title: Intraprotein electron transfer in a two-domain construct of neuronal nitric oxide synthase: the output state in nitric oxide formation.
abstract: Intersubunit intraprotein electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in nitric oxide (NO) synthesis by NO synthase (NOS). Previous crystal structures and functional studies primarily concerned an enzyme conformation, which serves as the input state for reduction of FMN by electrons from NADPH and flavin adenine dinucleotide (FAD) in the reductase domain. To favor the formation of the output state for the subsequent IET from FMN to heme in the oxygenase domain, a novel truncated two-domain oxyFMN construct of rat neuronal NOS (nNOS), in which only the FMN and heme domains were present, was designed and expressed. The kinetics of IET between the FMN and heme domains in the nNOS oxyFMN construct in the presence and absence of added calmodulin (CaM) were directly determined using laser flash photolysis of CO dissociation in comparative studies on partially reduced oxyFMN and single-domain heme oxygenase constructs. The IET rate constant in the presence of CaM (262 s(-)(1)) was increased approximately 10-fold compared to that in the absence of CaM (22 s(-)(1)). The effect of CaM on interdomain interactions was further evidenced by electron paramagnetic resonance (EPR) spectra. This work provides the first direct evidence of the CaM control of electron transfer (ET) between FMN and heme domains through facilitation of the FMN/heme interactions in the output state. Therefore, CaM controls IET between heme and FMN domains by a conformational gated mechanism. This is essential in coupling ET in the reductase domain in NOS with NO synthesis in the oxygenase domain.
authors.affiliation: College of Pharmacy, University of New Mexico, Albuquerque, New Mexico 87131, USA. cfeng@salud.unm.edu
authors.forename: Changjian
authors.initials: C
authors.lastname: Feng
authors.forename: Gordon
authors.initials: G
authors.lastname: Tollin
authors.forename: Michael A
authors.initials: MA
authors.lastname: Holliday
authors.forename: Clayton
authors.initials: C
authors.lastname: Thomas
authors.forename: John C
authors.initials: JC
authors.lastname: Salerno
authors.forename: John H
authors.initials: JH
authors.lastname: Enemark
authors.forename: Dipak K
authors.initials: DK
authors.lastname: Ghosh
mesh_terms: D000818:Animals; D016227:Benzoquinones / Q000378:metabolism / Q000494:pharmacology; D002147:Calmodulin / Q000378:metabolism / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D005486:Flavin Mononucleotide / Q000737:chemistry / Q000378:metabolism; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D007834:Lasers; D008954:Models, Biological; D009569:Nitric Oxide / Q000096:biosynthesis* / Q000737:chemistry; D052248:Nitric Oxide Synthase Type I / Q000737:chemistry* / Q000378:metabolism*; D010084:Oxidation-Reduction; D010777:Photochemistry; D010782:Photolysis; D017434:Protein Structure, Tertiary / Q000187:drug effects; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D012256:Riboflavin / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology
chemical_list: D016227:Benzoquinones; D002147:Calmodulin; D011993:Recombinant Fusion Proteins; C007228:5-deazariboflavin; D009569:Nitric Oxide; C025232:semiquinone radicals; D006418:Heme; D005486:Flavin Mononucleotide; D052248:Nitric Oxide Synthase Type I; D006419:Heme Oxygenase (Decyclizing); D012256:Riboflavin