Biology Reference
In-Depth Information
the latter measurement was made before awareness of the post-translational
modification and may be inaccurate. Regardless, a thermodynamic prefer-
ence for the ferric state is apparent in comparison with myoglobin. Addi-
tional oxidation-reduction measurements in
Synechocystis
6803 GlbN
have demonstrated that bis-histidine coordination is tighter in the ferric state
than in the ferrous state (
Halder et al., 2007
). Weaker haem affinity in the
ferrous state is also apparent in
Synechococcus
6803 GlbN.
5.4. Chemical reactivity and possible function
Section 4
exposed how little is known about cyanobacterial and algal globins
from focused investigations
in vivo
. Because of this lack of information, there
have been attempts to circumscribe the function of cyanobacterial and algal
globins using
in vitro
information. The approach suffers from the fact that
haem proteins have a great capacity for performing chemistry, so that an
activity that is detected in the test tube may not be effective
in vivo
. The cel-
lular context, including localization, determines which of the various pos-
sibilities are physiologically relevant.
5.4.1 Endogenous hexacoordination
The coordination of the distal histidine has consequences for ligand binding
and for electron transfer. Furthermore, blocking the distal site can result in
diminished reactivity. Thus, bis-histidine
Synechocystis
6803 and
Syn-
echococcus
7002 GlbNs react slowly with hydrogen peroxide, whereas variants
substituting a non-ligating residue for the distal histidine tend to be more
susceptible to haem damage (
Nothnagel et al., 2011
). Endogenous hexa-
coordination may be essential for proteins that function under high levels
of oxidative stress as encountered during oxygenic photosynthesis.
5.4.2 Haem post-translational modification
A hallmark of
Synechococcus
7002 and
Synechocystis
6803 GlbNs is their ability
to attach the haem covalently and irreversibly to the protein matrix. The
mechanism of this post-translational modification has interest not only from
a chemical point of view, but also because the modification conveys excep-
tional properties to the protein and therefore raises questions of molecular
evolution. Unlike cytochrome
c
, which requires a dedicated machinery
for maturation in the cell and complete formation of the correct thioether
linkages (
Kranz, Richard-Fogal, Taylor, & Frawley, 2009
), GlbN undergoes
the modification spontaneously in the ferrous state, without the aid of
