title: Direct measurement by laser flash photolysis of intraprotein electron transfer in a rat neuronal nitric oxide synthase.
abstract: Intraprotein interdomain electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in nitric oxide (NO) synthesis by NO synthase (NOS). Our previous laser flash photolysis studies have provided a direct determination of the kinetics of IET between the FMN and heme domains in truncated oxyFMN constructs of rat neuronal NOS (nNOS) and murine inducible NOS (iNOS), in which only the oxygenase and FMN domains along with the calmodulin (CaM) binding site are present [Feng, C. J.; Tollin, G.; Holliday, M. A.; Thomas, C.; Salerno, J. C.; Enemark, J. H.; Ghosh, D. K. Biochemistry 2006, 45, 6354-6362. Feng, C. J.; Thomas, C.; Holliday, M. A.; Tollin, G.; Salerno, J. C.; Ghosh, D. K.; Enemark, J. H. J. Am. Chem. Soc. 2006, 128, 3808-3811]. Here, we report the kinetics of IET between the FMN and heme domains in a rat nNOS holoenzyme in the presence and absence of added CaM using laser flash photolysis of CO dissociation in comparative studies on partially reduced NOS and a single domain NOS oxygenase construct. The IET rate constant in the presence of CaM is 36 s-1, whereas no IET was observed in the absence of CaM. The kinetics reported here are about an order of magnitude slower than the kinetics in a rat nNOS oxyFMN construct with added CaM (262 s-1). We attribute the slower IET between FMN and heme in the holoenzyme to the additional step of dissociation of the FMN domain from the reductase complex before reassociation with the oxygenase domain to form the electron-transfer competent output state complex. This work provides the first direct measurement of CaM-controlled electron transfer between catalytically significant redox couples of FMN and heme in a nNOS holoenzyme.
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: James T
authors.initials: JT
authors.lastname: Hazzard
authors.forename: Nickolas J
authors.initials: NJ
authors.lastname: Nahm
authors.forename: J Guy
authors.initials: JG
authors.lastname: Guillemette
authors.forename: John C
authors.initials: JC
authors.lastname: Salerno
authors.forename: Dipak K
authors.initials: DK
authors.lastname: Ghosh
mesh_terms: D000818:Animals; D002147:Calmodulin / Q000737:chemistry; D003001:Cloning, Molecular; D004583:Electrons; D004926:Escherichia coli / Q000378:metabolism; D005486:Flavin Mononucleotide / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D007700:Kinetics; D007834:Lasers; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism*; D010084:Oxidation-Reduction; D010777:Photochemistry; D010782:Photolysis; D051381:Rats; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012256:Riboflavin / Q000031:analogs & derivatives / Q000737:chemistry
chemical_list: D002147:Calmodulin; C007228:5-deazariboflavin; D006418:Heme; D005486:Flavin Mononucleotide; D052248:Nitric Oxide Synthase Type I; D012256:Riboflavin