Iron acquisition by the haem-binding Isd proteins in Staphylococcus aureus: studies of the mechanism using magnetic circular dichroism.

The bacterium Staphylococcus aureus is responsible for numerous hospital-acquired infections ranging from superficial wound lesions to more severe infections such as pneumonia, osteomyelitis and septicaemia and, in some cases, death. The Isd (iron-regulated surface determinant) proteins expressed by S. aureus and select other bacteria are anchored to the bacterial cell wall and membrane and are involved in extracting haem from haemoglobin as an iron source. Our knowledge of the overall haem-scavenging mechanism on the bacterial surface is limited. A detailed description of the haem-binding properties in the transport pathway is critical to our understanding of the mechanism for haem-iron scavenging in S. aureus. Our work involves using a combination of techniques to characterize both the dynamic and steady-state haem-binding properties of these proteins. UV-visible absorption and MCD (magnetic circular dichroism) spectroscopy provide diagnostic spectral data sensitive to the axial ligands, the spin state and oxidation state of the central haem-iron. Electrospray MS provides stoichiometric information on the numbers of haems bound, the effect of haem binding on the overall folding of each protein and kinetic information about the rate of haem binding. Together, these data allow us to address the outstanding questions regarding the mechanism of haem transport via the Isd protein chain in S. aureus.