Structures of the N-Hydroxy-L-Arginine Complex of Inducible Nitric Oxide Synthase Oxygenase

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• 作者：Brian R. Crane ; Andrew S. Arvai ; Sanjay Ghosh ; Elizabeth D. Getzoff ; Dennis J. Stuehr ; and John A. Tainer
• 刊名：Biochemistry
• 出版年：2000
• 出版时间：April 25, 2000
• 年：2000
• 卷：39
• 期：16
• 页码：4608 - 4621
• 全文大小：1486K
• 年卷期：v.39,no.16(April 25, 2000)
• ISSN：1520-4995

文摘

Nitric oxide synthases (NOSs) catalyze two mechanistically distinct, tetrahydrobiopterin (H₄B)-dependent, heme-based oxidations that first convert L-arginine (L-Arg) to N-hydroxy-L-arginine (NHA)and then NHA to L-citrulline and nitric oxide. Structures of the murine inducible NOS oxygenase domain(iNOS_ox) complexed with NHA indicate that NHA and L-Arg both bind with the same conformationadjacent to the heme iron and neither interacts directly with it nor with H₄B. Steric restriction of dioxygenbinding to the heme in the NHA complex suggests either small conformational adjustments in the ternarycomplex or a concerted reaction of dioxygen with NHA and the heme iron. Interactions of the NHAhydroxyl with active center -structure and the heme ring polarize and distort the hydroxyguanidinium toincrease substrate reactivity. Steric constraints in the active center rule against superoxo-iron accepting ahydrogen atom from the NHA hydroxyl in their initial reaction, but support an Fe(III)-peroxo-NHA radicalconjugate as an intermediate. However, our structures do not exclude an oxo-iron intermediate participatingin either L-Arg or NHA oxidation. Identical binding modes for active H₄B, the inactive quinonoid-dihydrobiopterin (q-H₂B), and inactive 4-amino-H₄B indicate that conformational differences cannot explainpterin inactivity. Different redox and/or protonation states of q-H₂B and 4-amino-H₄B relative to H₄Blikely affect their ability to electronically influence the heme and/or undergo redox reactions during NOScatalysis. On the basis of these structures, we propose a testable mechanism where neutral H₄B transfersboth an electron and a 3,4-amide proton to the heme during the first step of NO synthesis.