Biology Reference
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supported by the sealed nature of the haem cavity. The main out-of-plane
contribution to the
Ma
Pgb
*
haem distortion is ruffling (which leaves oppo-
site carbon atoms equally displaced and alternatively above and below the
mean porphyrin plane), while the
Ma
Pgb
*
haem in-plane distortion is
mainly ascribed to a strong breathing mode, which involves the symmetric
compression-expansion of the porphyrin ring (with expansion associated to
destabilisation of O
2
binding, whereas the opposite trend is found for com-
pression). Therefore, haem compression due to the restricted
Ma
Pgb
*
haem-binding pocket is expected to lead to a sizable stabilisation of O
2
bind-
ing, overcoming the destabilisation due to ruffling, thus resulting in
stabilisation of the haem-bound O
2
, as compared to the ideal planar refer-
ence haem model (
Bikiel et al., 2010
). This is in agreement with the unusu-
ally low
in vitro
O
2
dissociation rate reported for
Ma
Pgb
*
(0.092-0.0094 s
1
),
appearing particularly low considering that the haem-bound O
2
was found
not to be stabilised by any hydrogen bond to the protein (
Nardini et al.,
2008
). This unusually low O
2
dissociation rate constant in
Ma
Pgb
*
has been
correlated to the large deviations from planarity of the porphyrin system
(
Bikiel et al., 2010; Nardini et al., 2008
). Thus, the O
2
binding behaviour
of ferrous
Ma
Pgb
*
suggests a scenario whereby evolutionary events could
subtly regulate O
2
affinity by shaping the haem cavity to favour porphyrin
out-of-plane distortions in order to decrease ligand affinity, or to compress
the porphyrin ring, for instance, by the presence of bulky residues (usually
Leu and/or Phe), which would selectively push the pyrrole rings out of the
porphyrin plane, to promote the reverse effect (
Bikiel et al., 2010
).
Very recently, structural studies on ferric
Ma
Pgb
*
(and selected
mutants) have further characterised the distal site plasticity and the
ligand-binding modes in
Ma
Pgb
*
(
Pesce et al., 2011; Pesce, Tilleman,
et al., 2013
). The first intriguing finding was that ferric
Ma
Pgb
*
can pro-
ductively bind a plethora of ligands, comprising classical globin ligands
such as cyanide, azide and imidazole, and a less common ligand such as nic-
otinamide. Contrary to what reported for the oxygenated protein, in ferric
Ma
Pgb
*
allligandsareproductivelystabilisedbyhydrogenbonds.The
prototypical case is represented by the Fe-bound cyanide, which is sta-
bilised by two hydrogen bonds provided by Trp(60)B9 and Tyr(61)B10
side chains. While ligand stabilisation by the OH group of Tyr(61)B10
requires only one side chain rotation relative to the ligand-free ferric
Ma
Pgb
*
, the hydrogen bond provided by the N
e
2atomofTrp(60)B9
requires a complex re-arrangement of the haem distal site cavity
(
Fig. 3.2
). The presently available data identify three residues at key
