Chemistry Reference
In-Depth Information
FIGURE 13.3
The three classes of RNRs utilize their metallo-cofactors to generate an active-site thiyl radical (S
.
). The diferric-tyrosyl
radical in class I enzymes is on the R2 subunit, at best 35
˚
from the thiyl radical (on subunit R1).
(From
Stubbe et al., 2001
.
)
1.
haemoproteins in which an iron porphyrin is incorporated into different apo-proteins to give O
2
carriers, O
2
activators, or alternatively electron transfer proteins.
2.
iron
e
sulfur proteins, many of which are involved in electron transfer.
3.
non-haem, non-iron
sulfur, iron-containing proteins, which include proteins of iron storage and transport,
already described in Chapter 8.
e
Since the roles played by iron are so diverse, we clearly cannot cover all of them, so we have chosen only to
give a small sample of selected illustrations. The incorporation of iron into porphyrins and Fe
e
S clusters has
already been discussed in Chapter 4.
HAEMOPROTEINS
Oxygen Transport
Oxygen transport and storage in multicellular oganisms, whether they are mammals, insects, or worms, are
assured by haemoglobins and myoglobins. These were the first proteins to have their X-ray crystal structures
determined by John Kendrew and Max Perutz, for which they received the Nobel Prize for Chemistry in 1962;
shortly after, when the structures of insect and lamprey haemoglobins were determined, it became clear that all
these oxygen-binding proteins share a common tertiary structure, known as the globin fold. This is illustrated in
Figure 13.4
by sperm whale myoglobin. However, whereas the monomeric myoglobin with a single haem has
a hyperbolic oxygen-binding curve, the tetrameric haemoglobin with four haeme groups has a sigmoidal oxygen-
binding curve (
Figure 13.4
)
. This reflects the cooperativity of oxygen binding
the fourth O
2
molecule binds with
100-fold greater affinity than the first. We know that, like other allosteric proteins haemoglobin exists in two
e
