Molecular Mechanism of Myoglobin Autoxidation: Insights from Computer Simulations

Myoglobin (Mb) and hemoglobin have
the biological ability to carry/store
oxygen (O2), a property which requires its heme iron atom
to be in the ferrous -Fe­(II)- state. However, the thermodynamically
stable state in the presence of O2 is Fe­(III) and thus
the oxidation rate of a globin is a critical parameter related to
its function. Mb has been extensively studied and many mutants have
been characterized regarding its oxygen mediated oxidation (i.e.,
autoxidation) rates. Site directed mutants in residues 29 (B10), which
shapes the distal cavity, and 64 (E7), the well-known histidine gate,
have been shown to display a wide range of autoxidation rate constants.
In this work, we have thoroughly studied the mechanism underlying
the autoxidation process by means of state-of-the-art computer simulation
methodologies, using Mb and site directed mutants as benchmark cases.
Our results explain the observed autoxidation rate tendencies in different
variants of Mb, L29F < wt < L29A = H64Q < H64F < H64A,
and shed light on several aspects of the reaction at the atomic level.
First, water access to the distal pocket is a key event and the observed
acid catalysis relies on HisE7 protonation and opening of the His
gate to allow water access, rather than protonation of the oxy heme
itself. Our results also suggest that the basic mechanism, i.e., superoxide
displacement by hydroxide anion, is energetically more feasible. Finally,
we confirmed that distal hydrogen bonds protect the oxy complex from
autoxidation.