Biology Reference
In-Depth Information
against a role in O
2
delivery—it is hard to envisage a protein playing a role in
O
2
delivery if it is unlikely to dissociate and
Ouellet et al. (2003)
instead sug-
gest a role as a redox sensor.
After photodissociation of CO from CO-bound wild-type Mtb trHbO,
CO re-binds to the haem as expected; a high proportion of this can be attrib-
uted to geminate re-binding (
Guallar et al., 2009
). In fact, around 95% of
CO removed from the haem by photoexcitation re-binds (
Jasaitis et al.,
2012
). It is thought that the distal pocket residues are responsible for this fast
geminate re-binding of CO, and for the slow ligand on- and off-rates
(
Guallar et al., 2009
). A molecular dynamics study of the CO complex
showed that interactions between water molecules play an important role
in this high rate of re-binding, in addition to the rotational freedom
employed by the protein (
Jasaitis et al., 2012
). Again, the hydrogen bond
network is important in this aspect of the biochemistry of trHbO: CO is
orientated so that it points towards the haem. Interestingly, only 1% of pho-
toexcited O
2
re-binds, and less than 1% of NO (
Jasaitis et al., 2012
). In con-
cert with this fast re-binding, it follows that trHbO displays very little
exchange of ligands with the environment (
Jasaitis et al., 2012
).
Perhaps it is proper to discuss the biochemistry of Ml trHbO separately, as
it may performmultiple functions and therefore show slightly different func-
tional characteristics to other trHbO proteins. Oxy-ferrous Ml trHbO binds
NO and is itself oxidised in the production of stoichiometric amounts of
nitrate (
Ascenzi, Bocedi, et al., 2006; Fabozzi et al., 2006
) as is the case for
trHbN. Analysis of the transient species formed in this reaction suggests
the production of a peroxynitrite intermediate, Fe(II)OONO (
Ascenzi,
Bocedi, et al., 2006
). The
k
on
value for NO oxidation (2.1
0
6
M
1
s
1
)
is lower than for trHbN, perhaps reflecting the different function of trHbO,
and is probably due to the differences in haempocket arrangement and tunnel
availability. Due to this discovery, the peroxynitrite scavenging ability of Ml
trHbO was investigated. Upon exposure of ferrous-NO-bound Ml trHbO
with peroxynitrite, the protein immediately forms the ferric-NO-bound
form before switching to the ferric form with assumed release of NO
(
Ascenzi, Milani, & Visca, 2006
). After addition of peroxynitrite to oxy-
ferrous Ml trHbO, the oxy-ferryl state was formed; the authors concluded
that both oxy-ferrous and ferrous-NO-bound trHbO are able to scavenge
peroxynitrite (
Ascenzi, Milani, et al., 2006
). Investigations into the
denitrosylation and O
2
-mediated oxidation of ferrous-NO-bound trHbO
presented a possible reaction mechanism: NO binds to ferrous trHbO but
is then displaced by O
2
; the NOmay, however, stay trapped in a cavity close
