Abstract
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Myoglobin
(Mb) binds diatomic ligands, like O2, CO,
and NO, in a cavity that is only transiently accessible. Crystallography
and molecular simulations show that the ligands can migrate through
an extensive network of transiently connected cavities but disagree
on the locations and occupancy of internal hydration sites. Here,
we use water 2H and 17O magnetic relaxation
dispersion (MRD) to characterize the internal water molecules in Mb
under physiological conditions. We find that equine carbonmonoxy Mb
contains 4.5 ± 1.0 ordered internal water molecules with a mean
survival time of 5.6 ± 0.5 μs at 25 °C. The likely
locations of these water molecules are the four polar hydration sites,
including one of the xenon-binding cavities, that are fully occupied
in all high-resolution crystal structures of equine Mb. The finding
that water escapes from these sites, located 17–31 Å apart
in the protein, on the same μs time scale suggests a global
exchange mechanism. We propose that this mechanism involves transient
penetration of the protein by H-bonded water chains. Such a mechanism
could play a functional role by eliminating trapped ligands. In addition,
the MRD results indicate that 2 or 3 of the 11 histidine residues
of equine Mb undergo intramolecular hydrogen exchange on a μs
time scale.
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