Chemistry - A Molecular Science

Chapter 14 Inorganic Chemistry

Animals also utilize metal-containing proteins to transport, store, and use the O

(^2)
produced in photosynthesis. Heme is an iron(II) porphyrin (Figure 14.2b) that is the “active site” in
hemoglobin.
Hemoglobin is a protein found in red blood cells that
transports oxygen from the lungs to the rest of
the body, where it is used in respiration
.
The heme active site is shown in Figure 14.9. Fe
2+
has six coordination sites, but the
porphyrin ligand coordinates to only four. One of the two remaining sites is used to bind the heme unit to the polypeptide, represented by the ribbon. In
deoxy
hemoglobin (
without
O^2
), the remaining site is unoccupied (Fi
gure 14.9a). The resulting five-coordinate Fe
2+
ion has square pyramidal coordination geometry
(Figure 14.1d) in which the iron(II) ion is
pulled out of the plane of the porphyrin. In lung tissue, the high concentration of O
favors 2
the coordination of O
to the sixth Fe 2
2+ coordination site. This gives a six-coordinate iron
complex in which the iron is in the plane of the porphyrin ligand. The oxygen also hydrogen bonds to the protein, as shown in Figure 14.9b. When the
oxy
hemoglobin (
with
O^2
) reaches oxygen-poor tissue, the oxygen is rele
ased.* The released oxygen is then used
in the cell to produce energy by the
reaction described in Equation 14.1.
Carbon monoxide poisoning is the result of the coordination of CO to hemoglobin,
which destroys hemoglobin’s oxygen transport ability. CO is a strong-field ligand that binds to the heme irreversibly
(does not come off), while O
binds reversibly (the oxygen 2
is easily removed in the cell). Thus, when CO
is in the lungs, it binds to the heme iron in
place of O
, 2
and carbon monoxide asphyxiation results
because the hemoglobin can no
longer transport O
to the cells. 2
Hemoglobin, represented by a ribbon diagram in Figure 14.10, consists of
approximately 600 amino acids in four he
me-containing polypeptides held together by
intermolecular forces. Although the iron porph
yrin is the center of the oxygen transport
process, the polypeptide chains play critical roles in stabilizing the binding and release of the four O
molecules. Changes in protein structure have profound effects on the ability of 2
hemoglobin to carry O

. For example, consider sickle-cell anemia, a hereditary condition 2

affecting four in 1000 black people. In a pe

rson with sickle-cell anemia, two amino acids

(glutamates), out of the 600 amino acids that comprise hemoglobin, are replaced with two other amino acids (valines, see Figure 14.11). Th

e change results in a different charge on

the protein because valine is neutral while gl

utamate carries a negative charge. The ability

of the blood to transport oxygen is greatly re

duced as a consequence of this very small

change.

Fe-N bond bindsiron porphyrinto the protein

Hydrogen bonding between

protein and bound O

2

Protein backbone

+O

2

(a)

(b)

=H

=N

=O

=Fe

= bond in porphyrin

Figure 14.9 Active site of hemoglobin a) deoxyhemoglobin (hemoglobin without oxygen) and b) oxyhemoglobin (hemoglobin with bound O

) 2

* This is just what Le Châtelier’s

principle predicts: the concentration

of O

bound to iron increases when the concentration of O 2

(^2)
increases (oxyhemoglobin
U
deoxyhemoglobin + O
). 2
(a)
(b)
Figure 14.10 Hemoglobin structure a) Ribbon diagram of one quarter of the hemoglobin molecule, with the five-coordinate Fe(II) porphyr
in outlined. The active site
highlighted by the thick line is presented in more detail in Figure 14.9. b) Ribbon diagram of the full hemoglobin molecule with four similar units. The porphyrin portion of each unit is outlined.
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