Chemistry Reference
In-Depth Information
ferrous iron, which is some 0.6
˚
out of the plane of the domed porphyrin ring. A second His residue, His E7 (the
distal histidine), is too far away from the iron atom to coordinate with it in the deoxy state.
A comparison of the deoxy- and oxy-haemoglobin structures reveals a number of important differences.
Whereas in the T (deoxy) state the Fe atom is out of the haem plane, on oxygenation it moves into the plane of the
now undomed porpyrin, pulling the proximal His F8 and the F-helix, to which it is attached, with it (
Figure 13.6
),
FIGURE 13.6
The triggering mechanism for the T to R transition in haemoglobin.
as we will see shortly, thereby triggering the T to R transition. The major differences between R and T confor-
mations are at the
a
1
b
2
(and the corresponding
a
2
b
1
) subunit interfaces which consist of the C helix of
a
-subunits. These fit to one another in two distinct conformations, which
correspond to a 6
˚
relative shift at the interface. In the T state His FG4 is in contact with Thr C6, whereas in the
R state the same His is in contact with Thr C3, one turn further back along the C helix (
Figure 13.7
)
. Another series
of very important differences concern a network of salt bridges at subunit
-subunits and the FG interface of the
b
e
subunit interfaces which stabilize the T
state, but are broken in the more relaxed R state.
FIGURE 13.7
The
a
1
b
2
interface in (a) human deoxy haemoglobin and (b) oxy-haemoglobin.
(Adapted from
Voet & Voet, 2004
.
)
