--------- Dr. David Sept will be presenting a seminar titled Structure/Function Studies on Ion Channels and New Models for Ions. Refreshments will be provided at this event.
Abstract: Voltage and Ca2+ activated BK channels modulate neuronal activities. Previous studies found that the Ca2+ binding sites and the activation gate are spatially separated, but exactly how Ca2+ binding couples to gate opening is not clear. We address this question by studying how a mutation in BK channels, which is associated with generalized epilepsy and paroxysmal dyskinesia, enhances Ca2+ sensitivity. This epilepsy mutation is located in a structural domain (the AC region) that is close to a putative Ca2+ binding site, and mutagenesis studies show that the AC region is important in the coupling between Ca2+ binding and gate opening. Through a combination of experimental and computational studies, we find the epilepsy mutation enhances Ca2+ sensitivity by an allosteric mechanism affecting the coupling between Ca2+ binding and gate opening.
Our efforts to determine the details of the Ca2+-bound structure of the AC region took an unexpected turn that required us to develop a new model for divalent cations. Current ion models in molecular mechanics are simple spheres, and their interactions are solely determined from the radius of the sphere and the total charge. This set of parameters is chosen to closely reproduce the hydration free energy for the ion, but this exercise uses all the available degrees of freedom and our ability to reproduce the binding free energy to a protein or other thermodynamic quantities is therefore limited. In our new model we distribute the total charge of the ion into n-dummy centers that are placed in the direction of the coordinating atoms. We have parameterized this model for two divalent cations, Ca2+ and Mg2+, and have tested the model’s accuracy in a variety of simulations. With this model we are not only able to correctly predict the free energy and selectivity for cation binding sites in both proteins and nucleic acids, but we achieve better coordination geometries and can capture more subtle effects such as the exchange of inner shell waters. One further advantage of this model is that it does not use higher-order electrostatics and thus can be easily used with standard force fields.
