Nitric Oxide Synthase: Difference between revisions
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== The Oxygenase domain of NOS == | == The Oxygenase domain of NOS == | ||
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The oxygenase domain can also 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 BH4 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 also 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 BH4 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. | ||
== Substrate binding == | === Substrate binding === | ||
The active site is highly conserved in the different NOS species. Thus it is possible to discuss substrate binding i general terms. The NOS enzyme binds its substrate by hydrogen bindings both to the guanidino[http://en.wikipedia.org/wiki/Guanidino] end and the amino acid end (EVT FIG!). | The active site is highly conserved in the different NOS species. Thus it is possible to discuss substrate binding i general terms. The NOS enzyme binds its substrate by hydrogen bindings both to the guanidino[http://en.wikipedia.org/wiki/Guanidino] end and the amino acid end (EVT FIG!). | ||
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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>. | ||
== The reaction of NOS: == | === The reaction of NOS: === | ||
As previously described NOS is an enzyme split in to different domains; the N-terminal oxygenase domain and the C-terminal reductase domain. | As previously described NOS is an enzyme split in to different domains; the N-terminal oxygenase domain and the C-terminal reductase domain. | ||
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The reaction above therefore takes both the oxygenase and the reductase domain into account. | The reaction above therefore takes both the oxygenase and the reductase domain into account. | ||
==H<sub>4</sub>B== | ===H<sub>4</sub>B=== | ||
[[image:bh4.png|right|frame|Structure of tetrahydrobiopterin]] | [[image:bh4.png|right|frame|Structure of tetrahydrobiopterin]] | ||
{{STRUCTURE_2g6h | PDB=2g6h | SCENE= }} | {{STRUCTURE_2g6h | PDB=2g6h | SCENE= }} | ||
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It is also known that H<sub>4</sub>B works as a elctron donor to reduce a oxyferrous complex (HVAD ER DETTE) from Fe (III) to Fe(II), but as stated above, it does not reduce the ferric heme. (fra artiklen Nitric-oxide synthase: A cytochrome p450 family foster child, by A. C. F. Gorren og B. Mayer) | It is also known that H<sub>4</sub>B works as a elctron donor to reduce a oxyferrous complex (HVAD ER DETTE) from Fe (III) to Fe(II), but as stated above, it does not reduce the ferric heme. (fra artiklen Nitric-oxide synthase: A cytochrome p450 family foster child, by A. C. F. Gorren og B. Mayer) | ||
==Heme== | ===Heme=== | ||
===Zinc=== | |||
== The Reductase Domain of NOS == | |||
{{STRUCTURE_1tll|PDB=1tll|SCENE=Sandbox_5/Nos_reductase_cofactors/1}} | |||
The reductase domain of the NOS homodimer will not be discussed thoroughly at this page. However a short discussion of the electron transfer which occurs will be given along with an introduction to the bound cofactors and the general structure. | |||
The <scene name='Sandbox_5/Nos_reductase_cofactors/1'>reductase domain</scene> has three cofactors bound: | |||
<scene name='Sandbox_5/Nos_reductase_napd/1'>NADPH</scene> ([http://en.wikipedia.org/wiki/NADPH Nicotinamide adenine dinucleotide phosphate]) | |||
<scene name='Sandbox_5/Nos_reductase_fad/3'>FAD</scene> ([http://en.wikipedia.org/wiki/FAD Flavin adenine dinucleotide]) | |||
<scene name='Sandbox_5/Nos_reductase_fmn/2'>FMN</scene> ([http://en.wikipedia.org/wiki/Flavin_mononucleotide Flavin mononucleotide]) | |||
The reductase domain is, as mentioned, bound to an oxygenase domain by a calmodulin linker. This linker responds to Ca2+ -ions (constitutive NOS isoforms). The calmodulin linker is consists of 32 residues and contains a binding region for the Ca2+-ions. This binding is found to be crucial it induces a conformational change which is essential for the electron transfer. It is important to emphasize that the electron transfer occurs from the reductase domain of one subunit to the oxygenase domain of the opposite subunit (i.e. a trans transfer). The conformational change induced by Ca2+-ions brings the mentioned reductase and oxygenase domains closer together, therefore the linker acts like a hinge. The electron transfer occurs two times per produced NO molecule, first electrons are passed on for the conversion of L-Arginine to its intermediate, secondly for the conversion of the intermediate to produce Citruline and NO. In general the reductase domain can be divided into three binding domains: the NADPH binding domain, the FAD binding domain, and the FMN binding domain. The NADPH and FAD binding domains are associated whereas the FAD and FMN domains are connected by an α-helical binding domain. An electron is donated by NADPH, which passes the electron on to FAD. FAD shuttles on the electron to FMN. The FMN binding domain is a flexible domain and here the conformational change occurs, the Calmodulin linker rotates the reductase domain and oxygenase domain along a vertical axis, thus bringing the reductase domain closer to the opposite oxygenase domain. The electron can then due to shorter distance be passed on the the Heme group bound by the oxygenase domain. | |||