Nitric Oxide Synthase: Difference between revisions
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The NOS homodimer is composed of two types of domains: an oxygenase domain and a reductase domain. Each subunit is held together by a Zinc ion, which is bound by two cysteines from each oxygenase domain. Binding of the domains is caused by calmodulin (CaM). The reductase domain supplies electrons for the NOS reaction which takes place in the oxygenase domain. The reductase domain contains two redox-active prosthetic groups, flavin adenine dinucleotide (FAD) and Flavin mononucleotide (FMN). Nicotinamide adenine dinucleotide phosphate(NADPH) binds to the domain and passes on an electron to FAD which passes the electron on to FMN. FMN passes the electron on to the Heme in the oxygenase domain on the opposite subunit. The oxygenase domain contains H<sub>4</sub>B (5,6,7,8-tetrahydrobiopterin)and the already mentioned Heme ion (Fe(III)). These two are also redox active groups. H<sub>4</sub>B is required by NOS in order to produce NO and not H<sub>2</sub>O<sub>2</sub>. Besides Heme and H<sub>4</sub>B, the oxygenase domain binds the substrate L-arginine which takes part in the NO synthase reaction (see below). | The NOS homodimer is composed of two types of domains: an oxygenase domain and a reductase domain. Each subunit is held together by a Zinc ion, which is bound by two cysteines from each oxygenase domain. Binding of the domains is caused by calmodulin (CaM). The reductase domain supplies electrons for the NOS reaction which takes place in the oxygenase domain. The reductase domain contains two redox-active prosthetic groups, flavin adenine dinucleotide (FAD) and Flavin mononucleotide (FMN). Nicotinamide adenine dinucleotide phosphate(NADPH) binds to the domain and passes on an electron to FAD which passes the electron on to FMN. FMN passes the electron on to the Heme in the oxygenase domain on the opposite subunit. The oxygenase domain contains H<sub>4</sub>B (5,6,7,8-tetrahydrobiopterin)and the already mentioned Heme ion (Fe(III)). These two are also redox active groups. H<sub>4</sub>B is required by NOS in order to produce NO and not H<sub>2</sub>O<sub>2</sub>. Besides Heme and H<sub>4</sub>B, the oxygenase domain binds the substrate L-arginine which takes part in the NO synthase reaction (see below). | ||
== The reaction of NOS == | == The reaction of NOS == | ||
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<scene name='Nitric_oxide_synthase/Nos_oxygenase_zinc/2'>Zinc ion</scene> | <scene name='Nitric_oxide_synthase/Nos_oxygenase_zinc/2'>Zinc ion</scene> | ||
==== General Structure ==== | ==== General Structure ==== | ||
The oxygenase domain can be divided into three subdomains. Firstly, the substrate-binding subdomain, which is crescent in shape, binds the substrate in an interior pocket of the crescent. The Heme group is located between the tips of the crescent shape, thus closing the pocket in which the substrate is located. Secondly the the H4B binding subdomain serves as a cap for the for the cavity created by the substrate binding domain's crescent shape. Thirdly there is a subdomain with two helical bundles making up a hydrophobic core, however this subdomain is not thought to take part of the catalytic activity of the enzyme. | The oxygenase domain can be divided into three subdomains. Firstly, the substrate-binding subdomain, which is crescent in shape, binds the substrate in an interior pocket of the crescent. The Heme group is located between the tips of the crescent shape, thus closing the pocket in which the substrate is located. Secondly the the H4B binding subdomain serves as a cap for the for the cavity created by the substrate binding domain's crescent shape. Thirdly there is a subdomain with two helical bundles making up a hydrophobic core, however this subdomain is not thought to take part of the catalytic activity of the enzyme. | ||
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=== Substrate binding === | === Substrate binding === | ||
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binds its substrate (L-arginine) by coordinating CO to the heme at the site occupied by oxygen....<ref>PMID:9376373 </ref>. | binds its substrate (L-arginine) by coordinating CO to the heme at the site occupied by oxygen....<ref>PMID:9376373 </ref>. | ||
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===H<sub>4</sub>B=== | ===H<sub>4</sub>B=== | ||
[[image:bh4.png|left|frame|Structure of tetrahydrobiopterin]] | [[image:bh4.png|left|frame|Structure of tetrahydrobiopterin]] | ||