Biology Reference
In-Depth Information
CHAPTER 2
SATURATION OF HEMOGLOBIN WITH
OXYGEN AT EQUILIBRIUM
INTRODUCTION
Human beings, as well as many other animals, have developed a very
efficient protein, known as hemoglobin, to carry oxygen from the lungs to
the tissues. We also have another similar but simpler protein, known as
myoglobin, to store oxygen in our muscles. Due to the presence of large
amounts of hemoglobin in our red blood cells, it has been relatively easily
purified by biochemists. Similarly, myoglobin can also be purified from
muscles. Their biochemistry, physiology, genetics, evolutionary history,
three-dimensional structure, etc. have been extensively studied during the
past century. Indeed, myoglobin and hemoglobin are the two proteins, the
three-dimensional structures of which have first been determined by X-ray
diffraction studies. Many genetic diseases have been identified with
mutations in the hemoglobin gene. The most well known one is sickle cell
anemia.
The binding of myoglobin with oxygen at equilibrium is relatively simple.
One oxygen molecule can bind one myoglobin molecule. There is a specific
site on the myoglobin molecule containing a smaller molecule, known as
heme (Fig. 2-1), with an iron atom in its center capable of binding oxygen
molecule reversibly. In this case, the iron atom is not oxidized to the ferric
state in the presence of oxygen. However, myoglobin is not efficient
enough to transport oxygen from our lungs to our tissues, due to its simple
equilibrium kinetics. Probably through gene duplication as well as other
evolutionary processes, we have acquired a more complex macromolecule
consisting of four subunits, each of which resembles a molecule of
myoglobin. The equilibrium and time-dependent kinetics of hemoglobin
binding oxygen, as well as various molecular models, have been studied
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