Hemoglobin binding and catalytic heme extraction by the IsdB receptor of Staphylococcus aureus (583.7)

The Isd (Iron‐regulated surface determinant) system is a multi‐protein transporter that enables Staphylococcus aureus to take up iron from hemoglobin (Hb) during human infection. In this system, IsdB is a cell wall‐anchored surface protein that contains 2 NEAT domains and rapidly extracts heme from Hb. Using a series of recombinant IsdB constructs which included at least one NEAT domain, we demonstrated that both domains are required to bind Hb with high affinity (KD = 0.42 ± 0.05 μM) and to extract heme from Hb. Moreover, IsdB only extracted heme from oxidized metHb although it also bound oxyHb and the Hb‐CO complex. In a reconstituted model of the biological heme relay pathway, IsdB catalyzed heme transfer from metHb to IsdA with a Km for metHb of 0.75 ± 0.07 μN and a kcat of 0.22 ± 0.01 s‐1. The latter is consistent with the transfer of heme from metHb to IsdB as being the rate‐limiting step. Grant Funding Source : Supported by CIHR grant MOP‐49597