The Isd (iron-regulated surface determinant) system is a multiprotein transporter that allows Staphylococcus aureus to take up Fe from Hb during human infection. In this system, IsdB is a cell wall-anchored surface protein that contains 2 near iron transporter (NEAT) domains, one of which binds heme. IsdB rapidly exts. heme from Hb and transfers it to IsdA for relay into the bacterial cell. Using a series of recombinant IsdB constructs that included at least one NEAT domain, the authors demonstrated that both domains were required to bind Hb with high affinity (Kd = 0.42 μM) and to ext. heme from Hb. Moreover, IsdB extd. heme only from oxidized metHb, although it also bound oxyHb and the Hb-CO complex. In a reconstituted model of the biol. heme relay pathway, IsdB catalyzed the transfer of heme from metHb to IsdA with a Km for metHb of 0.75 μN and a kcat of 0.22 s-1. The latter was consistent with the transfer of heme from metHb to IsdB being the rate-limiting step. With both NEAT domains and the linker region present in a single contiguous polypeptide, high-affinity Hb binding was achieved, rapid heme uptake was obsd., and multiple turnovers of heme extn. from metHb and transfer to IsdA were conducted, representing all known Hb-heme uptake functions of the full-length IsdB protein. [on SciFinder(R)]