Biomaterial Database

Arginine conversion to nitroxide by tetrahydrobiopterin-free neuronal nitric-oxide synthase. Implications for mechanism. 2000

S Adak, and Q Wang, and D J Stuehr

Department of Immunology, Lerner Research Institute, Cleveland Clinic, Cleveland, Ohio 44195, USA. adaks@ccf.org

We studied catalysis by tetrahydrobiopterin (H4B)-free neuronal nitric-oxide synthase (nNOS) to understand how heme and H4B participate in nitric oxide (NO) synthesis. H4B-free nNOS catalyzed Arg oxidation to N(omega)-hydroxy-l-Arg (NOHA) and citrulline in both NADPH- and H(2)O(2)-driven reactions. Citrulline formation was time- and enzyme concentration-dependent but was uncoupled relative to NADPH oxidation, and generated nitrite and nitrate without forming NO. Similar results were observed when NOHA served as substrate. Steady-state and stopped-flow spectroscopy with the H4B-free enzyme revealed that a ferrous heme-NO complex built up after initiating catalysis in both NADPH- and H(2)O(2)-driven reactions, consistent with formation of nitroxyl as an immediate product. This differed from the H4B-replete enzyme, which formed a ferric heme-NO complex as an immediate product that could then release NO. We make the following conclusions. 1) H4B is not essential for Arg oxidation by nNOS, although it helps couple NADPH oxidation to product formation in both steps of NO synthesis. Thus, the NADPH- or H(2)O(2)-driven reactions form common heme-oxy species that can react with substrate in the presence or absence of H4B. 2) The sole essential role of H4B is to enable nNOS to generate NO instead of nitroxyl. On this basis we propose a new unified model for heme-dependent oxygen activation and H4B function in both steps of NO synthesis.

UI	MeSH Term	Description	Entries
D009249	NADP	Nicotinamide adenine dinucleotide phosphate. A coenzyme composed of ribosylnicotinamide 5'-phosphate (NMN) coupled by pyrophosphate linkage to the 5'-phosphate adenosine 2',5'-bisphosphate. It serves as an electron carrier in a number of reactions, being alternately oxidized (NADP+) and reduced (NADPH). (Dorland, 27th ed)	Coenzyme II,Nicotinamide-Adenine Dinucleotide Phosphate,Triphosphopyridine Nucleotide,NADPH,Dinucleotide Phosphate, Nicotinamide-Adenine,Nicotinamide Adenine Dinucleotide Phosphate,Nucleotide, Triphosphopyridine,Phosphate, Nicotinamide-Adenine Dinucleotide
D009569	Nitric Oxide	A free radical gas produced endogenously by a variety of mammalian cells, synthesized from ARGININE by NITRIC OXIDE SYNTHASE. Nitric oxide is one of the ENDOTHELIUM-DEPENDENT RELAXING FACTORS released by the vascular endothelium and mediates VASODILATION. It also inhibits platelet aggregation, induces disaggregation of aggregated platelets, and inhibits platelet adhesion to the vascular endothelium. Nitric oxide activates cytosolic GUANYLATE CYCLASE and thus elevates intracellular levels of CYCLIC GMP.	Endogenous Nitrate Vasodilator,Mononitrogen Monoxide,Nitric Oxide, Endothelium-Derived,Nitrogen Monoxide,Endothelium-Derived Nitric Oxide,Monoxide, Mononitrogen,Monoxide, Nitrogen,Nitrate Vasodilator, Endogenous,Nitric Oxide, Endothelium Derived,Oxide, Nitric,Vasodilator, Endogenous Nitrate
D010084	Oxidation-Reduction	A chemical reaction in which an electron is transferred from one molecule to another. The electron-donating molecule is the reducing agent or reductant; the electron-accepting molecule is the oxidizing agent or oxidant. Reducing and oxidizing agents function as conjugate reductant-oxidant pairs or redox pairs (Lehninger, Principles of Biochemistry, 1982, p471).	Redox,Oxidation Reduction
D002384	Catalysis	The facilitation of a chemical reaction by material (catalyst) that is not consumed by the reaction.	Catalyses
D002956	Citrulline
D006418	Heme	The color-furnishing portion of hemoglobin. It is found free in tissues and as the prosthetic group in many hemeproteins.	Ferroprotoporphyrin,Protoheme,Haem,Heme b,Protoheme IX
D006861	Hydrogen Peroxide	A strong oxidizing agent used in aqueous solution as a ripening agent, bleach, and topical anti-infective. It is relatively unstable and solutions deteriorate over time unless stabilized by the addition of acetanilide or similar organic materials.	Hydrogen Peroxide (H2O2),Hydroperoxide,Oxydol,Perhydrol,Superoxol,Peroxide, Hydrogen
D001120	Arginine	An essential amino acid that is physiologically active in the L-form.	Arginine Hydrochloride,Arginine, L-Isomer,DL-Arginine Acetate, Monohydrate,L-Arginine,Arginine, L Isomer,DL Arginine Acetate, Monohydrate,Hydrochloride, Arginine,L Arginine,L-Isomer Arginine,Monohydrate DL-Arginine Acetate
D001708	Biopterins	Pterin derivatives based on 2-amino-6-(1,2-dihydroxypropyl)-4(1H)-pteridinone. Biopterins are natural products that have been considered as growth factors for some insects. Biopterins are cofactors for the AROMATIC AMINO ACID hydroxylases and NITRIC OXIDE SYNTHASE. Deficiencies in BIOPTERINS metabolism (e.g., lowered TETRAHYDROBIOPTERIN) are associated with neurological deterioration (e.g., HYPERPHENYLALANINAEMIA).	2-Amino-6-((1S,2R)-1,2-dihydroxypropyl)-4(1H)-pteridinone,2-Amino-6-((1S,2S)-1,2-dihydroxypropyl)-4(1H)-pteridinone,2-Amino-6-(1,2-dihydroxypropyl)-4(8H)-pteridinone,2-amino-6-((1R,2R)-1,2-dihydroxypropyl)-4(3H)-pteridinone,4(1H)-Pteridinone, 2-amino-6-(1,2-dihydroxypropyl)-, (S-(R,S))-,6-Biopterin,Biopterin,D-threo-Biopterin,L-Biopterin,L-erythro-Biopterin,L-threo-Biopterin,2-Amino-6-(1,2-dihydroxypropyl)-4(1H)-pteridinone,Dictyopterin,Orinapterin,6 Biopterin,D threo Biopterin,L Biopterin,L erythro Biopterin,L threo Biopterin
D052248	Nitric Oxide Synthase Type I	A CALCIUM-dependent, constitutively-expressed form of nitric oxide synthase found primarily in NERVE TISSUE.	NCNOS Enzyme,NOS1 Protein,Neural Constitutive Nitric Oxide Synthase,Neuronal Nitric Oxide Synthase,Nitric Oxide Synthase, Type I,nNOS Enzyme