The Reactivity with CO of AHb1 and AHb2 from Arabidopsis thaliana is Controlled by the Distal HisE7 and Internal Hydrophobic Cavities

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• 作者：Stefano Bruno ; Serena Faggiano ; Francesca Spyrakis ; Andrea Mozzarelli ; Stefania Abbruzzetti ; Elena Grandi ; Cristiano Viappiani ; Alessandro Feis ; Stephan Mackowiak ; Giulietta Smulevich ; Elena Cacciator
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：March 14, 2007
• 年：2007
• 卷：129
• 期：10
• 页码：2880 - 2889
• 全文大小：590K
• 年卷期：v.129,no.10(March 14, 2007)
• ISSN：1520-5126

文摘

The nonsymbiotic hemoglobins, AHb1 and AHb2, have recently been isolated from Arabidopsisthaliana. Using steady-state and time-resolved spectroscopic methods, we show that Fe²⁺ AHb1 containsa mixture of penta- and hexacoordinated heme, while Fe²⁺ AHb2 is fully hexacoordinated. In the COcomplexes, polar interactions and H-bonds with the ligand are stronger for AHb1 than for AHb2. The ligandbinding kinetics are substantially different, reflecting the distribution between the penta- and hexacoordinatedspecies, and indicate that protein dynamics and ligand migration pathways are very specific for each ofthe two proteins. In particular, a very small, non-exponential geminate rebinding observed in AHb1 suggeststhat the distal heme cavity is connected with the exterior by a relatively open channel. The large, temperature-dependent geminate rebinding observed for AHb2 implies a major role of protein dynamics in the ligandmigration from the distal cavity to the solvent. The structures of AHb1 and AHb2, modeled on the basis ofthe homologous rice hemoglobin, exhibit a different cavity system that is fully compatible with the observedligand binding kinetics. Overall, these kinetic and structural data are consistent with the putativeNO-dioxygenase activity previously attributed to AHb1, whereas the role of AHb2 remains elusive.