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Papers In Press, published online ahead of print June 4, 2004
J. Biol. Chem, 10.1074/jbc.M400872200
Submitted on January 26, 2004
Revised on June 4, 2004
Accepted on June 4, 2004

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

Department of Immunology, Cleveland Clinic, Cleveland, OH 44195

Corresponding Author: stuehrd{at}ccf.org

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 to 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 sub-domain that controls reactivity of its FMN cofactor in electron transfer.


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