Cohen et al. claim gas migration pathways are well-defined through myoglobin

 Cohen, Jordi, Anton Arkhipov, Rosemary Braun, and Klaus Schulten. "Imaging the Migration Pathways for O2, CO, NO, and Xe Inside Myoglobin." Biophysical Journal 91, (2006): 1844-1857.

In their paper, Cohen et al. introduce a method for studying gas migration pathways in proteins based on free-energy perturbation calculations called implicit ligand sampling (ILS).  They present the theoretical foundation of ILS, which calculates the Gibb's free energy cost of having a particle occupy certain positions in a molecular dynamics simulation.  This calculation provides a three-dimensional map of the potential of mean force (PMF) inside the analyzed volume.  They used the technique to study sperm whale myoglobin.  First they reported that ILS accurately predicted the xenon binding sites in the whale myoglobin by comparing the results of their ILS to crystal structures of the protein co-crystallized with xenon gas, validating the technique.  They then mapped out gas migration pathways using ILS for other gases O2, CO, and NO finding that they all shared similar binding pockets and pathways.  Their results led them to claim that gas migration pathways are not randomly distributed in the protein but are located in well-defined regions.

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