646
CHAPTER 28
Hemoglobin
FIGURE 28-1
Arrangement of hemoglobin subunits. The four polypeptide subunits are
located at the corners of a tetrahedron to give a roughly spherical
structure.
der Waals bonds, and hydrophobic forces. About 60% of
the contact between subunits does not change during re-
versible oxygen binding (packing contacts), while about
35% does (sliding contacts),
aa
and ££ contacts form
about 5% of the total intersubunit contacts, the remainder
being a £ contacts. In deoxyhemoglobin, there is no ££
contact. This information was derived principally from
x-ray crystallographic studies of Perutz, Kendrew, and
coworkers and clarifies how 2,3-bisphosphoglycerate
(BPG) or (2,3-diphosphogIycerate, 2,3-DPG) regu-
lates oxygen transport (Note: 2,3-bisphosphoglycerate and
2,3-diphosphoglycerate are identical.)
If each subunit is labeled, the dissociation in dilute
solution becomes clearer:
It also shows the asymmetry in the tetramer, since the
a
i £,
contact differs from the
ct\
£2
contact, and «
2
/
3
, differs
from
a 2f32.
However, a i/1
1
and
a 2
£2
contacts are similar,
as are
a\P2
and «
2/61
contacts. During oxygenation and
deoxygenation, the
a \
£, and a
2£2
contacts do not change,
but the
cc\f32
and a
2
/3, contacts do. Thus, the dimers
a
i£,
and
a 2j32
function as a unit and can slide relative to each
other. Stable a £ dimers can be present as a result of
a mutation that prevents the formation of the tetramer.
An example in which hemoglobin dissociates into sta-
ble dimers in the liganded state is the variant hemoglobin
Rothschild, which is due to a mutation affecting the
(3
chain
(£37 Trp -> Arg).
Figures 28-2 and 28-3 are schematic diagrams of the
secondary structures of
a
and £ chains, respectively. Each
chain contains helical and nonhelical segments and sur-
rounds a heme group. Almost 80% of the amino acids
exist in an a-helical conformation.
Heme Group
Heme consists of a porphyrin ring system with an Fe2+
fixed in the center through complexation to the nitrogens
of four pyrrole rings. The porphyrin system is a nearly pla-
nar aromatic ring formed from four pyrrole rings linked
by =CH—
(methene) bridges. The pyrrole rings are substi-
tuted so that different porphyrins are distinguished by vari-
ations in their side chains (see also Chapters 14 and 29).
The heme porphyrin is protoporphyrin IX. Iron has a co-
ordination number of six, i.e., each atom of iron can bond
with six electron pairs. In heme, two coordination posi-
tions of iron are not occupied by porphyrin nitrogens.
When heme is associated with a globin chain, a histidyl ni-
trogen (from histidine F
8
, important in the allosteric mech-
anism) bonds with the fifth coordination position of iron,
while the sixth position is open for combination with oxy-
gen, water, carbon monoxide, or other ligands. Portions
of the seven or eight helices of a globin chain form a hy-
drophobic crevice near the surface of the subunit. Heme
lies in this crevice, between helices E and F. The low
dielectric constant of the crevice prevents permanent ox-
idation of iron (from Fe2+ to Fe3+) by oxygen and is re-
sponsible for the reversible binding of oxygen. When Fe2+
in solution combines with oxygen, it is oxidized to Fe3+,
which does not bind reversibly with oxygen. Evidence
from electron paramagnetic resonance studies shows that,
in oxyhemoglobin, the oxygen can be considered to ox-
idize the iron as long as the oxygen is bound. When the
oxygen is released, Fe3+ is again reduced to Fe2+. Re-
versible
0 2
binding is a unique property of hemoglobin.
If the Fe2+ in hemoglobin is permanently oxidized to the
ferric state (as in methemoglobin), the Fe3+ binds tightly
to a hydroxyl group, and oxygen will not bind.
Heme groups can be removed from hemoglobin by dia-
lysis, indicating that they are not covalently attached. Not
counting the proximal histidine [His £92 (F
8
) or «87 (F
8
)],
about 60 amino acid residues come within 0.4 nm of one
or more atoms of the heme group. This is approximately
the maximal length of an effective hydrogen bond or hy-
drophobic interaction. Of these contacts, only one in the
a
subunit and two in the £ subunit are polar, which empha-
sizes the highly hydrophobic environment of the hemes.
The three polar interactions involve the carboxyl groups
of the propionic acid side chains on heme.
28.2 Functional Aspects of Hemoglobin
Oxygen Transport
The primary function of hemoglobin is to transport oxy-
gen from the lungs to the tissues. Hemoglobin forms a
