Bacterial acquisition of heme-iron

Learning How S. aureus Steals Heme-Iron from Hemoglobin

In order to successfully mount an infection S. aureus and other bacterial pathogens need to actively procure iron from their host. Heme from Hemoglobin (Hb) is the most abundant source of iron in the human body and is captured by S. aureus using Iron regulated surface determinant (Isd) proteins (22-24). Isd proteins arrayed in the bacterial cell wall first capture Hb on the cell surface and extract its heme (Fig. 3). The heme is then passed across the cell wall envelop via a series of hemoreceptors, imported into the cytoplasm, and degraded to release free iron. Intriguingly, heme capture and transfer occur via protein-protein complexes, which may be required because alternative indirect transfer mechanisms in which the heme is first released into the solvent and subsequently captured by an acceptor protein occur extremely slowly. The rate enhancements gained by protein-protein association are impressive, with heme transfer via protein-protein complexes occurring up to ~80,000 times faster than indirect mechanisms (25, 26).

Fig. 3. Capturing heme-iron from hemoglobin (Hb). Left, Overview of the Iron regulated surface determinant (Isd) heme acquisition system. Hb is captured on the cell surface and its heme is removed by IsdB and IsdH. Heme then flows across the cell wall and into the cytoplasm via a series of protein-protein. Right, NMR structure of IsdC protein determined in the Clubb lab 19. IsdC is thought to be the central conduit through which heme flows.

Mechanistic Studies:

We are using a combination of structural, biochemical and cellular approaches to learn how Hb is captured on the cell surface, and how its heme is extracted and rapidly passed across the bacterial cell wall via protein-protein complexes. Our research has thus far studied the Hb and heme receptors, IsdH and IsdC, respectively (16-19). Interestingly, they both use related binding modules, called NEAT (NEAr Transporter) domains, to interact with distinct ligands in the pathway. To gain insights into the molecular basis of this functional dichotomy, we have conducted structure-function studies of each protein (16, 19). Our most recent results indicate that IsdH uses three NEAT domains with distinct functions to capture heme from Hb17. We are currently testing the hypothesis that the domains work together synergistically accelerate the rate of heme release from Hb.

NMR Methods Development:

Nuclear magnetic resonance (NMR) is used extensively in our research. We have recently developed a new a heteronuclear zero-quantum (and double-quantum) coherence Nz-exchange experiment that resolves 15N chemical shift degeneracy in the indirect dimension (18). The new experiment expands the utility of an existing Nz-exchange experiment and has enabled us to quantitatively measure the heme binding kinetics.

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