Biology Reference
In-Depth Information
Carbon monoxide (CO) and nitric oxide (NO) are excellent ligands for
the haem ferrous iron. In fact, CO has a significantly higher affinity for the
free haem than O
2
. The discrimination factor (or
M ¼K
CO
=K
O
2
) is greater
than 20,000 in a non-polar solvent (
Traylor et al., 1981
), a value so large that
if it were maintained by a globin, a function as reversible dioxygen binder
would be impaired by irreversible CO poisoning. Discrimination among the
three gaseous ligands, which have approximately the same size and are
apolar, is therefore an essential property conveyed by the haem environ-
ment. In sperm whale myoglobin, the
M
for CO value drops to 25. The
800-fold reduction is attributed to hydrogen bonding by the distal histidine.
The polarity of the distal histidine and other H-bonding residues in the distal
pocket is also at work for the discrimination of NO (
Olson & Phillips, 1997;
Spiro & Soldatova, 2012; Tsai, Berka, Martin, & Olson, 2012
).
Unfortunately, there is little information on the capacity of cyanobacterial
and algal globins to discriminate among ligands. The numbers presented in
Table 6.7
should be taken with caution as they are affected by the difficulty
in measuring ligand affinities for these proteins. At face value, no specific pat-
tern emerges, although it is interesting to note that
N. commune
GlbNdoes not
discriminate as efficiently as the other two proteins, a property likely to be
linked to the composition of the distal pocket mentioned earlier.
5.3. Auto-oxidation, redox potential and electron transfer
5.3.1 Auto-oxidation
A ferrous globin, as it combines with dioxygen, is susceptible to oxidation.
The products are the ferric protein, unable to bind O
2
, and the superoxide
anion (
Wever, Oudega, & Van Gelder, 1973
). For the globins that perform
transport and storage, the rate at which this process occurs is a physiologically
important chemical characteristic. The reaction, known as auto-oxidation
(
George & Stratmann, 1954
), has been the subject of numerous mechanistic
studies (
Brantley, Smerdon, Wilkinson, Singleton, &Olson, 1993; Shikama,
2006
). In general, the rate constant for auto-oxidation,
k
ox
, depends strongly
on pH and dioxygen concentration. The molecular explanation advanced
for myoglobin is as follows. At low oxygen concentration, if the hydrogen
bond between the bound O
2
molecule and the distal histidine breaks, dio-
xygen can dissociate from the iron. It leaves behind deoxymyoglobin, which
contains a loosely associated water molecule in the distal pocket. Iron oxi-
dation then occurs with free dioxygen in a bimolecular fashion resulting in
aquomet myoglobin. At a higher concentration of oxygen, the dominant
mechanism involves the protonation of bound dioxygen and dissociation
