Although iron is abundant in nature, the concentration of free iron is approximately 10-18 M in the human host, where the majority of iron is sequestered in host proteins such as transferrin and hemoglobin (Hb). To overcome iron scarcity, human bacterial pathogens have evolved numerous mechanisms to acquire iron. One such mechanism is the expression of cell surface receptors specific for host heme-containing proteins. Haemophilus ducreyi, the causative agent of the sexually transmitted genital ulcer disease chancroid, expresses only one iron-scavenging mechanism required for virulence. This iron-scavenging protein is a Hb receptor termed HgbA. HgbA belongs to a large family of outer membrane proteins that are dependent on TonB that link energy via the proton-motive force of the inner membrane to transport in the outer membrane. Although TonB-dependent receptors are structurally similar, the mechanisms to acquire iron or heme from different substrates appear to be different. Furthermore, no structures of Hb-binding/transport proteins have been determined so far, even though most pathogens express proteins to extract heme from Hb. Very little is known specifically about heme uptake from Hb in H. ducreyi. Determining the mechanism of iron/heme acquisition of H. ducreyi may lead to the development of new antibiotic strategies targeting iron acquisition, which may impact all pathogens, including those defined by the CDC as category A select agents. Our short-term goal is to determine how HgbA is able to bind its substrate Hb, how it removes heme from Hb, and how HgbA transports heme to the periplasm. Our long-term goal is to understand heme uptake from Hb using H. ducreyi as a model of heme scavenging from Hb in pathogenic bacteria. Using a combination of bacteriology and biophysical techniques, we will explore the structure and function of the HgbA heme receptor from H. ducreyi.
Iron is extremely scarce and so bacterial pathogens have developed mechanisms to steal iron from the human host. This proposal describes experiments to explore how a bacterium called H. ducreyi, is able to bind and remove the iron containing molecule heme from hemoglobin.
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