Featured Article - January 2013
Short description: Structural transitions in a bacterial hemophore suggest a mechanism that couples hemoglobin binding and heme capture.
Iron, endowed with unique redox properties utilized in metabolism, is essential in the broader competition for resources between pathogen and host. Pathogenic bacteria have evolved two distinct mechanisms to sequester host iron: direct iron capture by small-molecule siderophores and heme capture by hemophore proteins. Hemophores are secreted and, using a still undefined mechanism, extract heme from globins for subsequent transfer to bacterial receptors.
Maresso and colleagues (PSI UC4UCI) have solved crystal structures of the apo (heme-free) and holo (heme-bound) forms of the IsdX1 hemophore from Bacillus anthracis, the causative agent of anthrax. The IsdX1 protein contains a NEAT (near-iron transporter) domain that is conserved among Gram-positive bacteria. IsdX1 is also a valuable model protein since it binds both heme and hemoglobin, and thus may offer insights into whether hemoglobin recognition and heme capture are mechanistically coupled.
The 1.8 Å apo-IsdX1 NEAT domain structure has an immunoglobulin β-sandwich fold in which the heme-binding pocket is enclosed by a β-hairpin and an unexpectedly ordered 310-helix (residues Ser52 to Asn56) stabilized by hydrogen bonding and van der Waals interactions. The 2.15 Å holo-IsdX1 structure, solved by molecular replacement, is very similar to the apo structure (with a root mean square deviation of 0.51 Å). The conserved Tyr136 residue coordinates the heme iron, and stabilization is provided by π-stacking and hydrogen bonding by residues surrounding the heme pocket.
Closer inspection revealed conformational changes that suggest a dynamic role for the 310-helix during heme capture. While residues Ser52 and Ser53 do not change conformation, Arg54 shifts away from the pocket and Met55 in the apo form adopts two conformations, only one of which (shifted away) is evident in the holo form. Biochemical experiments using the R54A mutant further suggest a complex role for this residue during heme capture. This mutant anomalously captures elevated levels of heme from lysates and has significantly weaker binding to hemoglobin, suggesting that Arg54 sterically blocks the pocket in the apo form and that interaction with hemoglobin removes that block, allowing coupled binding and capture. This elegant mechanism is supported by the previous data on other mutants and hemophores.
The authors note that more work is needed to determine if the 310-helix or other residues also function in collective NEAT to NEAT heme transfer through the bacterial cell wall. Overall, the structures highlight the role that conformational dynamics may play in hemophore systems.
M. T. Ekworomadu et al. Differential function of lip residues in the mechanism and biology of an anthrax hemophore.
PLoS Pathog. 8, e1002559 (2012). doi:10.1371/journal.ppat.1002559