Reactivity of the Flavin Semiquinone of Nitric Oxide Synthase in the Oxygenation of Arginine to N^G-Hydroxyarginine, the First Step of Nitric Oxide Synthesis

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• 作者：Witteveen ; Cor F. B. ; Giovanelli ; John ; Yim ; Moon B. ; Gachhui ; Ratan ; Stuehr ; Dennis J. ; Kaufman ; Seymour
• 刊名：Biochemical and Biophysical Research Communications
• 出版年：1998
• 出版时间：September 8, 1998
• 年：1998
• 卷：250
• 期：1
• 页码：36-42
• 全文大小：157 K

文摘

Nitric oxide synthase (NOS) is a heme protein that catalyzes the oxygenation of L-arginine in the presence of NADPH to form nitric oxide, L-citrulline and NADP⁺, and proceeds via two partial reactions:L-Arginine → N^G-hydroxy-L-arginine (1)N^G-Hydroxy-L-arginine → L-citrulline + nitric oxide (2)Calmodulin, FAD, FMN and tetrahydrobiopterin are required for both reactions. Reactions 1 and 2 require the input of 2 and 1 electron equivalents, respectively. Under normal multiple turnover conditions, these electrons are ultimately derived from NADPH. We previously reported that NOS contains an endogenous reductant that, in the absence of NADPH, can support the single-turnover oxygenation of L-arginine to N^G-hydroxy-L-arginine and a relatively small amount of L-citrulline [Campos, K. L., Giovanelli, J., and Kaufman, S. (1995)J. Biol. Chem.270, 1721–1728]. This reductant has now been identified as the stable flavin semiquinone free radical (FSQ). Its oxidation appears to be coupled to the formation of N^G-hydroxy-L-arginine and L-citrulline. The rate of FSQ oxidation is two orders of magnitude slower than the flux of electrons from NADPH through NOS during normal turnover of the enzyme, indicating that FSQ is not the proximal electron donor for heme under these conditions.