$Staphylococcus aureus$ IsdB hemophore interaction with human hemoglobin: Structural and functional insights.

Iron is a necessary nutrient for bacteria and $Staphylococcus aureus$ acquires hemic iron from humans by forming a complex with hemoglobin (Hb) through its hemophore IsdB. Static Small (SAXS) and Wide (WAXS) Angle X-ray Scattering measurements were performed to characterize the IsdB interaction with both metHb, the dominant form $in vivo$, and oxyHb, which is insensitive to heme removal and was exploited as a tool to investigate an intermediate state along the extraction process. Static SAXS/WAXS patterns allowed us to confirm complex stoichiometry and to characterize the Hb dimerization and monomerization induced by IsdB. A novel setup featured with a stopped-flow apparatus online with WAXS beamline at the European Synchrotron Radiation Facility was successfully applied to follow the structural changes induced by rapid mixing of IsdB with metHb. Coupled with time-resolved fluorescence and absorption spectroscopy, data allowed to a) define a sequential model describing the observed kinetics and determine its microscopic constants and b) explore how the function of this receptor is dependent on dynamic interactions that guarantee the fast and efficient heme transfer to the bacterium.