Because myoglobin and hemoglobin each bind oxygen, we can assume that there should be some similarities between their structures. However, there should also be some dissimilarities because of their different roles as oxygen binding proteins. One obvious difference between the two proteins is that myoglobin is a single polypeptide chain while vertebrate hemoglobins are tetrameric.
Human myoglobin has 153 amino acid residues in a highly folded and compact structure with eight separate and distinct alpha helical secondary structures (labeled A through H).
The surfaces of proteins that, like myoglobin, are in an aqueous (watery) environment are composed of mostly polar residues, while the interior is largely non-polar. This animation will cut cross-sectional slices through myoglobin, allowing you to see the hydrphobic interior, protected from water by the predominately hydrophilic exterior.
Click and drag on myoglobin to move it to any orientation you choose, then take cross-sections:
Human hemoglobin is a more complex molecule than myoglobin, having four polypeptide subunits.
Human hemoglobin is composed of two alpha subunits, of 144 residues each, and two beta subunits, of 146 residues each. The alpha and beta subunits associate more strongly with each other than with similar subunits (alpha-alpha or beta-beta). For this reason, hemoglobin is sometimes referred to as a "dimer of dimers". Both the alpha and beta subunits have structural characteristics similar to that of myoglobin. This is interesting in light of the fact that if you compare the amino acid sequences of the hemoglobin alpha, beta, and myoglobin chains, only 27% of the residues are identical.
The Heme Prosthetic Group
The heme is a small but important non-protein molecule, or prosthetic group, that is associated with these oxygen binding proteins. The heme prosthetic group is made up of a complicated-looking ring structure called protoporphyrin IX that binds an iron atom in the ferrous (+2) oxidation state. A metal ion bound by a ringed structure like this is called a chelate.
Incidentally, when a metal ion binds to a porphyrin, which has a great number of conjugated double bonds, it will lead to a compound that is highly colored. This is why blood is red. In plants, the porphyrin is chelated with a magnesium ion, giving chlorophyll a greenish-blue color.
The Heme Binding Site
The heme resides in a small hydrophobic cleft within each polypeptide.
The Proximal Histidine
The heme is held in the cleft both by hydrophobic interactions and by a covalent bond between the iron and a nitrogen atom of a nearby histidine side chain. This histidine is referred to as the proximal histidine.
Oxygen-binding and the Distal Histidine
The side of the heme opposite the proximal histidine is where oxygen binds. Nearby is another histidine residue called the distal histidine. This residue serves two very important functions in the polypeptide. First, it prevents oxidation of the iron by any number of possible oxidizing agents. At a higher oxidation state, the iron is unable to bind oxygen. Secondly, the position of this histidine side chain blocks carbon monoxide (CO) from binding to the iron, while allowing oxygen to bind easily. Left to itself, the heme group has a much greater bonding affinity for carbon monoxide than for oxygen. If the distal histidine was absent, even low levels of CO would out-compete oxygen for the iron binding site, resulting in suffocation.
Binding and releasing oxygen has a marked effect on hemoglobin's three-dimensional structure. When oxygen binds to a single subunit of deoxyhemoglobin, it leads to subtle changes in the quaternary structure of the protein, that is, in the way that the four subunits fit together. This in turn affects the intertior stucture of each subunit, making it easier for a subsequent molecule of oxygen to bind to the next subunit. Thus, with the binding of an initial oxygen to one subunit, the remaining unbound subunits become more receptive to oxygen. This phenomenon of a change in one protein subunit affecting the shape and behavior of another subunit is called an allosteric (through space) interaction.
The binding of oxygen to hemoglobin can be dramatically altered by a small group of substances called allosteric effectors. Hydrogen ions (protons), carbon dioxide, and 2,3-bisphosphoglycerate are effectors that can promote the release of oxygen by favoring the deoxygenated form of hemoglobin. Since these allosteric effectors bind to sites that are specific to each kind of compound, their effects are cumulative.
Hydrogen ions and carbon dioxide are found in high concentrations around actively metabolizing tissues. In the capillaries, the environment favors the release of oxygen from hemoglobin and the binding of these allosteric effectors. The overall result is to facilitate oxygen release into blood plasma and subsequent uptake of oxygen by the high affinity myoglobin in the tissues. The specific reactions of the hydrogen ions and carbon dioxide with hemoglobin causing the release of additional oxygen is called the Bohr effect .
The reactions of the Bohr effect are reversible. When deoxygenated hemoglobin returns to the lungs, the concentration of the hydrogen ions and the partial pressure of carbon dioxide is low. This causes these compounds to be released from hemoglobin. The carbon dioxide is expelled out of the body through expired air. So hemoglobin not only carries oxygen to the cells, it also carries waste products from the cells to the lungs, eventually to be eliminated out of the body.
In addition to hydrogen ions and carbon dioxide, a very important allosteric effector is 2,3-bisphosphoglycerate (2,3-BPG). It is a small, organic molecule that is synthesized in red blood cells from 1,3-BPG, an intermediate in glycolysis. This "costs" the red blood cell 1 ATP that it would have gained from converting 1,3-BPG to 3-phosphoglycerate, but 2,3-BPG has a major and critical effect on the affinity of hemoglobin for oxygen.
BPG affects oxygen binding affinity by binding in a small cavity at the center axis of deoxygenated hemoglobin. In oxygenated hemoglobin, this cavity is too small to effectively accommodate 2,3-BPG. When bound, 2,3-BPG stabilizes the deoxygenated conformation of hemoglobin, greatly diminishing the binding of oxygen and facilitating oxygen unloading to actively respiring tissues. At high altitude, when the proportion of oxygen in the atmosphere is lower and hence oxygen is harder to deliver to the tissues, the synthesis of 2,3-BPG is upregulated significantly. It takes about 24 hours for 2,3-BPG levels to rise, and over longer periods of time, the levels continue to increase. This is why athletes can train at high altitudes to temporarily increase their aeorbic capacity.
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