Biology Reference
In-Depth Information
operation within high O
2
environments. Indeed, the oxygen requirements
for these two Archaea are prohibitively small (nanomolar level) or null; in
fact, the obligate aerobe
A
.
pernix
is also a hyperthermophile and only
25
m
MofO
2
can dissolve in water (1 atm pressure) at its optimal growth
temperature of 95
C, and
M
.
acetivorans
is a strict anaerobe.
For
Ma
Pgb, a possible involvement in either facilitating O
2
detoxifica-
tion or acting as CO sensor/supplier in methanogenesis has been proposed
(
Freitas et al., 2003, 2004, 2005; Hou et al., 2001
). Very unusually,
Ma
Pgb
shows a selectivity ratio for O
2
/CO binding that favours O
2
ligation and
anti-cooperativity in ligand binding (
Abbruzzetti et al., 2012; Nardini
et al., 2008
). Such property could be related to the fact that
M
.
acetivorans
takes advantage of acetate, methanol, CO
2
and CO as carbon sources for
methanogenesis; methane production occurs simultaneously with the for-
mation of a proton gradient that is essential for energy harvesting (
Lessner
et al., 2006; Oelgeschl¨ger & Rother, 2008; Rother & Metcalf, 2004
).
Therefore, the ability to convert CO to methane might indicate that CO
is the actual ligand of
Ma
Pgb
in vivo
, supporting the hypothesis of a very
ancient origin for such metabolic pathway(s) (
Ferry & House, 2006;
Oelgeschl¨ger & Rother, 2008
).
The possibility for Pgb to play a yet unclear role in CO metabolism of
Archaea has driven efforts for the biochemical and kinetic characterisation
of CO binding and dissociation, also taking advantage of the stability of the
CO complexes, as shown for many other globins in the literature
(
Brunori & Gibson, 2001
). Early rapid mixing experiments have shown
that CO (and O
2
) binding to
Ma
Pgb
*
is a biphasic process (
Nardini
et al., 2008
). This feature raises the question whether such heterogeneity
arises from the existence of two molecular populations in equilibrium, or
whether it is linked to the presence of the reported two-tunnel system. By
means of vibrational and time-resolved spectroscopy experiments, it was
shown that the protein exists in two distinct conformations displaying dif-
ferent affinities for CO, based on an equilibrium influenced by the ligation
state of the protein (
Abbruzzetti et al., 2012
). Ligation shifts the equilib-
rium towards a high-affinity species
(in the following indicated as
r
) characterised by low dissociation rates. In the absence of the
ligand, the protein adopts preferentially a lower affinity conformation
(in the following indicated as
Ma
Pgb
*
Ma
Pgb
*
t
), displaying high dissociation rates.
Upon ligand dissociation, switching from the high- to the low-affinity
conformation occurs on the sub-millisecond time scale. Such conforma-
tional change appears to involve only the tertiary structure of each subunit,
