The role of conformational motions in the control of ligand binding to myoglobin and hemoglobin
Philipp, Robert Georg
This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.
Permalink
https://hdl.handle.net/2142/18876
Description
Title
The role of conformational motions in the control of ligand binding to myoglobin and hemoglobin
Author(s)
Philipp, Robert Georg
Issue Date
1994-05
Doctoral Committee Chair(s)
Frauenfelder, Hans
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
myoglobin
hemoglobin
Biophysics
ligand binding
conformational motions
Language
en
Abstract
Myoglobin and hemoglobin are dioxygen storage and transport
proteins. They bind small molecules (ligands) such as dioxygen (02) and
carbon monoxide (CO) reversibly. The active site is the heme, a disc shaped
molecule which sits in a pocket of the protein (heme pocket). At the center
of the heme is an iron (Fe) atom, to which the ligands bind reversibly when
the Fe is in the ferrous state. The ligand binding rates of these proteins
depend on the type of ligand. For example, myoglobin and hemoglobin bind
02 faster, but with lower affinity, than they bind CO. The mechanism with
which these proteins control ligand binding is not fully known, and
remains a fundamental issue, if the general principles of ligand binding are
to be understood.
Protein motions are essential for myoglobin and hemoglobin to
perform their function. X-ray crystal structures show no pathways through
which the ligands can enter and exit the protein. Without the ability of
ligand entry and exit, myoglobin and hemoglobin could not deliver 02 to
the required tissues. Large scale motions are required to open pathways for
ligand entry and exit. Additional motions have been shown to be important
for control of the ligand binding barrier. The bound and deoxy structures of
myoglobin and hemoglobin are different, respectively. One important
difference is the position of the heme-iron relative to the heme plane. Upon
ligand dissociation, the Fe relaxes to its unbound position. Movement of the
Fe out of the mean heme-plane has been correlated to the height of the
rebinding enthalpy barrier.
We have conducted flash photolysis experiments on R and T state
carp hemoglobin and sperm whale myoglobin over a wide range in
temperature (lOK to 300K) and time (30ns to lOOs). Our data show that the
conformational relaxation of the Fe out-of-plane distance proceeds in
discrete steps which can be correlated with peaks in the rebinding lifetime
distribution, j(logt). In addition we show that this conformational
relaxation can be enhanced through photons absorbed by the heme. A
model is presented in which the ligand must wait for a conformational
rearrangement of the Fe, to the approximate bound position, before bond
formation can occur. This conformational rearrangement is controlled on
the proximal side (opposite side to the ligand binding site) of the heme.
The consequences of this model on the binding of 02 by myoglobin
are discussed. Many of the features of 02 rebinding are predicted by this
model, and the model suggests a mechanism by which these proteins
discriminate between CO and 02. Additional research on the binding of 02
must be performed.
Use this login method if you
don't
have an
@illinois.edu
email address.
(Oops, I do have one)
IDEALS migrated to a new platform on June 23, 2022. If you created
your account prior to this date, you will have to reset your password
using the forgot-password link below.
