The Relationship Between Structure and Function of the Heme-Scavenging Protein IsdX1

In biological systems, iron is extremely important due to its use in electron transport, redox centers, oxygen transport, dismutation of reactive oxygen species, and structural stabilization. Among bacteria, iron is essential for healthy growth and many mechanisms exist for its acquisition. A prominent example is the iron-regulated surface determinant (Isd) system, which is found in many pathogens including the causative agent of anthrax, Bacillus anthracis. In the Isd system of B. anthracis, the protein IsdX1 is responsible for scavenging heme from host hemoglobin so that iron can be acquired through heme degradation. This process is not novel among bacteria that express Isd proteins, but IsdX1 is unique as the only example of an extracellular hemophore in Gram-positive bacteria. In order to understand the functional mechanics of IsdX1, it is necessary to characterize the protein with and without heme bound. This is accomplished by assessing proteolytic and thermal stability with limited proteolysis and circular dichroism. Using these tools, it is clear that IsdX1 is a stable protein in both the apo and holo forms that is quite resistant to proteolytic degradation. The binding of heme increases thermal stability to a moderate extent. These results suggest that IsdX1 is important for the growth of B. anthracis, and that heme binding influences the secondary structure content, possibly through a stabilizing conformational change