Iron is an essential nutrient for the bacterial pathogen Staphylococcus aureus .
Heme in
hemoglobin (Hb) is the most abundant source of
iron in the human body and during
infections is captured by S. aureus using
iron-regulated surface determinant (Isd)
proteins. A central step in this process is the transfer of
heme between the cell wall associated IsdA and IsdC hemoproteins. Biochemical evidence indicates that
heme is transferred via an activated IsdA:
heme:IsdC
heme complex. Transfer is rapid and occurs up to 70,000 times faster than indirect mechanisms in which
heme is released into the
solvent. To gain insight into the mechanism of transfer, we modeled the structure of the complex using NMR paramagnetic relaxation enhancement (PRE) methods. Our results indicate that IsdA and IsdC transfer
heme via an ultraweak affinity "handclasp" complex that juxtaposes their respective 3(10) helices and β7/β8 loops. Interestingly, PRE also identified a set of transient complexes that could represent high-energy pre-equilibrium encounter species that form prior to the stereospecific handclasp complex. Targeted
amino acid mutagenesis and stopped-flow measurements substantiate the functional relevance of a PRE-derived model, as mutation of interfacial side chains significantly slows the rate of transfer. IsdA and IsdC bind
heme using NEAr Transporter (NEAT) domains that are conserved in many species of pathogenic Gram-positive bacteria.
Heme transfer in these microbes may also occur through structurally similar transient stereospecific complexes.