Arginine oxygenase

Arginine oxygenase Bacteria Arginine y-Guanidine butyra- Thoai and Olomuchki... 

Although the NOS oxygenase domain does not contain clear consensus sequences that identify heme, H4biopterin, of L-arginine binding domains, it is... 

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). 

Unlike cytochrome P-450 reductase, NOS is a self-sufficient enzyme in that the oxygenation of its substrate, L-arginine, occurs at the heme-site in the N-terminal portion, termed the oxygenase domain, of the protein. Stoichiometric amounts of heme are present in NOS and are required for catalytic activity. 

NO synthases are oxygenases that carry out a two-step oxidation of L-arginine to L-citrulline with production of NO. In the first step, a normal monooxygenase reaction, i -N -hydroxyarginine is formed (Eq. 18-65, step a). In the second step (Eq. 18-65, step b) NO is formed in a three-electron oxidation. In this equation the symbols and + indicate positions of incorporation of labeled 02 atoms in the intermediate and final products. 

Degradation of L-arginine by Streptomyces griseus is initiated by a hydroxylase that causes decarboxylation and conversion of the amino acid into an amide (Eq. 24-26), a reaction analogous to that catalyzed by the flavin-dependent lysine oxygenase (Eq. 18-41). The... 

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 bioinorganic chemistry of NO has only developed after the publication of the first edition of this book. Because this book deals with metal catalysis, a discussion of NO does not belong here. However, its reduction with an NO reductase [33] and its synthesis from L-arginine with an NO synthase oxygenase [34] clearly deal with metal heme systems and therefore should be mentioned here as well. 

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. 

Mammalian NOS is a P-450-like enzyme that catalyzes the oxidation of L-arginine to L-citrulline and NO. This process is a two-step reaction that leads to a five-electron oxidation of L-arginine. The enzyme requires NADPH and O2 as substrates for both reaction steps, and iron protoporphyrin IX (heme), FMN, FAD, and tetrahydrobiopterin (H4B) as protein-bound cofactors. NOS is active as a homodimer and contains an N-terminal oxygenase (or heme) domain, a C-terminal flavopro-tein reductase domain, and a central calmodulin binding region... 

Figure 9. Map of the primary amino acid sequence of nNOS, eNOS, and iNOS. A calcium-calmodulin binding region separates the oxygenase and reductase domains. The reductase domain contains binding sites for two flavin cofactors (FAD and FMN) as well as a binding site for the electron donor, NADPH. The oxygenase domain contains binding sites for the heme, the substrate (L-arginine), and tetrahyrobiopterin (H4B). (Adapted from Ref. [90].)...

Figure 11. Schematic ribbon drawing of the iNOS oxygenase dimer generated from the X-ray coordinates [107] illustrating the locations of heme, L-arginine, tetrahydrobiopterin, and the zinc ion. The zinc ion is tetrahedrally coordinated to its protein ligands.

Figure 4.2. Prostaglandin synthesis and inhibition in COX-1 and COX-2. (a)Initial stages of prostaglandin synthesis, (b) Binding stages of standard NSAIDs to arginine 120 to inhibit prostaglandin synthesis by direct blockade of cyclo-oxygenase charmel. (c) Differences between COX-1 and COX-2, (d) Specific blockade of COX-2.

---