Insights into heme-based O2 sensing from structure-function relationships in the FixL proteins

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• FixL proteins are heme-containing bacterial signal transducers crucial for sensing environmental oxygen concentrations.
• In *Sinorhizobium meliloti* and *Bradyrhizobium japonicum*, FixL acts as a protein histidine kinase, forming an oxygen-sensitive switch with FixJ to regulate nitrogen fixation and respiration genes during symbiosis.

Purpose of the Study:

• To investigate the molecular mechanisms by which FixL proteins sense oxygen and transduce this signal to regulate gene expression.
• To characterize the kinetic and thermodynamic properties of ligand binding to FixL and elucidate the structural basis for oxygen sensitivity.

Main Methods:

• UV-visible spectroscopy
• Resonance Raman spectroscopy
• Extended X-ray absorption fine structure (EXAFS)
• X-ray crystallography
• Kinetic and thermodynamic studies of ligand binding

Main Results:

• FixL exhibits low ligand affinity, primarily due to slow ligand binding rates.
• Oxygenation of the heme group in FixL leads to a transition from a six-coordinate low-spin to a five-coordinate high-spin state.
• Distal effects in the heme pocket are critical for communicating the heme's ligation state to the kinase domain, involving conformational changes.

Conclusions:

• The FixLJ system functions as an oxygen-sensitive switch, with FixL's heme pocket dynamics mediating signal transduction.
• Understanding these mechanisms provides insight into bacterial adaptation to varying oxygen environments, particularly during symbiotic interactions.