Hemoglobin
Lisa Natzke, '98
References:
Liddington, R. et al. 1992.
High resolution crystal structures and comparisons of T state deoxyhemoglobin
and two liganded T-state hemoglobins. J. Mol. Biol. 228: 551.
Perutz, M. F. 1970. Stereoschemistry
of cooperative effects in haemoglobin. Nature. 228: 726-734.
Shaanan, B. 1982. The iron-oxygen
bond in human oxyhaemoglobin. Nature. 296: 683.
Shaanan, B. 1983. Structure
of human oxyhaemoglobin at 2.1 resolution. J. Mol. Biol. 171: 31.
I. Introduction
Approximately
one third of the mass of a mammalian red blood cell is hemoglobin. Its major function
is to carry oxygen from the lungs through the arteries to the tissues and help
to carry carbon dioxide through the veins back to the lungs. The process whereby
hemoglobin performs this essential physiological role is characterized by a cooperative
interaction among its constituent subunits. Hemoglobin has thus assumed the
role of a model system whose study acquires ramifications extending far beyond
its own function as an oxygen transport system.
II. Protein Structure
The hemoglobin
molecule is made up of four polypeptide chains: two alpha
chains <
< < <
>. The iron atom in heme binds to the four nitrogens in the center of
the protoporphyrin ring. The hemoglobin molecule is nearly spherical, with a diameter
of 55 angstroms . The four chains are packed together to form a tetramer. The
heme groups are located in crevices near the exterior of the molecule, one in
each subunit. Each alpha chain is in contact with
both beta chains<
>. However, there are few interactions between
the two alpha chains or between the two beta chains
>.
Each
polypeptide chain is made up of eight or nine alpha-helical
segments <
>and an equal number of nonhelical ones placed at the corners between
them and at the ends of the chain. The helices are named A-H, starting from
the amino acid terminus, and the nonhelical segments that lie between the helices
are named AB, BC, CD, etc. The nonhelical segments at the ends of the chain
are called NA at the amino terminus and HC at the carboxyl terminus.
To
form the tetramer <
>, each of the subunits is joined to its partner around a twofold symmetry
axis, so that a rotation of 180 degrees brings one subunit into congruence with
its partner. One pair of chains is then inverted and placed on top of the other
pair so that the four chains lie at the corners of a tetrahedron. The four subunits
are held together mainly by nonpolar interactions and hydrogen bonds. There
are no covalent bonds between subunits. The twofold symmetry axis that relates
the pairs of alpha and beta chains runs through a water-filled
cavity
>at the center of the molecule. This cavity widens upon transition
form the R
structure to the T structure to form a receptor site for the allosteric
effector DPG (2,3
diphosphoglycerate) between the two beta chains. The heme group is wedged
into a pocket of the globin with its hydrocarbon side chains interior and its
polar propionate side chains exterior.
There
are nine positions in the amino acid sequence that contain the same amino acid
in all or nearly all species studied thus far. These conserved positions are
especially important for the function of the hemoglobin molecule. Several of
them, such as histidines F8 (His87)<
> and E7 (His63)<
>, are directly involved in the oxygen-binding
site<
> . Phenylalanine CD1 (Phe43) <
> and leucine F4 (Leu83) <
> are also in direct contact with the heme group<
>. Tyrosine HC2 (Tyr140) <
>stabilizes the molecule by forming a hydrogen bond between the H<
> and F helices<
>. Glycine B6 (Gly25)<
>is conserved because of its small size: a side chain larger than a
hydrogen atom would not allow theB<
> and E helices<
> to approach each other as closely as they do. Proline
C2 (Pro37)<
> is important because it terminates the C
helix. Threonine C4 (Thr39) and lysine H10 (Lys127) are also conserved residues,
but their roles are uncertain.
III. Transition from the T Structure
to the R Structure
There are
two kinds of contact regions between the alpha and beta chains: the alpha1beta1
and the alpha1beta2 contacts. Upon transmission from the
deoxy (T) structure to the oxy (R) structure, the alpha1beta2
dimer rotates relative to the other by 15 degrees. Some atoms at this interface
shift by as much as 6 angstroms . The alpha1beta2 contact
region is designed to act as a switch between two alternative structures. The
T structure is constrained by additional bonds between the subunits, which oppose
the changes in tertiary structure needed to flatten the hemes upon combination
with oxygen. These bonds take the form of salt
bridges.
Transition
from the T structure<
> to the R structure<
> is triggered by stereochemical changes at the hemes. In deoxyhemoglobin,
the iron atom is about 0.6 angstroms out of the heme plane because of steric
repulsion between the proximal histidine and the nitrogen atoms of the porphyrin.
The heme group and proximal histidine make intimate contact with some fifteen
side chains and so the structures of the F helix, the EF corner, and the FG
corner change on oxygenation. These changes are then transmitted to the subunit
interfaces. The expulsion of the tyrosine HC2 from the pocket between the F
and H helices leads to the rupture of interchain salt bridges. Consequently,
the equilibrium between the two quaternary structures is shifted to the R form
on oxygenation.
IV. Cooperative Binding of Oxygen
The binding
of oxygen to the heme
group of one subunit has the effect of increasing the affinity of a neighboring
subunit (on the same molecule) for oxygen<
>. Deoxyhemoglobin
is a taut moleucule, contrained by its eight salt links between the four subunits.
Oxygenation cannot occur unless some of these salt links are broken so that the
iron atom can move into the plane of the heme group. The number of salt links
that need to be broken for the binding of an oxygen molecule depends on whether
it is the first, second, third, or fourth to be bound. More salt links must be
broken to permit the entry of the first oxygen molecule than of subsequent ones.
Because energy is required to break salt links, the binding of the first oxygen
molecule is energetically less favorable than that of subsequent oxygen molecules.
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