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282 Tissues and organs

Gas transport B. Hemoglobin and CO 2 transport
Hemoglobin is also decisively involved in the
Most tissues are constantly dependent on a
supply of molecular oxygen (O 2 ) to maintain transport of carbon dioxide (CO 2 )from the
tissues to the lungs.
their oxidative metabolism. Due to its poor
Some 5% of the CO 2 arising in the tissues is
solubility, O 2 is bound to hemoglobin for covalently bound to the N terminus of hemo-
transport in the blood (see p. 280). This not
only increases the oxygen transport capacity, globin and transported as carbaminohemoglo-
bin (not shown). About 90% of the CO 2 is first
but also allows regulation of O 2 uptake in the
lungs and O 2 release into tissues. converted in the periphery into hydrogen car-
–
bonate (HCO 3 ), which is more soluble (bot-
tom). In the lungs (top), CO 2 is regenerated
–
A. Regulation of O 2 transport again from HCO 3 and canthenbe exhaled.
These two processes are coupled to the
When an enzyme reacts to effectors (sub- oxygenation and deoxygenation of Hb.
strates, activators, or inhibitors) with confor-
mational changes that increase or reduce its Deoxy–Hb is a stronger base than oxy–Hb. It
therefore binds additional protons (about
activity, it is said to show allosteric behavior +
(see p. 116). Allosteric enzymes are usually 0.7 H per tetramer), which promotes the for-
–
oligomers with several subunits that mutually mation of HCO 3 from CO 2 in the peripheral
–
tissues. The resulting HCO 3 is released into
influence each other.
Although hemoglobin is not an enzyme (it the plasma via an antiporter in the erythro-
–
releases the bound oxygen without changing cyte membrane in exchange for Cl ,and
it), it has all the characteristics of an allosteric passes from the plasma to the lungs. In the
lungs, the reactions described above then
protein. Its effectors include oxygen, which as proceed in reverse order: deoxy-Hb is oxy-
a positive homotropic effector promotes its
own binding. The O 2 saturation curve of he- genated and releases protons. The protons
shift the HCO 3 /CO 2 equilibrium to the left
moglobin is therefore markedly sigmoidal in
shape (2, curve 2). The non-sigmoidal satura- and thereby promote CO 2 release. +
O 2 binding to Hb is regulated by H ions
tion curve of the muscular protein myoglobin
is shown for comparison (curve 1). The struc- (i. e., by thepHvalue)via thesamemecha-
ture of myoglobin (see p. 336) is similar to nism. High concentrations of CO 2 such as
those in tissues with intensive metabolism
that of a subunit of hemoglobin, but as a +
monomer it does not exhibit any allosteric locally increase the H concentration and
thereby reduce hemoglobin’s O 2 af nity
behavior. (Bohr effect; see above). This leads to in-
+
CO 2 ,H , and a special metabolite of ery-
throcytes—2,3-bisphosphoglycerate (BPG)— creased O 2 release and thus to an improved
oxygen supply.
act as heterotropic effectors of hemoglobin.
The adjustment of the equilibrium be-
BPG is synthesized from 1,3–bisphosphogly- tween CO 2 and HCO 3 is relatively slow in
–
cerate, an intermediate of glycolysis (see the uncatalyzed state. It is therefore acceler-
p. 150), and it can be returned to glycolysis
again by breakdown into 2–phosphoglycerate ated in the erythrocytes by carbonate dehy-
dratase (carbonic anhydrase) [1])—an enzyme
(1), with loss of an ATP.
BPG binds selectively to deoxy–Hb, thereby that occurs in high concentrations in the
increasing its amount of equilibrium. The re- erythrocytes.
sult is increased O 2 release at constant pΟ 2 .In
the diagram, this corresponds to a right shift
of the saturation curve (2,curve 3).CO 2 and
+
H act inthe same directionas BPG. Their
influence on the position of the curve has
long been known as the Bohr effect.
The effects of CO 2 and BPG are additive. In
thepresenceofboth effectors, the saturation
curve of isolated Hb is similar to that of whole
blood (curve 4).

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