TY - JOUR
T1 - The FAD-shielding residue Phe1395 regulates neuronal nitric-oxide synthase catalysis by controlling NADP+ affinity and a conformational equilibrium within the flavoprotein domain
AU - Konas, David W.
AU - Zhu, Keng
AU - Sharma, Manisha
AU - Aulak, Kulwant S.
AU - Brudvig, Gary W.
AU - Stuehr, Dennis J.
N1 - Copyright:
Copyright 2008 Elsevier B.V., All rights reserved.
PY - 2004/8/20
Y1 - 2004/8/20
N2 - 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 Phe 1395 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.
AB - 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 Phe 1395 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.
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U2 - 10.1074/jbc.M400872200
DO - 10.1074/jbc.M400872200
M3 - Article
C2 - 15180983
AN - SCOPUS:4143083758
VL - 279
SP - 35412
EP - 35425
JO - Journal of Biological Chemistry
JF - Journal of Biological Chemistry
SN - 0021-9258
IS - 34
ER -