Biology Reference
In-Depth Information
A. H¨fner's Model
In 1884, H ¨ fner reported that hemoglobin bound 1.34 cc of gas per
gram of hemoglobin, or 32 grams of O
2
per 16,700 grams of hemoglobin.
A few years later, it was shown that various hemoglobins contained
approximately 0.335% iron, corresponding to a minimal molecular
weight per iron of approximately 16,670 daltons. Consequently,
oxygenated hemoglobin contained one mole of O
2
per mole of iron.
Based upon these observations, in 1889 H ¨ fner proposed the first model
for hemoglobin-oxygen binding. In his model, one molecule of O
2
is
bound to one molecule of hemoglobin (Hb) according to Eqs. (7-6)
and (7-7). This predicts a nonsigmoid binding curve like myoglobin
(i.e., M in Figure 7-3):
Hb
þ
O
2
$
HbO
2
:
(7-7)
However, H ¨ fner made a serious mistake. He believed the
simple mathematical model must be correct and reasoned that only
a single data point near half-saturation on the oxygen-binding curve
was needed to evaluate the binding affinity. He collected one
experimental data point and determined the binding affinity based
upon that single point, without testing its validity. Therefore, H ¨ fner
failed to observe the quintessential sigmoid-shaped binding curves (i.e.,
H in Figure 7-3) characteristic of the cooperative hemoglobin-oxygen
binding system. In H ¨ fner's defense, his work was done 40 years before
human hemoglobin was known to be a unique molecule.
B. Bohr's Approach
Christian Bohr (1855-1911) had a different approach to the study of
hemoglobin-oxygen binding. Bohr was an experimentalist. As Edsall
wrote, ''Bohr's motto was that every experiment had a value, nothing
which was obtained as the result of a test in the laboratory was set aside
on the grounds of its inherent unlikelihood, of its failure to fit general
principles or theories'' (Edsall [1972]). Bohr's curves, obtained point by
point from experimental measurements of oxygen pressure relative to
oxygenated and deoxygenated amounts of hemoglobin, were reported in
Bohr et al. (1904). They differed fundamentally from H
¨
fner's curve,
having the characteristic sigmoid shape of a cooperative interaction (see
the H curve in Figure 7-3). Although the sigmoid nature of the oxygen-
binding curves for hemoglobin was an extremely important observation,
it was not what made Bohr famous. In the same paper, Bohr also
documented the effect CO
2
binding has on O
2
binding. He showed that
the increased amount of CO
2
bound to the hemoglobin in the tissues
lowers the affinity of the hemoglobin for the O
2
it is carrying and
consequently aids the transfer of O
2
to tissues. This is now known as the
Bohr effect.
Bohr and his collaborators tried to measure whether the binding of O
2
alters the binding of CO
2
. They were unable to measure this reciprocal
