Biology Reference
In-Depth Information
repetitive NO additions, which suggests that the redox state of the protein
can be regenerated after the reaction to recycle the protein into the ferrous
state (
Pathania, Navani, Gardner, et al., 2002
).
Salmonella enterica
serovar Typhimurium (hereafter
S. typhimurium
) also
has NO detoxification abilities in the form of the Hmp protein (
Stevanin,
Poole, Demoncheaux, & Read, 2002
); when an
S. typhimurium hmp
strain
was transformed with a vector expressing
glbN
from
M. tuberculosis
,no
growth advantage was conferred under normal laboratory conditions
(
Pawaria et al., 2007
). The NO uptake activity of
S. typhimurium hmp
at
ambient O
2
levels (0.02 nmol haem
1
s
1
) was significantly improved upon
expression of Mtb trHbN (19.7 nmol haem
1
s
1
) and Mtb trHbO
(18.4 nmol haem
1
s
1
); however, under low O
2
levels, trHbN continues
to efficiently remove NO, whereas trHbO does not (
Pawaria et al., 2007
).
These data suggest that the function of trHbN is NO removal even under
low O
2
levels, which perhaps reflect the hypoxic conditions in which it will
be required by the cell to function, and while trHbO can remove NO from
solution, it cannot do so when O
2
levels are low. Growth curves were per-
formed in the presence of acidified nitrite, which should mimic nitrosative
stress within the cell. When exposed to 30 mM acidified nitrite at pH 7, the
growth of
S. typhimurium
was inhibited, showing a long lag phase before
recovering after about 20 h, but when expressing trHbN, the lag phase
was shortened and cells recovered from the stress after around 7.5 h,
suggesting that trHbN is aiding in the response to acidified nitrite. Under
the same conditions, trHbO provides no protection (
Pawaria et al.,
2007
). When
S. typhimurium
cells carrying the globins were used to infect
activated macrophages, expression of
glbN
but not
glbO
improved survival
after 30, 60 and 90 min (
Pawaria et al., 2007
), again implicating trHbN but
not trHbO in NO detoxification, as the activated macrophages were shown
to contain elevated levels of NO.
As already suggested, the reductive evolution of the
M. leprae
genome has
led to the loss of trHbN and the expression of only trHbO. It is conceivable
that in this organism, trHbO has evolved to detoxify NO, replacing the lost
detoxification activity provided by trHbN. Indeed, purified oxy-ferrous Ml
trHbO can detoxify NO and produce nitrate (
Fabozzi et al., 2006
). Expres-
sion of
glbO
from
M. leprae
in an
E. coli hmp
strain protected cells from the
toxicity of SNP and SNAP, but only when induced by IPTG, suggesting
that the levels of trHbO inside the cell must be high in order for NO detox-
ification to occur. There are two reasons that this could happen: as trHbO is
being expressed in
E. coli
, it is unlikely that any partner proteins would be
