Molecular Horizons Seminar - Professor Martin Stillman

Iron is a critically important nutrient for all species. A major role is in heme proteins and almost all organisms rely of an array of different heme proteins for metabolic health. While reversible dioxygen binding and monooxygenases have been well studied, there remain the intriguing chemistries of heme transport and the sophistication of the binding site tuning of the spin state of ferric hemes. In this paper, I will describe two studies involving heme proteins where a combination of MCD spectroscopy and ESI-mass spectrometry has allowed complex chemistries to be unravelled (1-3). The first example is that of the trafficking of heme in bacteria. Bacteria have evolved specialist survival systems to chelate and transport iron across the wall and membrane into the cytoplasm. In the human pathogenic bacteria Staphylococcus aureus involves extracting heme from hemoglobin, then transporting the intact heme across the wall and membrane of the cell via a series of different proteins. Using detailed time-resolved mass spectral and magnetic circular dichroism spectral data we have determined that the full Isd-B1B2 protein is required to extract heme from hemoglobin. In the next step the heme is transferred via 3 proteins through the cell wall. Real time ESI-MS data recorded of 3 apirs of apo- and holo- protein species as heme was transferred unidirectionally from holo-IsdA to apo-IsdE via IsdC provides a complete mechanistic description of the pathways involved (1). The data show that a unique role for IsdC is acting as the central cog wheel that facilitates heme transfer from IsdA to IsdE. The 2nd case involves the specialist reactivity of cytochrome c’ from Alcaligenes xylosoxidans (AxCyt c’). Here, ESI-MS data recorded over a remarkably wide pH range of 2-13, combined with MCD spectral data allowed the protein conformation in the heme binding site to be correlated with the electronic structure of the ferric heme as the spin state changed from high spin to low spin (3).

1. "Multiprotein heme shuttle pathway in Staphylococcus aureus: iron-regulated surface determinant cog-wheel kinetics”, MT Tiedemann, DE Heinrichs, MJ Stillman. J. Am. Chem. Soc. 134 (40), 16578-16585.
2. “The heme-sensitive regulator SbnI has a bifunctional role in staphyloferrin B production by Staphylococcus aureus” MM Verstraete1, et al. J. Biol. Chem. doi:10.1074/jbc.RA119.007757
3. "The pH Dependent Protein Structure Transitions and Related Spin-State Transition of Cytochrome c′ from Alcaligenes xylosoxidans NCIMB 11015#”, A Takashina, et al. Bull. Chem. Soc. Jpn. 2017, 90, 169–177.