section 28.2
Functional Aspects of Hemoglobin
651
F I G U R E 2 8 - 5
Schematic representation of the geometry of the unliganded (a) and the
liganded (b) heme group, and the F helix in hemoglobin. The iron atom in
the unliganded heme (a) is located out of the mean heme plane (distance
a)
and the iron and the porphyrin ring are slightly domed toward histidine (R).
The histidine is tilted as indicated by the bond angle
a
(8-10 degrees) and
the distance
b > c.
When O
2
binds to the iron (b), the iron atom is closer to
the heme plane (distance
a
decreases), the N-Fe bond becomes more nearly
perpendicular
(a
=
1
degree and
b
= c), and the heme acquires a less
domed conformation. These events pull helix F across the heme group,
setting off a series of reactions leading to the T-to-R transition. (Modified
and redrawn with permission from I. Geis and R. E. Dickerson.)
type of subunit (a or
P)
binds oxygen first is unclear,
although a three- to fourfold difference in oxygen affinity
exists between the subunits in the tetramer. It is assumed
that oxygen binds first to the
a
subunit because its heme
is more readily accessible.
The T-to-R transition occurs after the binding of two
or three oxygen molecules, when the noncovalent interac-
tions are too few to stabilize the deoxy form. The oxygen
binding breaks the remaining noncovalent bonds that sta-
bilize the deoxy structure, relieves the strain on the oxy-
genated subunits, and increases the affinity for oxygen of
the unliganded subunits. The higher oxygen affinity of the
oxy form and the formation of new noncovalent interac-
tions account for the cooperative binding of oxygen.
The sum of the motions of individual side chains on
binding and release of oxygen produces a relative reori-
entation of the four subunits (Figure 28-6). The motion
can be considered as rotation of
aj3
pairs, joined by
a \P l -
type interactions, around the
aa
axis. Movement occurs
primarily at the
a\P2
interfaces. The effect is to move the
P
subunits closer to each other. Thus, a change in the ter-
tiary structure of the subunits brings about the change in
quaternary structure of the tetramer.
At pH 7.4, deoxyhemoglobin releases 0.7 mol of H+
for each mole of oxygen it binds, the process being re-
versible. However, under physiological conditions, deoxy-
hemoglobin in whole blood releases only 0.31 mol of H+
per mole of O
2
bound. This phenomenon is the alkaline
Bohr effect (decrease in oxygen affinity of hemoglobin
with decrease in pH or increase in Pco2)i these H+ ions are
known as
Bohr protons.
An equivalent statement would
be that oxyhemoglobin is a stronger acid than the deoxy
form. In fact, the pK' for oxyhemoglobin is 6.62, while
that for deoxyhemoglobin is 8.18. This change in pK' oc-
curs because basic side chains involved in the salt bridges
are broken during the T-to-R transition. Release of the pro-
tonated side chains decreases their pK's, making it easier
for the H+ to separate. It is estimated that, at pH 7.4,40%
of the Bohr protons come from the C-terminal histidines
on the
p
chains, 25% from the N-terminal amino groups on
the
a
chain, and 35% from many amino acids, particularly
F I G U R E 2 8 -6
Relative motion of the
subunits of hemoglobin upon oxygenation and
deoxygenation.
