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Iron in the body - Biochemistry
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BIOCHEMISTRY – LECTURE 11 PART 2iron in the body
Iron is vitally important in many molecules – oxygen binding proteins (haem
prosthetic group) = haemoglobin and myoglobin, electron transport chain
proteins and in many enzymes = haem-based iron centre and non-haem-based
iron (sulfur based) such as ETC.
Iron is in the ‘d block’. It is a transition element. Like all transition elements it is a
metal that has multiple oxidation states (variable valency). Transition elements
(including iron) have their characteristics because of the complexity in the filling
of electron shells above 3p (4s shells are occupied before 3d, and are lost first).
This contributes to the multiple, stable oxidation states and characteristic
different colours at these states. At pH 7 and above, iron is usually found in the
3+ (ferric) state, whereas at acidic conditions is typically found in the 2 + state
(ferrous).
Although the redox promiscuous nature of iron is vital for many of its functions, it
presents potential problems:
In 3+ state will form large complexes with anions and precipitate out of
solution
Free iron binds to molecules and can affect their function
Free iron is also used by some bacteria and can allow for pathological
microbial proliferation
The body has many iron-binding proteins to protect it from these
dangerous effects.
Fe2+ can catalyse the cleavage of the O-O bond in hydrogen peroxide to produce
hydroxyl radical and a hydroxide ion. This is known as the Fenton reaction.
Hydroxyl radicals are more damaging than superoxide.
Fe2+ + H2O2 HO· + HO- + Fe3+
Superoxide can react with the hydroxyl radical to form singlet oxygen and
although technically a termination step, the resultant 1O2* is reactive.
O2·- + HO· 1O2* + HO-
Generally, radicals react with other molecules by free-radical substitution, in a
chain reaction of reduction/oxidation. The chain reaction or propagation effect
explains why radicals are so damaging, even at low concentrations.
The haem prosthetic group is made of a tetrapyrrole ring with the iron ion in the
middle. 4 of these prosthetic groups are incorporated into the haemoglobin
protein. Haemoglobin is a tetramer; it consists of two α-globin chains and two β-
globin chains.
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