344 IRON-CONTAINING PROTEINS AND ENZYMES
myoglobin (Mb) and hemoglobin (Hb), is critical to the maintenance of bio-
logical function. While dioxygen ’ s solubility in water is quite low (6.6 cm^3 per
liter or 3 × 10 − 4 M), myoglobin and hemoglobin increase O 2 ’ s solubility in
blood approximately 30 times — that is, to 200 cm^3 per liter or 9 × 10 − 3 M.^2
Myoglobin and hemoglobin complex iron through use of a prosthetic heme
group, a planar four - coordinate porphyrin ligands such as that shown in Figure
7.1. This heme group is classifi ed as a heme b, the same porphyrin ligand
system that ligates iron in cytochrome P450 (Section 7.4 ), cytochrome b(6)f
(Section 7.5 ), and cytochrome bc 1 (Section 7.6 ). Protein side - chain ε - nitrogen
atoms of histidine complete iron ’ s ligand coordination sphere in myoglobin
and hemoglobin.
The structure of hemoglobin and myoglobin were among the fi rst to be
solved by X - ray crystallography: for deoxymyoglobin by J. C. Kendrew and
H. C. Watson beginning in 1966,^3 and for deoxyhemoglobin by G. Fermi begin-
ning in 1975.^4 These fi rst deoxyhemoglobin structures have been declared
obsolete in the Research Collaboratory for Structural Bioinformatics (RCSB)
Protein Data Bank (PDB, http://www.rcsb.org/pdb/ ). In 1984 Fermi and co -
workers published the deoxyhemoglobin quaternary structure visualized in
Figure 7.2.^5 In Figure 7.2 the four hemoglobin protein chains are visualized in
cartoon, cylindrical helical format, and the four hemes are shown in sphere
format using the software program Pymol. TM The Hb α 1 chain (right, front,
lower) is visualized in blue with its heme carbon atoms in gray, oxygens in red,
and nitrogens in blue. The Hb α 2 chain (left, rear, lower) is visualized in red
with its heme carbon atoms in red. The Hb β 1 chain (right, rear, upper) is
visualized in green with its heme carbon atoms in red. The Hb β 2 chain (left,
front, uppper) is visualized in cyan with its heme carbon atoms in gray. The
X - ray crystallographic data, deposited in the Protein Data Bank as PDB:
2HHB, 3HHB and 4HHB, have been updated by many later structures of
deoxy - and oxyhemoglobins, many having been modifi ed by site - directed
mutagenesis of important aa residues that affect the position of the iron heme
Figure 7.1 Protoporphyrin IX, heme b, as found in Mb and Hb.N NN NO OH O OHA BD CFeγβαδ12 3
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