Nitrosyl Myoglobins
and Their Nitrite Precursors:
Crystal Structural and Quantum Mechanics and Molecular Mechanics Theoretical
Investigations of Preferred Fe–NO Ligand Orientations
in Myoglobin Distal Pockets
posted on 2018-07-12, 00:00authored byBing Wang, Yelu Shi, Jesús Tejero, Samantha M. Powell, Leonard M. Thomas, Mark T. Gladwin, Sruti Shiva, Yong Zhang, George B. Richter-Addo
The
globular dioxygen binding heme protein myoglobin (Mb) is present
in several species. Its interactions with the simple nitrogen oxides,
namely, nitric oxide (NO) and nitrite, have been known for decades,
but the physiological relevance has only recently become more fully
appreciated. We previously reported the O-nitrito
mode of binding of nitrite to ferric horse heart wild-type (wt) MbIII and human hemoglobin. We have expanded on this work and
report the interactions of nitrite with wt sperm whale (sw) MbIII and its H64A, H64Q, and V68A/I107Y mutants whose dissociation
constants increase in the following order: H64Q < wt < V68A/I107Y
< H64A. We also report their X-ray crystal structures that reveal
the O-nitrito mode of binding of nitrite to these
derivatives. The MbII-mediated reductions of nitrite to
NO and structural data for the wt and mutant MbII–NOs
are described. We show that their FeNO orientations vary with distal
pocket identity, with the FeNO moieties pointing toward the hydrophobic
interiors when the His64 residue is present but toward the hydrophilic
exterior when this His64 residue is absent in this set of mutants.
This correlates with the nature of H-bonding to the bound NO ligand
(nitrosyl O vs N atom). Quantum mechanics and hybrid quantum mechanics
and molecular mechanics calculations help elucidate the origin of
the experimentally preferred NO orientations. In a few cases, the
calculations reproduce the experimentally observed orientations only
when the whole protein is taken into consideration.