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Myoglobin (Mb) binds diatomic ligands, like O$_2$, CO, and NO, in a cavitythat is only transiently accessible. Crystallography and molecular simulationsshow that the ligands can migrate through an extensive network of transientlyconnected cavities, but disagree on the locations and occupancy of internalhydration sites. Here, we use water $^2$H and $^{17}$O magnetic relaxationdispersion (MRD) to characterize the internal water molecules in Mb underphysiological conditions. We find that equine carbonmonoxy Mb contains 4.5$\pm$ 1.0 ordered internal water molecules with a mean survival time of 5.6$\pm$ 0.5 $\mu$s at 25 $^\circ$C. The likely location of these water moleculesare the four polar hydration sites, including one of the xenon-bindingcavities, that are fully occupied in all high-resolution crystal structures ofequine Mb. The finding that water escapes from these sites, located 17 -- 31{\AA} apart in the protein, on the same $\mu$s time scale suggests a globalexchange mechanism. We propose that this mechanism involves transientpenetration of the protein by H-bonded water chains. Such a mechanism couldplay a functional role by eliminating trapped ligands. In addition, the MRDresults indicate that two or three of the 11 histidine residues of equine Mbundergo intramolecular hydrogen exchange on a $\mu$s time scale.

Internal water and microsecond dynamics in myoglobin