ATLF

The anthrax toxin lethal factor (ATLF) secreted by anthrax bacilli is responsible for much of the pathology of anthrax even after living bacteria have been killed. Our work, funded by a grant from the NIH NIAID (1 R01 AI083234-01) awarded to Principal Investigator Elizabeth Ambrose, initially revolved around structural characterizations of compounds identified as active inhibitors of the proteolytic activity of ATLF. After validating our ability to structurally characterize known ligands, we pursued co-crystallization studies with compounds identified through virtual screening or high-throughput in vitro screening[1], and other synthetic analogs of potent hydroxamate inhibitors created in the Ambrose lab [2].

The take-away message from our characterization of a dozen complexes with diverse inhibitors is that ATLF possesses remarkable conformational flexibility. Different inhibitors can induce three different quaternary states[3]. Some of these are consistent with the proteolysis of peptide substrates, while others seem to lock the enzyme in conformations that deny substrate binding. Computational modeling should prioritize the search for novel compounds targeting an inactive state, as these seem no less stable thermodynamically. We were also able to identify ligands with constrained binding geometry that are able to induce the opening of a totally new hydrophobic subsite (S1*) that looks like an attractive target for future inhibitor design (left) [4].