| Bacteria require iron for survival when residing within a human host. Two general strategies for iron acquisition by bacteria have evolved: non-heme and heme iron uptake. Non-heme iron uptake relies on low molecular weight molecules called siderophores, whereas heme uptake is achieved through a series of heme scavenging and heme transport proteins. The mechanism of heme uptake differs between gram-positive and gram-negative bacteria and the major features of the two pathways are described herein. Mycobacterium tuberculosis (Mtb) and Bacillus anthracis both contain heme uptake pathways. The Mtb heme uptake pathway consists of at least three proteins specific to mycobacteria, Rv0203, MmpL3 and MmpL11. While the secreted protein Rv0203 is proposed to be a heme scavenging or heme transport protein, MmpL3 and MmpL11 are proposed transmembrane heme transporters. In B. anthracis, heme scavenging is mediated by IsdX1 and IsdX2, which subsequently transfer heme to downstream receptors. In this work we aim to biochemically and biophysically characterize the Mtb heme uptake proteins and elucidate their respective roles in the heme acquisition pathway. We have characterized the heme binding properties of Rv0203, and the soluble E1 domains of MmpL3 and MmpL11 and demonstrate that heme transfer from Rv0203 to both soluble E1 domains occurs via protein-protein interactions. Furthermore, we present the structures of B. anthracis IsdX1 and the NEAT5 domain of IsdX2, which offers insights into the structural characteristics that determine the respective protein's heme binding and heme scavenging ability. |
