Gating mechanism, active conformation

NADPH oxidation and NO synthesis by the enzyme, it supports a role for reduction of the heme iron in catalysis, and may explain why NOS functions only as an NADPH-dependent reductase in the absence of bound calmodulin (Klatt et ai, 1993). The mechanism of calmodulin gating is envisioned to involve a conformational change between the reductase and oxygenase domains of NOS, such that an electron transfer between the terminal flavin and heme iron becomes possible. Calmodulin may also have a distinct role within the NOS reductase domain, in that its binding dramatically increases reductase activity of the enzyme toward cytochrome c (Klatt et al., 1993 Heinzel et al., 1992). However, it is clear that several other NOS functions occur independent of calmodulin, including the binding of L-arginine and NADPH, and transfer of NADPH-derived electrons into the flavins (Abu-Soud and Stuehr, 1993). 

Conformational gating processes may also lead to different mechanisms for oxidation and reduction reactions. Reduction of O to R can follow two alternate pathways O — P R and O — Q — R. Direct conversion of O -> R is unlikely, as Hoffman and Ratner (122) have demonstrated that the activation energy for the concerted O to R reaction is higher than for the two sequential reactions. It is possible that the reduction of O to R and the oxidation of R to O could follow different pathways, depending on the rate constants for the various steps. Thus,... 

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