Simulating protein dynamics to find binding-competent states

Mutation to the tumor-suppressor protein p53 is a common feature in most cancers. MDM2–a protein whose job it is to downregulate p53 via their direct binding interaction–has therefore become a prime target for cancer therapeutics. Normally, a small helical region of p53 binds the MDM2 receptor site, but if a molecule with a similar shape can bind the receptor site of MDM2 strongly enough, it can prevent p53 from binding, thus making more p53 available to perform its tumor suppression abilities.

Many different kinds of molecular mimics of the p53 helix have been designed to disrupt the p53-MDM2 interaction, including stapled peptides, cyclic hairpin peptides, beta-peptides, peptoids (N-substituted oligoglycines), oligoarylamides, and spiroligomers, just to name a few. These molecules are much larger than typical small-molecule drugs, and have interesting folding properties that must be overcome to achieve tight binding. Stapled peptides, for instance, feature a hydrocarbon “staple” that helps rigidify the helical conformation in solution, which in turn enhances the binding affinity.

Using molecular simulations on Folding@home, we have been studying the coupled folding and binding of the p53 helix to MDM2 to address several key questions. One goal is to understand the roles the conformational dynamics in shaping the binding mechanism – such information can ultimately help to design better-binding molecular mimics.

Another question is whether or not molecular dynamics simulations can be used to discover binding-competent receptor conformations of MDM2 in the absence of a bound crystal structure. In new work from our lab (Pantelopulos et al. Proteins 2015), we show that ligand-free simulations of MDM2 starting from conformations with a closed binding cleft can sample open-cleft conformations capable of binding. We also tested the performance of several recent force field models in predicting experimental NMR measurements. We found that that all of the force fields perform similarly well, but that longer simulations (out to a microsecond) result in better agreement with experiment.

You can read about our work in the latest issue of Proteins:

Microsecond simulations of MDM2 and its complex with p53 yield insight into force field accuracy and conformational dynamics George A. Pantelopulos and Vincent A. Voelz. Proteins: Structure, Function and Bioinformatics, Accepted (2015)