Nitric-oxide synthases heme iron

Geng, Y. J., Petersson, A. S., Wennmalm, A., and Hansson, G. K. (1994). Cytokine-induced expression of nitric oxide synthase results in nitrosylation of heme and nonheme iron proteins in vascular smooth muscle cells. Exp. Cell Res. 214, 418-428. 

In 1989, BH4 was found to be a cofactor for nitric oxide synthase (NOS) [ 126, 127]. BH4 is also involved in dimerization of NOS, as NOS is catalytically active in a homodimer structure. Three isoforms of NOS exist neuronal NOS (NOS 1), inducible NOS (NOS 2) and endothelial NOS (NOS 3). BH4 is essential for all NOS isoforms. The NOS isoforms share approximately 50-60% sequence homology. Each NOS polypeptide is comprised of oxygenase and reductase domains. An N-terminal oxygenase domain contains iron protoporphyrin IX (heme), BH4 and an arginine binding site, and a C-terminal reductase domain contains flavin mononucleotide (FMN), and a reduced nicotin-amide adenine dinucleotide phosphate (NADPH) binding site. 

The amino acid Arginine is a precursor to a novel second messenger and neurotransmitter, which is a gas, nitric oxide. Nitric oxide is produced from arginine in an unusual five-electron oxidation that also yields citrulline (see Figure 21.3). The enzyme catalyzing the reaction, nitric oxide synthase, contains bound FMN, FAD, non-heme iron, and tetrahydrobiopterin. 

Figure 1.1. Iron(lII) protoporphyrin IX with a cysteinate as the axial ligand (1), which is typical of cytochrome P450, chloroperoxidase (CPO), and nitric oxide synthase (NOS) enzymes. The active oxygen species of these proteins and related heme enzymes is an oxoiron(IV) porphyrin cation radical (2), often called compound I.

FIGURE I Role for calmodulin (CaM) in triggering interdomain electron transfer to the nitric oxide synthase (NOS) heme iron. Electrons derived from NADPH can transfer only into the flavin centers of CaM-free neuronal NOS (A). CaM binding to NOS occurs in response to elevated Ca concentrations, and this enables electrons to transfer from the flavins to the heme iron. Heme iron reduction is associated with increased NADPH oxidation and results in (B) superoxide (O2) production in the absence of L-arginine or (C) nitric oxide (NO) synthesis in the presence of L-arginine. FAD, Flavin-adenine dinucleotide FMN, flavin mononucleotide. 

FIGURE 2 Proposed dual mode for calmodulin (CaM) control of nitric oxide synthase (NOS) electron transfer. Neuronal NOS is composed of a reductase and an oxygenase domain, shown as two circles. CaM binding to NOS activates at two points in the electron transfer sequence (1) It increases the rate at which NADPH-derived electrons are transferred into the flavins, and (2) it enables the flavins to pass electrons to the oxygenase domain of NOS. Activation at the first point is associated with an increase in reductase domain-specific catalytic activities, such as electron transfer to cytochrome c or ferricyanide (FeCN ). Activation at the second point is associated with a reduction of NOS heme iron, an initiation of NO synthesis from L-arginine (Arg), or a reduction of Oj to form superoxide (O2) in the absence of substrate. FAD, Flavin-adenine dinucleotide FMN, flavin mononucleotide NO, nitric oxide. 

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