Cryo-EM reveals how Staphylococcus aureus extracts hemin from hemoglobin-haptoglobin complex
Researchers used cryo-electron microscopy to determine at 3.1 Å resolution how the Staphylococcus aureus surface receptor IsdH extracts hemin from the hemoglobin-haptoglobin complex, a major circulating form of hemoglobin in the bloodstream. The study found that haptoglobin's N-linked glycans redirect IsdH's extraction unit toward β-hemoglobin, and that receptor dynamics actively accelerate hemin release by transiently perturbing the F-helix. These findings clarify a key bacterial iron-acquisition mechanism and suggest IsdH may also interfere with the host immune system's clearance of the hemoglobin-haptoglobin complex.
A new structural biology study published on bioRxiv resolves how Staphylococcus aureus overcomes host iron restriction by stealing hemin from the hemoglobin:haptoglobin (Hb:Hp) complex. Using cryo-electron microscopy, biophysical measurements, and solution kinetics, the researchers determined a 3.1 Å structure of Hb:Hp bound to the full-length IsdH receptor, showing that IsdH's N-terminal NEAT domain (N1) anchors the receptor to hemoglobin while the downstream N2N3 extraction unit targets βHb for hemin removal. Notably, N-linked glycans on haptoglobin sterically block the extraction unit from accessing αHb, biasing it toward βHb — a different engagement mode than IsdH uses on isolated hemoglobin. Kinetic assays confirmed that IsdH actively accelerates hemin release rather than passively capturing it, and three-dimensional variability analysis suggests this occurs through dynamic receptor motions that reposition the extraction unit to perturb the F-helix of βHb. Structural alignment with a previously determined CD163:Hb:Hp complex further indicates that IsdH binding may competitively disrupt recognition by the macrophage and monocyte CD163 scavenger receptor, potentially impairing host-mediated clearance of Hb:Hp from circulation and prolonging bacterial access to this iron source.
What's missing
As a preprint, this study has not yet undergone formal peer review. The authors do not address whether IsdH-mediated disruption of CD163 recognition has been validated in cellular or in vivo models. The physiological relevance of the glycan-steering effect under varying haptoglobin glycoforms found in human populations is also not examined.
What different sources said
- bioRxivCenter
Cryo-EM provides insight into how the Staphylococcus aureus IsdH receptor removes hemin from the hemoglobin:haptoglobin complex
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