section
7.2
Allosteric Enzyme Regulation
115
FIGURE 7-7
Profiles of oxygenation of myoglobin and hemoglobin as a function of
partial pressure of oxygen. Myoglobin shows a typical Michaelis-Menten
type of rectangular hyperbolic saturation curve, whereas hemoglobin
shows a sigmoidal saturation curve, consistent with its allosteric properties.
Myoglobin at any partial pressure of oxygen has much higher affinity for
oxygen than does hemoglobin. [Reproduced with permission from
A. Lehninger,
P rin cip les o f B iochem istry.
Worth, New York, 1982.]
2,3-bisphosphoglycerate (BPG or DPG). The
cooperativity of oxygen binding to hemoglobin and
the alterations of hemoglobin by various ligands
provide the most extensively investigated molecular
regulation of a biological process.
Hemoglobin carries oxygen from the lungs to
the tissues and carries CO
2
and H+ back from the
tissues to the lungs (Chapter 1), whereas myoglobin
functions as an oxygen store in muscle. Consistent
with its function, myoglobin has a higher affinity for
oxygen at any partial pressure of oxygen than does
hemoglobin (Figure 7-7). Thus, oxygen can be
transferred easily from hemoglobin to myoglobin.
Hemoglobin is a
tetramer
consisting of two different
subunit types (e.g.,
a
and
ß
in hemoglobin A). Each
polypeptide contains one heme group (an
iron-porphyrin prosthetic group) that binds to one
oxygen molecule by a cooperative process.
Myoglobin, a monomeric protein with one heme
group, remains monomeric under a wide range of
concentrations and does not show cooperative binding
with oxygen. The polypeptides of myoglobin and
hemoglobin exhibit many differences with respect to
their primary structures. For example, the many
amino acid residues present on the surface of
myoglobin are polar, whereas many of those in the
individual hemoglobin polypeptide chains are capable
of participating in nonpolar interactions with other
subunits. Despite these differences, critical regions
are conserved in the polypeptide chains of myoglobin
and hemoglobin, namely, the proximal and distal
histidyl residues that interact with the heme iron, the
hydrophobic amino acid residues that surround the
heme group, and certain prolyl residues that interrupt
the helical regions to allow the chain to fold back
upon itself. The region of the polypeptide chain in
contact with the heme group is known as the heme
pocket. The amino acid residues in this pocket
maintain the heme iron in the divalent state, which is
the functional oxidation state of iron in both
myoglobin and hemoglobin. Thus, the single
polypeptide chain of myoglobin and the two different
chains of hemoglobin are remarkably similar in
secondary and tertiary structures (Figure 7-8). These
similarities support the hypothesis that myoglobin
and hemoglobin evolved by gene duplication and
subsequent mutation from a common ancestral
oxygen-binding heme protein.
The binding of oxygen to myoglobin is not cooperative,
but the binding of oxygen to hemoglobin is cooperative.
This difference can be accounted for kinetically by con-
sidering the equilibrium for dissociation of oxymyoglobin
(Mb02) to deoxymyoglobin (Mb) and oxygen (O
2
):
M b02 ^ M b + 0
2
(7.1)
The equilibrium constant,
[Mb][p2]
[Mb02]
(7.2)
is expressed in moles per liter, and its value depends on
pH, ionic strength, and temperature. Since myoglobin has
a single oxygen binding site, a single equilibrium defines
the dissociation of oxymyoglobin.
So that we can deal with measurable parameters, Equa-
tion (7.2) needs to be modified by the introduction of
two terms,
Y
and P
5 0
.
Y
is defined as the fractional sat-
uration of myoglobin, e.g., when
Y —
0.3, 30% of the
available sites on the myoglobin are occupied by oxygen.
Thus,
number of binding sites occupied by
0
2
total number of binding sites available for binding
0
2
or
[MbP2]
[Mb] + [Mb02]
(7.3)
