Biology Reference
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However, it is clear that the saturation curve is ā€œSā€ shaped. The location of
the steep slope depends on the pH of the solution, known as the Bohr effect,
some small molecules, e.g. 2,3-diphosphoglycerate, etc. Over the years,
numerous theoretical models have been proposed to explain this interesting
behavior of hemoglobin. Three of them will be discussed here.
ONE-CONSTANT MODEL
The simplest theoretical model is that proposed by Hill (1910), as an
extension to the oxygen saturation of myoglobin discussed above, i.e.:
since hemoglobin has two -subunits and two subunits. He realized that
this was not possible, since a five molecular collision would be required.
However, one can carry out the formality and derive an equation for the
percentage saturation, y, just as in the case of myoglobin. We get:
Indeed, this equation can fit the experimental data plotted in Fig. 2-6 very
well. At very low oxygen pressure, y will increase as the fourth power of
[O 2 ]. At higher oxygen pressure, y will approach 1, and can exhibit the
property of an ā€œSā€ shaped curve. The equilibrium constant K can also be
determined as before, and is equal to [O 2 ] 1/2 4 .
Hill proposed that instead of the y vs. [O 2 ] plot, a more sensitive plot would
be needed. That plot is now known as the Hill's plot. Instead of y, he
calculated:
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