An Additional Role for CaM

Of the four catalytic activities stimulated by CaM, two activities (cytochrome c and ferricyanide reductions) were similarly stimulated in apo-NOS when compared to native NOS (Table I). This indicated that CaM s activation of these processes occurred through a mechanism not involving the flavin-to-heme electron transfer. Further analysis showed that CaM binding increased the rate of electron transfer from NADPH into the flavin centers by a factor of 20, revealing a direct activation of the reductase domain by CaM. 

TABLE I Calmodulin (CaM) Activation of Catalytic Functions in Native and apo-NOS° 

In contrast, CaM s activation of NO synthesis and substrate-independent NADPH oxidase activity did appear to involve flavin-to-heme electron transfer, because these reactions were not activated in apo-NOS and were blocked in native NOS by agents that prevent heme iron reduction (Abu-Soud et al., 1994a). We conclude that CaM activates neuronal NOS at two points (Fig. 2) electron transfer into the flavins and interdomain electron transfer between the flavins and heme. Activation at each point is associated with an up-regulation of domain-specific catalytic functions. The dual regulation by CaM is unique and represents a new means by which electron transfer can be controlled in a metal-containing flavoprotein. 

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