Download Molecule of the Month on Hemoglobin

Hemoglobin
Hemoglobin uses a change in shape to increase the efficiency of oxygen transport
Red Blood, Blue Blood
Ever wondered why blood vessels appear blue?
Oxygenated blood is bright red: when you are cut,
the blood you see is brilliant red oxygenated blood.
Deoxygenated blood is deep purple: when you
donate blood or give a blood sample at the doctor's
office, it is drawn into a storage tube away from
oxygen, so you can see this dark purple color.
However, deep purple deoxygenated blood appears
blue as it flows through our veins, especially in
people with fair skin. This is due to the way that
different colors of light travel through skin: blue light
is reflected in the surface layers of the skin, whereas
red light penetrates more deeply. The dark blood in
the vein absorbs most of this red light (as well as
any blue light that makes it in that far), so what we
see is the blue light that is reflected at the skin's
surface. Some organisms like snails and crabs, on
the other hand, use copper to transport oxygen, so
they truly have blue blood.
Hemoglobin, with hemes in red.
Hemoglobin is the protein that makes blood red. It is composed of four protein chains, two alpha
chains and two beta chains, each with a ring-like heme group containing an iron atom. Oxygen binds
reversibly to these iron atoms and is transported through blood. Each of the protein chains is similar
in structure to myoglobin, the protein used to store oxygen in muscles and other tissues. However,
the four chains of hemoglobin give it some extra advantages, as described below.
Use and Abuse of Hemoglobin
Aside from oxygen transport, hemoglobin can bind and transport other molecules like nitric oxide
and carbon monoxide. Nitric oxide affects the walls of blood vessels, causing them to relax. This in
turn reduces the blood pressure. Recent studies have shown that nitric oxide can bind to specific
cysteine residues in hemoglobin and also to the irons in the heme groups, as shown in PDB entry
1buw . Thus, hemoglobin contributes to the regulation of blood pressure by distributing nitric oxide
through blood. Carbon monoxide, on the other hand, is a toxic gas. It readily replaces oxygen at the
heme groups, as seen in PDB entry 2hco and many others, forming stable complexes that are
difficult to remove. This abuse of the heme groups blocks normal oxygen binding and transport,
suffocating the surrounding cells.
Artificial Blood
Blood transfusions have saved countless lives. However, the need for matching blood type, the short
life of stored blood, and the possibility of contamination are still major concerns. An understanding of
how hemoglobin works, based on decades of biochemical study and many crystallographic
structures, has prompted a search for blood substitutes and artificial blood. The most obvious
approach is to use a solution of pure hemoglobin to replace lost blood. The main challenge is
keeping the four protein chains of hemoglobin together. In the absence of the protective casing of
the red blood cell, the four chains rapidly fall apart. To avoid this problem, novel hemoglobin
molecules have been designed where two of the four chains are physically linked together, as shown
in PDB entry 1c7d . In that structure, two additional glycine residues form a link between two of the
chains, preventing their separation in solution.
Hemoglobin Cousins
Looking through the PDB, you will find many
different hemoglobin molecules. You can find Max
Perutz's groundbreaking structure of horse
hemoglobin in entry 2dhb , shown in the picture
here. There are structures of human
hemoglobins, both adult and fetal. You can also
find unusual hemoglobins like leghemoglobin,
which is found in legumes. It is thought to protect
the oxygen-sensitive bacteria that fix nitrogen in
leguminous plant roots. In the past few years
some hemoglobin cousins called the "truncated
hemoglobins" have been identified, such as the
hemoglobin in PDB entry 1idr , which have
several portions of the classic structure edited
out. The only feature that is absolutely conserved
in this subfamily of proteins is the histidine amino
acid that binds to the heme iron.
Cooperation Makes It Easier
Hemoglobin is a remarkable molecular machine that uses motion and small structural changes to
regulate its action. Oxygen binding at the four heme sites in hemoglobin does not happen
simultaneously. Once the first heme binds oxygen, it introduces small changes in the structure of the
corresponding protein chain. These changes nudge the neighboring chains into a different shape,
making them bind oxygen more easily. Thus, it is difficult to add the first oxygen molecule, but
binding the second, third and fourth oxygen molecules gets progressively easier and easier. This
provides a great advantage in hemoglobin function. When blood is in the lungs, where oxygen is
plentiful, oxygen easily binds to the first subunit and then quickly fills up the remaining ones. Then,
as blood circulates through the body, the oxygen level drops while that of carbon dioxide increases.
