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J. Biol. Chem., Vol. 279, Issue 34, 35412-35425, August 20, 2004

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The FAD-shielding Residue Phe¹³⁹⁵ Regulates Neuronal Nitric-oxide Synthase Catalysis by Controlling NADP⁺ Affinity and a Conformational Equilibrium within the Flavoprotein Domain^*

David W. Konas, Keng Zhu¶, Manisha Sharma, Kulwant S. Aulak, Gary W. Brudvig¶, and Dennis J. Stuehr||

From the Department of Immunology, The Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44195 and the ^¶Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107

Phe¹³⁹⁵ stacks parallel to the FAD isoalloxazine ring in neuronal nitric-oxide synthase (nNOS) and is representative of conserved aromatic amino acids found in structurally related flavoproteins. This laboratory previously showed that Phe¹³⁹⁵ was required to obtain the electron transfer properties and calmodulin (CaM) response normally observed in wild-type nNOS. Here we characterized the F1395S mutant of the nNOS flavoprotein domain (nNOSr) regarding its physical properties, NADP⁺ binding characteristics, flavin reduction kinetics, steady-state and pre-steady-state cytochrome c reduction kinetics, and ability to shield its FMN cofactor in response to CaM or NADP(H) binding. F1395S nNOSr bound NADP⁺ with 65% more of the nicotinamide ring in a productive conformation with FAD for hydride transfer and had an 8-fold slower rate of NADP⁺ dissociation. CaM stimulated the rates of NADPH-dependent flavin reduction in wild-type nNOSr but not in the F1395S mutant, which had flavin reduction kinetics similar to those of CaM-free wild-type nNOSr. CaM-free F1395S nNOSr lacked repression of cytochrome c reductase activity that is typically observed in nNOSr. The combined results from pre-steady-state and EPR experiments revealed that this was associated with a lesser degree of FMN shielding in the NADP⁺-bound state as compared with wild type. We conclude that Phe¹³⁹⁵ regulates nNOSr catalysis in two ways. It facilitates NADP⁺ release to prevent this step from being rate-limiting, and it enables NADP(H) to properly regulate a conformational equilibrium involving the FMN subdomain that controls reactivity of the FMN cofactor in electron transfer.

Received for publication, January 26, 2004 , and in revised form, June 4, 2004.

^* This work was supported by National Institutes of Health Grants GM51491 (to D. J. S.) and GM36442 (to G. W. B). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^|| To whom correspondence may be addressed: Dept. of Immunology NB-30, The Lerner Research Institute, The Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195. Tel.: 216-445-6950; Fax: 216-444-9329; E-mail: stuehrd{at}ccf.org.

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