Protein Structure and Function: Applications of Bioinformatics Methods - John Rigden 2014

Prediction of Protein Function Based on Theoretical Models
Practical Applications
Mutation Mapping

Rare Mutations occurring in key Proteins underlie many hereditary disorders. Other differences in protein sequences, such as allelic variants of drug target proteins, can result in altered drug binding and, consequently, diverse patient responses. Thus, structural mapping of mutations—a key Application of Molecular modeling—proves invaluable for elucidating the MOLECULAR MECHANISMS OF diseases and predicting patient responses, representing a significant step toward personalized medicine.

ATP-sensitive potassium channels play a pivotal role in many Tissues by coupling cellular METABOLISM to electrical activity. These channels are octameric complexes composed of two distinct proteins, Kir6.2 and SUR. The binding of ATP or ADP to the channels leads to their inhibition. The discovery of a series of mutations in Kir6.2 that reduce channel sensitivity to ATP has shed light on the Etiology of neonatal Diabetes Mellitus (Hattersley and Ashcroft 2005). Inhibited channels cause membrane hyperpolarization in pancreatic ß-Cells, which in turn leads to reduced Insulin secretion and, consequently, diabetes. Understanding the genetic etiology of the disease has revolutionized therapy for patients with neonatal diabetes caused by Kir6.2 mutations, since these channels can be kept closed by drugs such as sulfonylureas or glinides, allowing insulin therapy to be limited or suspended.

The comparative model of the Kir6.2 subunit enabled the spatial mapping of residues mutated in neonatal diabetes, thereby illustrating the molecular mechanism underlying the diminished sensitivity of ATP-sensitive potassium channels (Hattersley and Ashcroft 2005). Patients with Kir6.2 mutations exhibit a range of phenotypes that clearly correlate with The Nature of the mutation. For instance, patients presenting with neurological symptoms harbor mutations that are not directly involved in ATP binding, yet strongly favor the open state of the channel, thereby impairing the ability of ATP to block it (as ATP stabilizes the closed state of the channel).

Recent studies have shown that it is possible to identify a subset of patients with permanent neonatal diabetes who carry the L164P mutation in Kir6.2 and are unresponsive to sulfonylurea therapy (Tammaro et al. 2008). An Analysis of the spatial Location of L164 revealed that this residue lies deep within the Cell/13.html">Protein Structure, 35 Å away from the ATP-binding site, making its direct involvement in reducing ATP binding unlikely. Instead, the L164P mutation likely destabilizes the closed state of the channel—the state preferentially bound by sulfonylurea derivatives and rarely achieved in channels with an increased open-state probability. Taken together, these findings demonstrate that drug susceptibility depends on the nature of a specific mutation, yet it can be predicted through a detailed analysis of the protein model.