Biology Reference
In-Depth Information
Therefore, Hill proposed to plot ln{y/(1-y)} against ln{[O
2
] / [O
2
]
1/2
}. In this
case, his one-constant model of hemoglobin saturation would give a straight
line of slope of four, since
This is the well known Hill's plot.
To compare with the experiment results of Roughton
et al.
(1955) in Table
2-2, we need to estimate the value of [O
2
]
1/2
from the plot in Fig. 2-6. Using
the two experimental measurements closest to 50% saturation, we have:
Therefore, [O
2
]
1/2
is around 4.46 mm Hg.
The experimental Hill's plot is shown in Fig. 2-7, including error bars of
0.5% for all ten experimental measurements of percentage saturation. It is
immediately obvious that this error renders the measurements at very low
and very high percentage saturation useless. For this reason, Roughton
et
al.
were extremely careful of reducing the errors for these measurements
down to 0.05%.
The slope of the experimental result around 50% saturation is usually
between 2.8 and 3.1, indicating clearly that the one-constant model of
hemoglobin saturation is not correct. In fact, at one time, someone even
suggested that hemoglobin might have only three subunits. The three-
dimensional structure determined by X-ray diffraction studies obviously
eliminated that possibility (Fig. 2-2). Many models based on two molecular
collisions, i.e. sequential binding of oxygen molecules to hemoglobin, have
thus been proposed subseqeuntly.
