BIOINORGANIC CHEMISTRY A Short Course Second Edition

348 IRON-CONTAINING PROTEINS AND ENZYMES

dioxygen is bound to iron on the so - called distal side of the porphyrin ring.
Switching from the T state to the R state in the Hb tetramer takes place during
or after binding of approximately two dioxygen molecules. Upon binding of
dioxygen, constraints within the surrounding protein matrix are relaxed, allow-
ing the iron ion to move toward and nearly into the porphyrin plane.
Binding of the sixth ligand (O 2 ) and consequent movement of the iron atom
into the porphyrin plane induces a tertiary structure change as the proximal
histidine changes its bond angle with the iron atom. The F helix containing the
proximal histidine also changes position. In hemoglobin these factors in turn
change the quaternary structure of the Hb tetramer and infl uence the affi nity
of the four hemes for dioxygen. The metal ion ’ s movement is accompanied by
a change from a high ( S = 2) to a low - spin state ( S = 1/2) and from the Fe(II)
to Fe(III) oxidation states. In the change from the high - to low - spin states and
from Fe(II) to Fe(III), the Fe ion becomes smaller and is thus able to fi t better
into the porphyrin ring ’ s cavity. (See Figures 7.4 and 7.5 .) The dioxygen mole-
cule is guided by the protein pocket surrounding it to attach in a bent structure
with an Fe – O – O bond angle of 115 °. As Fe(II) is oxidized to Fe(III), dioxygen
becomes the superoxide ion ( O 2 −). Evidence for this behavior is found in the
Fe – O, Fe – O – O, and O – O distances and bond angles determined by X - ray
crystallography and from O – O bond orders determined by infrared and reso-
nance Raman spectroscopy. Additional evidence for the Fe III O()− − 2 moiety is
its experimental spin stateS = 0 indicating magnetic coupling of the Fe(III)
ion ’ s unpaired electron with that of the superoxide ion.

7.2.3 Behavior of Dioxygen Bound to Metals,

As can be seen from its ground - state molecular orbital diagram in Figure 7.6 ,
dioxygen has a paramagnetic ground state. It is the only stable homonuclear
diatomic molecule with this property.

Figure 7.5 Change from the high, Fe(II), to low, Fe(III), spin states allows iron ion to
enter porphyrin plane.

high spin ferrous, Fe(II)

S = 2

deoxy form

xz, yz

z^2

x^2 - y^2

xy

low spin ferric, Fe(III)

S = 1/2

oxy form

xz, yz

z^2

x^2 - y^2

xy