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picosecond to nanosecond timescale, though additional motions are
detected in the CE turn, the EF loop, and the kink in the H helix. Motions
on the microsecond to millisecond timescale appear to depend on the distal
ligand and the oxidation state of the iron. The ferrous states, in particular, are
endowed with higher flexibility.
5.1.5 Summary of structural information
There are now almost a dozen cyanobacterial and green algal globin struc-
tures in the Protein Data Bank (PDB), but they are from only three different
organisms and all are TrHb1s (
Table 6.5
). They offer a narrow view of the
globins found in unicellular photosynthetic organisms. Nevertheless, along
with related proteins from different sources, they illustrate essential features
that can be kept in mind when inspecting other globins.
5.1.5.1 Two-over-two fold
In addition to the three TrHb1 structures mentioned earlier, the PDB holds
coordinates for several T globins. Variations in loops and helix length do
occur, generally leaving the 2/2 topology intact. Thus, it is reasonable to
expect that the available structures are adequate templates for the modelling
of the three-dimensional properties of other cyanobacterial and algal
T globins. This, however, can only serve as a rough approach because of
the structural plasticity mentioned earlier.
5.1.5.2 Endogenous hexacoordination
Sequence alignment of cyanobacterial TrHb1s related to
N. commune
GlbN
reveals that the histidine at position E10 is conserved in many instances
(
Fig. 6.13
). In view of the behaviour of
Synechococcus
7002 GlbN (30% iden-
tity with
N. commune
GlbN) and
Synechocystis
6803 GlbN (40% identity with
N. commune
GlbN), it can be proposed that the spurious haemichrome
Table 6.4 Forms of Synechococcus sp. PCC 7002 GlbN inspected by
15
N NMR relaxation
a
Cross-linked
Spin state
Non-cross-linked
Spin state
Fe(III)GlbN-A-His
1/2
Fe(III)GlbN-R-His
1/2
Fe(III)GlbN-A-CN
1/2
Fe(III)GlbN-R-CN
1/2
Fe(II)GlbN-A-His
0
-
Fe(II)GlbN-A-CO
0
-
a
The post-translationally modified protein is referred to as 'cross-linked' or GlbN-A. The unmodified
protein is referred to as GlbN-R (
Pond et al., 2012
).
