Biology Reference
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chaperones or enzymes. The reaction is adequately described as an electro-
philic addition (
Nothnagel et al., 2011
). Interestingly, because electron
transfer occurs readily in the bis-histidine state, mixtures of ferrous and ferric
proteins convert fully to the post-translationally modified form via a linear
redox chain reaction (
Preimesberger et al., 2012
). Thus far, the only known
requirement for reaction is the ability of the reactive histidine and the haem
to adopt a relative orientation such that sp
3
geometry is achieved at the vinyl
site (
Preimesberger et al., 2013
). Sequence inspection (
Fig. 6.13
) suggests
strongly that several other cyanobacterial GlbNs are capable of the same
chemistry because it is likely that the topology of
Synechococcus
7002 and
Syn-
echocystis
6803 GlbN is compatible with the listed primary structures.
The post-translational modification of the haem occurs in
Synechococcus
7002 cells (
Scott et al., 2010
). A possible benefit of the covalent attachment is
the retention of the haem in the haem pocket (
Hoy, Smagghe, et al., 2007
),
although it is not clear that in the cell, haem loss is a problem that needs rem-
edy. Also, as demonstrated by many TrHb1s, the 2/2 fold can be decorated
by side chains that ensure high haem-binding constants. Low haem affinity,
however, may be particularly problematic for photosynthetic organisms
because
b
haem is used for the synthesis of linear tetrapyrrole pigments.
Attaching the haem to the globin would prevent haem dioxygenases to
act on this particular source of starting material. It is also possible that the
attached haem is less prone to oxidative damage, another advantage in
organisms that must tolerate high light and high O
2
conditions.
5.4.3 NO dioxygenation
Nitric oxide and other RNS are known to elicit a variety of responses in
microbes (
Bowman, McLean, Poole, & Fukuto, 2011
). These responses
involve a number of haem proteins, often including globins.
In vitro
, prac-
tically all globins have the ability to perform some degree of NO
dioxygenation, a reaction that converts NO to nitrate as shown in the fol-
lowing equation
1e
!
NO
3
O
2
þ
NO
þ
ð
6
:
1
Þ
This is the main function of FHbs (
Gardner, 2005
), a large set of bacterial
proteins that continue to inform on globin chemistry, electron transfer and
flavin-containing haem enzymes (
Mowat, Gazur, Campbell, & Chapman,
2010
). NO dioxygenation appears to be an ancestral activity that was eroded
over the years in the globins responsible for reversible dioxygen binding.
