Nitric Oxide Synthase: Difference between revisions

The reaction is:

[[image:Makroprojekt-chemdraw-1-.gif|frame|left|thumb|450px]]

The reaction occurs in two steps. In the first step L-arginine is turned in to N-hydroxy-arginine. In the second step L-citrulline and NO are formed. The reaction utilizes 2O₂ and 3 electrons from 3/2 NADPH.<ref>PMID:17014963</ref>

===H₄B===

[[image:bh4.png|left|frame|Structure of tetrahydrobiopterin]]

<scene name='Sandbox_5/Nos_oxygenase_bh4/11'>H4B</scene> is an essential cofactor in NOS and in the [[aromatic amino acid hydroxylases]]. NOS contains two molecules of <scene name='Sandbox_5/Begge_h4b/1'>H4B</scene>, one in each monomer. The active site forms part of the cavity already described. This cavity can be visualized as a <scene name='Nitric_oxide_synthase/Substratebinding_distal_pocket/2'>tunnel</scene>. Here H₄B helps substrate interactions by lining the active-center tunnel and hydrogen bonding to the heme propionate and to alfa helix 7. The propionate group and alpha helix 7 are also involved in the L-Arg binding. This gives H₄B the opportunity to play an important role in the control of subunit interactions and active site formation. H₄B is therefore more or less a structural cofactor and has a stabilizing effect. Its structural importance is further reconned to play a role in dimer formation (dimerization requires bound zinc ion along with H₄B), and major conformational changes leading to the formation of the active site channelform<ref name="Raman">PMID:9875848</ref>.

[[image:H4b_hydrogenbindinger2.png|thumb|left|Hydrogenbondings in H₄B binding site ([[2nse]]) ]]

But H₄B is not only a structural cofactor, it also plays a very important role in NO synthesis, donating an electron to the heme.<ref name="heme">PMID:12237227</ref> H₄B can deliver an electron to the heme much faster than the reductase domain can, therefor H₄B is used by NOS in the Arg hydroxylation, activating O₂ by providing the second electron. Thus, H₄B is a kinetically prefered electron donor. As shown in the reaction (bottom right, click for enlargement) the second electron, that H₄B donates, helps the Fe^IIO₂ intermediate to be reduced to oxidants that are able to react with Arg and N-hydroxy-L-arginine (NOHA) <ref name="heme"/> If H₄B was not present the Fe^IIO₂ intermediate would decay to superoxide and ferric enzyme due to the reductase domain being slower to deliver an electron than the proces of decay is to happen. H₄B is faster than both of these processes<ref name="heme"/>.

PDB structures used in the section above: [[3nos]], [[2g6h]]