The Production of Nitrous Oxide by the Heme/Nonheme Diiron Center of Engineered Myoglobins (FeBMbs) Proceeds through a trans-Iron-Nitrosyl Dimer

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• 作者：Hirotoshi Matsumura ; Takahiro Hayashi ; Saumen Chakraborty ; Yi Lu ; Pierre Mo?nne-Loccoz
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：February 12, 2014
• 年：2014
• 卷：136
• 期：6
• 页码：2420-2431
• 全文大小：701K
• ISSN：1520-5126

文摘

Denitrifying NO reductases are transmembrane protein complexes that are evolutionarily related to heme/copper terminal oxidases. They utilize a heme/nonheme diiron center to reduce two NO molecules to N₂O. Engineering a nonheme Fe_B site within the heme distal pocket of sperm whale myoglobin has offered well-defined diiron clusters for the investigation of the mechanism of NO reduction in these unique active sites. In this study, we use FTIR spectroscopy to monitor the production of N₂O in solution and to show that the presence of a distal Fe_B^II is not sufficient to produce the expected product. However, the addition of a glutamate side chain peripheral to the diiron site allows for 50% of a productive single-turnover reaction. Unproductive reactions are characterized by resonance Raman spectroscopy as dinitrosyl complexes, where one NO molecule is bound to the heme iron to form a five-coordinate low-spin {FeNO}⁷ species with 谓(FeNO)_heme and 谓(NO)_heme at 522 and 1660 cm^鈥?, and a second NO molecule is bound to the nonheme Fe_B site with a 谓(NO)_FeB at 1755 cm^鈥?. Stopped-flow UV鈥搗is absorption coupled with rapid-freeze-quench resonance Raman spectroscopy provide a detailed map of the reaction coordinates leading to the unproductive iron-nitrosyl dimer. Unexpectedly, NO binding to Fe_B is kinetically favored and occurs prior to the binding of a second NO to the heme iron, leading to a (six-coordinate low-spin heme-nitrosyl/Fe_B-nitrosyl) transient dinitrosyl complex with characteristic 谓(FeNO)_heme at 570 卤 2 cm^鈥? and 谓(NO)_FeB at 1755 cm^鈥?. Without the addition of a peripheral glutamate, the dinitrosyl complex is converted to a dead-end product after the dissociation of the proximal histidine of the heme iron, but the added peripheral glutamate side chain in Fe_BMb2 lowers the rate of dissociation of the promixal histidine which in turn allows the (six-coordinate low-spin heme-nitrosyl/Fe_B-nitrosyl) transient dinitrosyl complex to decay with production of N₂O at a rate of 0.7 s^鈥? at 4 掳C. Taken together, our results support the proposed trans mechanism of NO reduction in NORs.