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of HO
2
, leaving behind a ferric ion (
Brantley et al., 1993
), which then binds
a water molecule. For reference, the auto-oxidation of sperm whale myo-
globin occurs with a rate constant,
k
ox
, of 0.055 h
1
at 37
C and neutral pH
(
Springer, Sligar, Olson, & Philips, 1994
). This corresponds to a half-life of
12 h. Details of the mechanism vary according to, among other features,
the nature of the residues in the distal pocket and the ease of water access.
N. commune
GlbN has a relatively high rate of auto-oxidation (half-life
shorter than 12 h,
Thorsteinsson et al., 1996
). CtrHb remains stable in the
oxy state for longer periods (half-life of 7 days at pH 8,
Couture & Guertin,
1996
). Replacement of Tyr B10 accelerates the rate of dioxygen dissociation
and also enhances the rate of auto-oxidation. This is in line with the observa-
tion in other globins that weaker oxygen affinity is accompanied with faster
auto-oxidation (
Shibata et al., 2012
). Auto-oxidation rates for
Synechococcus
and
Synechocystis
GlbNs are not available, but it should be noted that endog-
enous hexacoordination is expected to enhance the rate (
Weiland, Kundu,
Trent, Hoy, & Hargrove, 2004
). The direct significance of auto-oxidation
rates for globins that do not transport or store oxygen and may not exhibit
a stringent necessity to remain in the ferrous state is not clear.
5.3.2 Electron transfer and redox potential
Canonical globins, which utilize the proximal histidine as the only protein
ligand, have slow electron transfer rates due at least in part to the necessary
change in exogenous ligand accompanying the switch between ferric and
ferrous states. To return to the sperm whale example, the ferric state is stable
as an aquomet complex,whereas the ferrous state has no distal ligand. This
raises the re-organization energy associated with a change in redox state.
Endogenous hexacoordination, however, can reduce the energetic cost.
This is apparently the case in the bis-histidyl GlbNs (
Preimesberger,
Pond, Majumdar, & Lecomte, 2012
). NMR measurements have deter-
mined the rate of electron self-exchange for these proteins to be slower than
for cytochrome
c
but still sufficient to consider electron transfer as a possible
component of function. Thus, it is conceivable that in a catalytic mechanism
involving electron transfer, ferric and ferrous bis-histidine states are interme-
diates that can undergo facile reduction or oxidation.
The standard reduction potential of the cyanobacterial and algal proteins
discussed here has not been exhaustively studied. Values of
195 mV
(reduction midpoint potentials vs. the standard hydrogen electrode;
Halder, Trent, & Hargrove, 2007
) and
150 mV (
Lecomte, Scott, Vu, &
Falzone, 2001
) have been reported for
Synechocystis
6803 GlbN, although
