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J Phys Chem B. 2013 Dec 12;117(49):15786-91. doi: 10.1021/jp406709e. Epub 2013 Aug 26.

Ultraviolet resonance Raman observations of the structural dynamics of rhizobial oxygen sensor FixL on ligand recognition.

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  • 1Department of Chemistry, Graduate School of Science, Osaka University , 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan.


FixL is a heme-based oxygen-sensing histidine kinase that induces expression of nitrogen fixation genes under hypoxic conditions. Oxygen binding to heme iron in the sensor domain of FixL initiates protein conformational changes that are transmitted to the histidine kinase domain, inactivating autophosphorylation activity. Although FixL also can bind other diatomic ligands such as CO, the CO-bound FixL represents incomplete inhibition of kinase activity. Ultraviolet resonance Raman (UVRR) spectra revealed that oxygen binding to the truncated sensor domain of FixL markedly decreased the intensity of the Y8a band arising from Fα-10 Tyr. In contrast, no appreciable change in intensity of the Y8a band occurred after CO binding, and time-resolved UVRR spectra of the sensor domain of FixL upon O2 dissociation indicated that structural changes near Fα-10 Tyr occurred at ∼0.1 μs. These results suggest that O2 dissociation from FixL changes the protein conformation near the Fα-10 Tyr residue within a microsecond. The conformational changes of FixL upon O2 dissociation and the underlying sensing mechanism also are discussed.

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