Kinetic Control of O2 Reactivity in H-NOX Domains

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• 作者：Yuhan Sun ; Abdelkrim Benabbas ; Weiqiao Zeng ; Sandhya Muralidharan ; Elizabeth M. Boon ; Paul M. Champion
• 刊名：Journal of Physical Chemistry B
• 出版年：2016
• 出版时间：June 23, 2016
• 年：2016
• 卷：120
• 期：24
• 页码：5351-5358
• 全文大小：491K
• 年卷期：0
• ISSN：1520-5207

文摘

Transient absorption, resonance Raman, and vibrational coherence spectroscopies are used to investigate the mechanisms of NO and O₂ binding to WT Tt H-NOX and its P115A mutant. Vibrational coherence spectra of the oxy complexes provide clear evidence for the enhancement of an iron–histidine mode near 217 cm^–1 following photoexcitation, which indicates that O₂ can be dissociated in these proteins. However, the quantum yield of O₂ photolysis is low, particularly in the wild type (≲3%). Geminate recombination of O₂ and NO in both of these proteins is very fast (∼1.4 × 10¹¹ s^–1) and highly efficient. We show that the distal heme pocket of the H-NOX system forms an efficient trap that limits the O₂ off-rate and determines the overall affinity. The distal pocket hydrogen bond, which appears to be stronger in the P115A mutant, may help retard the O₂ ligand from escaping into the solvent following either photoinduced or thermal dissociation. This, along with a strengthening of the Fe–O₂ bond that is correlated with the significant heme ruffing and saddling distortions, explains the unusually high O₂ affinity of WT Tt H-NOX and the even higher affinity found in the P115A mutant.