General Chemistry Principles, Patterns, and Applications, 2011

transport system is available to provide an adequate supply of oxygen for the interior cells, organisms that
contain more than a few cells cannot exist. In addition, O2 is such a powerful oxidant that the oxidation
reactions used to obtain metabolic energy must be carefully controlled to avoid releasing so much heat
that the water in the cell boils. Consequently, in higher-level organisms, the respiratory apparatus is
located in internal compartments called mitochondria, which are the power plants of a cell. Oxygen must
therefore be transported not only to a cell but also to the proper compartment within a cell.
Three different chemical solutions to the problem of oxygen transport have developed independently in
the course of evolution, as indicated in Table 23.13 "Some Properties of the Three Classes of Oxygen-
Transport Proteins". Mammals, birds, reptiles, fish, and some insects use a heme protein
called hemoglobin to transport oxygen from the lungs to the cells, and they use a related protein
called myoglobin to temporarily store oxygen in the tissues. Several classes of invertebrates, including
marine worms, use an iron-containing protein called hemerythrin to transport oxygen, whereas other
classes of invertebrates (arthropods and mollusks) use a copper-containing protein calledhemocyanin.
Despite the presence of the hem- prefix, hemerythrin and hemocyanin do not contain a metal–porphyrin
complex.
Table 23.13 Some Properties of the Three Classes of Oxygen-Transport Proteins
Protein
Source
M per
Subunit
M per
O2Bound
Color (deoxy
form)
Color (oxy
form)
hemoglobin
mammals, birds, fish,
reptiles, some insects 1 Fe 1 Fe red-purple red
hemerythrin marine worms 2 Fe 2 Fe colorless red
hemocyanin
mollusks, crustaceans,
spiders 2 Cu 2 Cu colorless blue
Myoglobin and Hemoglobin
Myoglobin is a relatively small protein that contains 150 amino acids. The functional unit of myoglobin is
an iron–porphyrin complex that is embedded in the protein (Figure 23.23 "The Structure of
Deoxymyoglobin, Showing the Heme Group"). In myoglobin, the heme iron is five-coordinate, with only a
single histidine imidazole ligand from the protein (called the proximal histidine because it is near the
iron) in addition to the four nitrogen atoms of the porphyrin. A second histidine imidazole (the distal
histidinebecause it is more distant from the iron) is located on the other side of the heme group, too far
from the iron to be bonded to it. Consequently, the iron atom has a vacant coordination site, which is
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