Research Technologist/graduate student Mayo Clinic Jacksonville, Florida, United States
Matt Coban (Mayo Clinic)| Thomas Caulfield (Mayo Clinic)| Evette Radisky (Mayo Clinic)
There is evidence in the literature that many serine proteases, as typified by thrombin, possess an autoinhibitory loop (AIL) that controls substrate access to the active site. The presence of an AIL has not been documented in trypsins. Mesotrypsin is a human trypsin implicated in tumor cell invasiveness for pancreatic and other cancers. However, it is a challenging therapeutic target because of high structural similarity to other trypsin-like serine proteases, typically lower affinity for inhibitors compared to related serine proteases, and unique ability to cleave proteinaceous serine protease inhibitors. We have recently solved a high-resolution crystal structure of unbound mesotrypsin which revealed that residues 215-220 partially occluded the substrate binding site. Comparison of this new structure with previous inhibitor-bound crystal structures of mesotrypsin demonstrate the conformational transition of the AIL between closed and open states. These two conformational states were investigated with molecular dynamics simulations to discern the dynamical behavior of the AIL in different conformations. The AIL has increased local dynamics in the closed position, as well as a larger number of conformational clusters; this suggests there is a more well-defined open state ready to accept substrate, while there are a dynamic variety of closed states that preclude substrate binding to various extents. Additionally, we identified novel binding sites that are exposed by the conformational shift of the AIL to the closed state. We have conducted initial in silico screening for compounds predicted to bind preferentially to the AIL closed (but not open) structure and stabilize the closed conformation. These efforts have resulted in more than 40 compounds as initial hits. Exploiting the AIL as a novel allosteric binding site represents a new mechanism for targeting mesotrypsin and preventing its protumorigenic function.