Heme oxygenase catalytic oxidation

Fig. 2. The distinct steps in the oxidation of heme to biliverdin catalyzed by heme oxygenase. The substituted carbons of the porphyrin ring are labeled, as are the meso-positions. The oxygens introduced in the catalytic process are shown in bold t3rpe.

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. 

Three isoforms of NOS are produced in mammalian cells neuronal (nNOS), endothelial (eNOS), and inducible (iNOS) [55]. All NOS isoforms exist as homodimers with a C-terminal FMN-FAD fused reductase domain, an N-terminal oxygenase domain, and a calmodulin binding sequence at the interface of the two domains. The NOS catalytic mechanism is complicated and requires O2, NADPH, FMN, FAD, Ca2+, calmodulin, tetrahydro-biopterin (BH4), and heme to effect the five-electron oxidation of L-arginine to L-citrulline and NO. Consumed in this process are 1.5 equivalents of NADPH and 2 equivalents of O2. 

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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