Biology Reference
In-Depth Information
activity(ies) to facilitate the conversion of CO to methane, as an adaptation
to different gaseous environments.
Furthermore, it has been demonstrated that
A
.
pernix
Pgb and the globin
domains of GCSs use a common signalling mechanism, which would allow
Ap
Pgb to communicate with heterologous C-terminal transmitter domains
to perform signal transduction and gene regulation. Chimeric receptors
consisting of the GCS or
Ap
Pgb globin domains and the C-terminal
methyl-accepting chemotaxis protein (MCP) signalling domain of the
E
.
coli
chemotaxis transducer Tsr reversibly bind oxygen and mediate aerotactic
responses in
E
.
coli
(
Saito, Wan, Lee, Hou, & Alam, 2008
). The observation
that single-domain
Ap
Pgb is functionally compatible with the MCP signal-
ling domain for aerotactic signalling is intriguing, since it suggests that,
besides sequence and structural similarities, a functional relationship exists
between Pgbs and GCSs, and that
Ap
Pgb has an inherent capacity for signal
transduction. Thus, the existence of Pgb as a single-domain globin could
allow Pgbs for the flexibility to perform multiple functions.
6. CONCLUSIONS
The globin domain has been adopted by nature to host the haem in an
increasing number of successful engineering experiments. Pgb is a fascinat-
ing example of structural modulation of the classical 3/3 globin fold, which
translates into new access routes to the haem, into a unique distortion from
planarity of the porphyrin system that affects the protein reactivity versus O
2
,
and into a specific quaternary assembly shared with the phylogenetic related
GCS proteins (
Nardini et al., 2008
). What makes Pgb unique in the globin
panorama is the striking flexibility/adaptability of the haem distal site resi-
dues, which allow a structural crosstalk between ligand recognition/binding
and haem accessibility through the two-tunnel system, and the coupling of
potential reactivity in the haem cavity with ligation (
Pesce et al., 2011;
Pesce, Tilleman, et al., 2013
).
Thus, the accessibility to the haem cavity, at least through tunnel 1, is
linked to (and possibly modulated by) the ligation state of the protein
through an inter-twinned mechanism of side chain re-arrangements, which
involve three conserved residues at the key topological B9, B10 and E11
sites. Moreover, the nature of the haem ligand appears to be capable of driv-
ing the distal site architecture, the dynamics of the B-, E- and G-helices and
the open/closed states of tunnel 1 through conformational relocation of
