Nitric Oxide Synthase: Difference between revisions

[[image:mette.png|400 px|left|model for NOS oxygen activation]]

The H₄B is bound by hydrogen-bonds to several of the molekules surrounding it, including the substrate L-Arg. The substrate is H-bonded to the 4-keto group of pterin, and to one of the heme propionate groups, that has two carboxylate oxygens in use for H-bonds. These oxygens are further H-bonded to the 4-keto group of pterin, through water, and directly to N(3) and NH₂ on C (2). The big picture of all the H-bonds can be seen by clicking on the figure on the bottom right.  

But H₄B is not only a structurel cofactor, it also plays a very important role in NO synthesis, donating an electron to the heme.<ref>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. So H₄B is a kinetically prefered electron donor. (indsæt fig.3 i artiklen) As shown in the reaction (fig.3) the second electron, that H₄B donates helps the Fe^IIO₂ intermadiate to be reduced in to oxidants that can react with Arg and N-hydroxy-L-arginine (NOHA) <ref>PMID: 12237227 </ref> If H₄B was not present the Fe^IIO₂ intermediate would decay to superoxide and ferric enzyme, because the reductase domain is slower to deliver an electron, than the proces of decay is to happen. But H₄B is faster than both of these processes. <ref>PMID: 12237227 </ref>