Chemistry Reference
In-Depth Information
peroxide (O
2
2
−
), which are hazardous to biochemical systems if they 'escape'. Special
metalloenzymes 'mop up' these ions.
Free superoxide is dealt with by
superoxide dismutase
(SOD) via a dismutation reaction
(8.2) in which, for two molecules of the same type, one is oxidized and the other reduced.
2O
2
−
+
4H
+
→
O
2
+
2H
2
O
2
(8.2)
People with inherited motor neurone disease have mutations of SOD; over 100 mutations
have been identified.
Free peroxide is inherently unstable to a disproportionation reaction (8.3) to form water
and oxygen.
2H
2
O
2
→
2H
2
O
+
O
2
(8.3)
This is accelerated by the iron heme protein
catalase,
a particularly efficient enzyme with
one of the highest turnover numbers of all known enzymes (at
10
7
molecules per
second). This high rate reflects the important role for the enzyme, and its capacity for
detoxifying hydrogen peroxide.
∼
4
×
8.2.1.4
Electron Carriers
There are a family of nonheme iron proteins that participate in electron transfer that all
contain iron bound to sulfur of cysteine (cys, HS CH
2
CH(NH
2
) COOH) amino acid
residues present in a protein backbone. The simplest of these is the small protein (MW
∼
6000)
rubredoxin
, found in sulfur-containing bacteria, that consists of a protein containing
about 50 amino acids and one iron bound by the S atoms of four cysteine amino acid residues.
The iron is bound to four S atoms in a distorted tetrahedral arrangement. It has a Fe
II/III
redox potential of
0 V, meaning it can be oxidized and reduced readily by biological
redox reagents. The iron centre lies close to the surface of each protein (Figure 8.7),
providing good access for interaction with and electron transfer to other compounds. This
location of the metal redox centre near the exterior of a protein is common in those proteins
with a redox role. This makes outer sphere electron transfer easier and faster.
In addition to this simple compound, there are a number of related compounds, the
ferredoxins
, which contain several iron centres closely linked in small Fe
m
S
n
clusters.
In addition to cys-S, they contain 'labile S' that can be released as H
2
S on addition of
acid, being present in the biomolecules as coordinated and bridging S
2
−
. The 2-Fe cluster
contains 2 labile S ions, and is often designated as Fe
2
S
2
(although 4 other cys-S are
also coordinated), whereas the 4-Fe cluster contains four labile S ions (the cluster is thus
designated as Fe
4
S
4
) with a central Fe
4
S
4
core with a distorted cubic box-like structure.
A form of the latter structure with one iron 'missing' from a corner of the cube, termed
aFe
3
S
4
species, is also known. In all cases, each iron centre lie in a distorted tetrahedral
environment of four S-donor ligands.
∼
8.2.1.5
Iron Storage in Higher Animals
Iron is the most important and used metal in higher animals, and thus having a ready supply
of bioavailable iron is essential to their proper function. To achieve this, higher animals have
developed a way of storing iron. Iron is bound and transported in the body via
transferrin
and stored in
ferritin
protein, made of carboxylate-rich peptide subunits assembled into