In this environment, hemoglobin releases its bound oxygen. As soon as the first oxygen molecule
drops off, the protein starts changing its shape. This prompts the remaining three oxygens to be
quickly released. In this way, hemoglobin picks up the largest possible load of oxygen in the lungs,
and delivers all of it where and when needed.
In this animated figure, the heme group of one subunit, shown in the little circular window, is kept in
one place so that you can see how the protein moves around it when oxygen binds. The oxygen
molecule is shown in blue green. As it binds to the iron atom in the center of the heme, it pulls a
histidine amino acid upwards on the bottom side of the heme. This shifts the position of an entire
alpha helix, shown here in orange below the heme. This motion is propagated throughout the protein
chain and on to the other chains, ultimately causing the large rocking motion of the two subunits
shown in blue. The two structures shown are PDB entries 2hhb and 1hho .
Troubled Hemoglobins
The genes for the protein chains of hemoglobin show
small differences within different human populations,
so the amino acid sequence of hemoglobin is slightly
different from person to person. In most cases the
changes do not affect protein function and are often
not even noticed. However, in some cases these
different amino acids lead to major structural changes.
One such example is that of the sickle cell
hemoglobin, where glutamate 6 in the beta chain is
mutated to valine. This change allows the
deoxygenated form of the hemoglobin to stick to each
other, as seen in PDB entry 2hbs , producing stiff
fibers of hemoglobin inside red blood cells. This in turn
deforms the red blood cell, which is normally a smooth
disk shape, into a C or sickle shape. The distorted
cells are fragile and often rupture, leading to loss of
hemoglobin. This may seem like a uniformly terrible
thing, but in one circumstance, it is actually an
advantage. The parasites that cause the tropical
disease malaria, which spend part of their life cycle
inside red blood cells, cannot live in the fiber-filled
sickle cells. Thus people with sickle cell hemoglobin
are somewhat resistant to malaria. Other
circumstances leading to troubled hemoglobins arise
from a mismatch in the production of the alpha and
beta proteins. The structure requires equal production
of both proteins. If one of these proteins is missing, it
leads to conditions called Thalassemia.
Fiber of sickle cell hemoglobin, showing the site of mutation.
Download high quality TIFF image
Exploring the Structure
You can look at the binding of oxygen up close in two structures of human hemoglobin. PDB entry
2hhb shows hemoglobin with no oxygen bound. In this picture, the heme is seen edge-on with the
iron atom colored in gold. You can see the key histidine reaching up on the bottom side to bind to
the iron atom. In PDB entry 1hho , oxygen has bound to the iron, pulling it upwards. This in turn,
pulls on the histidine below, which then shifts the location of the entire protein chain. These changes
are transmitted throughout the protein, ultimately causing the big shift in shape that changes the
binding strength of the neighboring sites.
This picture was created with Rasmol. You can create similar pictures by clicking on the accession
codes and choosing one of the options for 3D viewing. Note that the PDB entry 1hho only contains
two of the four chains in the hemoglobin structure, so be sure to view the biological assembly if you
want to see the whole protein.
References
1. Perutz, M.F. (1978): Hemoglobin Structure and Respiratory Transport. Scientific American, 239
(6).
2. Squires, J.E. (2002): Artificial Blood. Science 295, p.1002.
3. Vichinsky, E. (2002): New therapies in sickle cell disease. Lancet 24, p. 629.
4. The Skinny on Blue Blood. Discover Magazine, December 1996.
May 2003, Shuchismita Dutta, David Goodsell
doi:10.2210/rcsb_pdb/mom_2003_5
About Molecule of the Month
The RCSB PDB Molecule of the Month by David S. Goodsell (The Scripps Research Institute and the RCSB PDB)
presents short accounts on selected molecules from the Protein Data Bank. Each installment includes an introduction
to the structure and function of the molecule, a discussion of the relevance of the molecule to human health and
welfare, and suggestions for how visitors might view these structures and access further details. More