bi5b00369_si_001.pdf (1.8 MB)
O2 and Water Migration Pathways between the Solvent and Heme Pockets of Hemoglobin with Open and Closed Conformations of the Distal HisE7
journal contribution
posted on 2015-09-01, 00:00 authored by Maria
S. Shadrina, Gilles H. Peslherbe, Ann M. EnglishHemoglobin transports O2 by binding the gas at its four
hemes. Hydrogen bonding between the distal histidine (HisE7) and heme-bound
O2 significantly increases the affinity of human hemoglobin
(HbA) for this ligand. HisE7 is also proposed to regulate the release
of O2 to the solvent via a transient E7 channel. To reveal
the O2 escape routes controlled by HisE7 and to evaluate
its role in gating heme access, we compare simulations of O2 diffusion from the distal heme pockets of the T and R states of
HbA performed with HisE7 in its open (protonated) and closed (neutral)
conformations. Irrespective of HisE7’s conformation, we observe
the same four or five escape routes leading directly from the α-
or β-distal heme pockets to the solvent. Only 21–53%
of O2 escapes occur via these routes, with the remainder
escaping through routes that encompass multiple internal cavities
in HbA. The conformation of the distal HisE7 controls the escape of
O2 from the heme by altering the distal pocket architecture
in a pH-dependent manner, not by gating the E7 channel. Removal of
the HisE7 side chain in the GlyE7 variant exposes the distal pockets
to the solvent, and the percentage of O2 escapes to the
solvent directly from the α- or β-distal pockets of the
mutant increases to 70–88%. In contrast to O2, the
dominant water route from the bulk solvent is gated by HisE7 because
protonation and opening of this residue dramatically increase the
rate of influx of water into the empty distal heme pockets. The occupancy
of the distal heme site by a water molecule, which functions as an
additional nonprotein barrier to binding of the ligand to the heme,
is also controlled by HisE7. Overall, analysis of gas and water diffusion
routes in the subunits of HbA and its GlyE7 variant sheds light on
the contribution of distal HisE7 in controlling polar and nonpolar
ligand movement between the solvent and the hemes.