Pulsed ENDOR Determination of the Arginine Location in the Ferrous鈥揘O Form of Neuronal NOS

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• 作者：Andrei V. Astashkin ; Bradley O. Elmore ; Li Chen ; Weihong Fan ; J. Guy Guillemette ; Changjian Feng
• 刊名：The Journal of Physical Chemistry A
• 出版年：2012
• 出版时间：June 28, 2012
• 年：2012
• 卷：116
• 期：25
• 页码：6731-6739
• 全文大小：441K
• 年卷期：v.116,no.25(June 28, 2012)
• ISSN：1520-5215

文摘

Mammalian nitric oxide synthases (NOSs) are enzymes responsible for oxidation of l-arginine (l-Arg) to nitric oxide (NO). Mechanisms of reactions at the catalytic heme site are not well understood, and it is of current interest to study structures of the heme species that activates O₂ and transforms the substrate. The NOS ferrous鈥揘O complex is a close mimic of the obligatory ferric (hydro)peroxo intermediate in NOS catalysis. In this work, pulsed electron鈥搉uclear double resonance (ENDOR) was used to probe the position of the l-Arg substrate at the NO^{鈥?/sup>-coordinated ferrous heme center(s) in the oxygenase domain of rat neuronal NOS (nNOS). The analysis of 2H and 15N ENDOR spectra of samples containing d₇- or guanidino-15N₂ labeled l-Arg has resulted in distance estimates for the nearby guanidino nitrogen and the nearby proton (deuteron) at C_未. The l-Arg position was found to be noticeably different from that in the X-ray crystal structure of nNOS ferrous鈥揘O complex [Li et al. J. Biol. Inorg. Chem.2006, 11, 753鈥?68], with the nearby guanidino nitrogen being 0.5 脜 closer to, and the nearby H_未 about 1 脜 further from, the NO ligand than in the X-ray structure. The difference might be related to the structural constraints imposed on the protein by the crystal. Importantly, in spite of its closer position, the guanidino nitrogen does not form a hydrogen bond with the NO ligand, as evidenced by the absence of significant isotropic hfi constant for N_g1. This is consistent with the previous reports that it is not the l-Arg substrate itself that would most likely serve as a direct proton donor to the diatomic ligands (NO and O₂) bound to the heme.}