346 IRON-CONTAINING PROTEINS AND ENZYMES
α - helical segments (labeled A – G) and six nonhelical segments. An example of
oxymyoglobin ’ s tertiary structure is found in the work of Phillips, who refi ned
X - ray crystallographic data collected for oxymyoglobin at 1.6 - Å resolution
(PDB: 1MBO).^7 Myoglobin ’ s heme prosthetic center contains an iron ion
complexed by a porphyrin known as protoporphyrin IX. (See Figure 7.1 .) The
Fe(II) protoporphyrin cofactor is held in place in the protein principally by
noncovalent hydrophobic interactions of some 80 or so residues, principally
leucine, isoleucine, valine, and phenylalanine aa residues, and one covalent
linkage at the proximal His F8 (his93) residue. The terminology His F8 refers
to the eighth residue of the Fα - helical region of the protein ’ s tertiary structure.
In newer publications this histidine will usually be referred to as his93, count-
ing aa residues sequentially beginning at the N - terminal end. The so - called
distal histidine, described more fully below, is identifi ed as His E7 or his64. Mb
stores oxygen in muscle and other cellular tissue binding one oxygen molecule
per protein subunit. Hemoglobin (Hb), a tetramer of four globular protein
subunits, each of which is nearly identical to a Mb unit (see Figure 7.2 ), trans-
ports oxygen through the blood plasma. Hb ’ s four subunits are comprised of
twoα chains of 141 residues and two β chains of 146 residues with a total
molecular weight of 64.5 kDa. In hemoglobin, α and β chains differ slightly,
especially in the manner in which the porphyrin is held within the protein.
Bond lengths and angles reported in Tables 7.1 and 7.2 illustrate these
differences.
Hemoglobin binds dioxygen in a cooperative manner — that is, once one O 2
molecule is bound to the enzyme, the second, third, and fourth attach them-
selves more readily. Both Mb and Hb bind dioxygen only when the iron ion
is in its reduced state as iron(II). The terminology oxy - and deoxy - Mb and Hb
refer to the enzyme in its oxygenated or deoxygenated forms, respectively,
both with iron(II) metal centers, while met - describes oxidized heme proteins
containing iron(III) centers. Comparison of reduction potentials in equations
7.1 and 7.2 indicates that dioxygen should oxidize iron(II) under most expected
concentration conditions.^8
OH 22 ++→ 442 +−eEHO^0 =+ 082. V (7.1)
Hb Fe()^320 +−+→eEHb Fe()+ =+ 017 .V (7.2)
Therefore the stability of biological heme – O 2 complexes must arise from
kinetic rather than thermodynamic considerations. Some circumstances favor-
ing heme – O 2 stability include: (1) placement of the heme in a hydrophobic
pocket within the enzyme that is inaccessible to water molecules and protons;
(2) a bent binding mode for dioxygen favored by the prosthetic group ’ s pocket
shape that prevents μ - oxo dimer formation; (3) σ - bonding donation from an
sp^2 - rehybridized superoxide ion to an empty dz^2 Fe(II) orbital facilitated by a
bent orientation of the bound dioxygen (see Figures 7.3, 7.4, and 7.5 ); and (4)
the formation ofπ - back - bonds through interaction of a half - fi lled dxz orbital
of Fe(II) with a half - fi lled π * orbital of the superoxide ion (see Figure 7.3 ).^9