Cooperative ligand binding in a bacterial heme-based oxygen sensor

Bacteria modulate essential phenotypes in response to external signals such as the availability of molecular oxygen (O₂). A class of direct O₂-sensing heme proteins, globin-coupled sensors, have been implicated in O₂-dependent regulation of pathogenic phenotypes, including biofilm formation, motility, and virulence. While cooperative O₂ binding is well known in both mammalian and prokaryotic hemoglobins, cooperative ligand binding previously has not been observed in bacterial sensor globins. This study explores the O₂-dependent allosteric communication between globin domains in the globin-coupled sensor protein from Pectobacterium carotovorum (PccGCS) through equilibrium O₂-binding measurements, X-ray crystallography, resonance Raman spectroscopy, and hydrogen-deuterium exchange mass spectrometry. Based on these experiments, we propose a model of allosteric regulation of O₂ binding that is directed by subtle changes in distal heme pocket protein conformation and transduced through dynamics of helices at the dimer interface of the PccGCS sensor globin. Together, this work identifies cooperative ligand binding in a family of bacterial heme proteins, which could allow the bacteria to more robustly respond to small changes in O₂ levels. Furthermore, this work highlights the importance of heme pocket residues in transducing the O₂ binding event within the dimer and suggests a pathway for signal transduction in dimeric myoglobin-like sensor proteins.