Biology Reference
In-Depth Information
present and perhaps trHbO requires a partner, for example a reductase, for
efficient function. Second, perhaps trHbO is not as efficient as trHbN at
removing NO and so has to be present at high concentrations to be effective.
As has already been discussed, homologues of trHbN are less likely to be
found in the genomes of other bacteria than trHbO;
Pathania, Navani,
Gardner, et al. (2002)
suggest that this is because NO scavenging is a more
niche function than the proposed function of trHbO, possibly O
2
scaveng-
ing or delivery (see
Section 5.2
).
5.2. Oxygen metabolism
Due to its unusual O
2
binding features, trHbO has been implicated in O
2
metabolism. When Mtb trHbO was expressed in a
S. typhimurium hmp
mutant, O
2
uptake was slightly higher than in wild-type cells
(
Pawaria et al., 2007
). Uptake of O
2
was increased when Mtb trHbO
was expressed in
E.
mol min
1
10
10
cells
1
, plus
coli
(control: 4.4
m
mol min
1
10
10
cells
1
)
trHbO:
5.6
m
and
M.
smegmatis
(control:
mol min
1
10
10
cells
1
); this
was also the case with
E. coli
membrane vesicles expressing trHbO (
Pathania,
Navani, Rajamohan, et al., 2002
). The O
2
uptake of mutants
of E. coli
which
expressed only cytochrome
bo
0
(
cyd
) or only cytochrome
bd
-type (
cyo
)was
also determined. In
E. coli cyo
,O
2
uptake rates were increased from
76
m
mmin
1
mg protein
1
in control cells to 88
m
mmin
1
mg protein
1
in those cells expressing trHbO; in the
E. coli cyd
strain, rates went from
111
mol min
1
10
10
cells
1
,plus:trHbO3.5
2.6
m
m
mmin
1
mg protein
1
, suggesting
that trHbO is able to sustain and even improve respiration in these single
mutants (
Pathania, Navani, Rajamohan, et al., 2002
). Another study found
that Mtb trHbO was only able to improve growth in the stationary phase
when expressed in wild-type
E. coli
cells, and that this improvement was
not seen in a
cyo
mutant (
Liu, He, & Chang, 2004
). These results both point
towards an interaction with cytochrome
bo
0
rather than
bd
, and this conclu-
sion was confirmed when it was shown that purified trHbO can interact,
albeit weakly, with purified
E. coli
CyoB; CyoB is however not required
for the initial interaction as trHbO can bind to artificial phospholipid mem-
branes as well (
Liu et al., 2004
). During purification of trHbO from
E. coli
cells, large amounts of the protein were found to be located with the mem-
brane fraction and were only removed after treatment with 0.2 M potassium
thiocyanate which interrupts hydrophobic interactions, suggesting that the
function of trHbO is involved with the membrane; cells expressing trHbO
mmin
1
mg protein
1
m
to 192
m
