Hemoglobin is the oxygen carrier
protein in red blood cells. It
is also the protein, which gives red blood cells their red color.
Hemoglobin consists of four
subunits, two a
subunit (refer to image) forms a dimer. Often,
hemoglobin is referred to as a 'dimer of
dimers.' The a
subunits are only slightly different from each other with the main
difference arising from the length of the
polypeptide chains; the
chain is 5
amino acid residues longer than the
chain. There are also some
differences in the composition of the amino acid residues.
This figure depicts the b subunit of
hemoglobin, which consists of 8
labelled A-H. Each a-helix is shown
in a different color. The protein chain begins with the
A-helix (blue) and ends with the H-helix (lavender). The heme
group is shown in red and the bound oxygen is shown in light
The figure at left shows the heme group, which
contains an Iron (Fe) atom at the center (orange). In the oxy state, oxygen
binds to the Fe in the heme group.
Each hemoglobin subunit contains a heme group.
The heme group is the site of oxygen (O2) binding.
When all four heme moieties bind O2, the structure of
hemoglobin changes. This
structural change involves a rearrangement of the ab
dimers with respect to each other, where one ab
dimer rotates approximately 18°
and translates 1 Å
with respect to the other dimer.
= 10-10m = 3.937 x 10-9 inches
3.3 Graphic of T-R
transition of HbS
When the protein is in the structure that binds O2, the
R-state, it binds O2 readily and when it is in the structure
that has no O2 bound, the deoxy or T-state(Fig 3.3 and Fig
3.4), it doesn’t bind O2
very well. This
difference in the ability to bind O2 depending on its
structural state is what allows hemoglobin to be so efficient in
delivering O2 to the tissues.
Once it delivers O2, the structural state changes and it
will not bind the delivered O2.
When the Red Blood Cells (RBCs) return to the lungs, the O2 concentration
is higher and hemoglobin binds O2 again and changes its
structural state. Understanding
how this structural change is effected is a central question in current
Relative Motion of Dimers
This animated figure illustrates the motion of the
dimer (thick coils towards front) relative to the a1b1
dimer (thin coils
towards rear) in the oxy-to-deoxy transition. Here, the coordinates
of oxy- and deoxy-Hb have been superimposed at the a1b1
interface so that
dimer remains stationary. The a2b2
dimer rotates by 15 degrees about an axis passing through the a
image from Jonathan Lukin, Dept. of Biological Sciences, Carnegie
Mellon University http://www.andrew.cmu.edu/user/jl2p/Hb_html/gallery.html
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suffers from it?)
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