time Factors
chemical_list: D010105:Oxygenases; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496317:Nos2 protein, mouse; D006419:Heme Oxygenase (Decyclizing)