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Among the properties that are consistent with efficient NO di-
oxygenation are high O 2 affinity and high superoxo character to the bound
oxygen, ability to limit the amount of bound NO competing with O 2 for the
ferrous iron, high auto-oxidation rates and internal electron transfer rates
appropriate for efficient catalysis ( Gardner, 2012; Mowat et al., 2010 ).
The set of favourable characteristics can be traced with some success to
specific structural features, including a distal tyrosine interacting with the
bound oxygen ( Gardner, Martin, Gardner, Dou, & Olson, 2000;
Mukai, Mills, Poole, & Yeh, 2001 ). It must be noted, however, that the
nitric oxide dioxygenase (NOD) function requires the participation of a
reductase or a reducing agent in order to return the iron to the ferrous state
after dioxygenation.
NO dioxygenation naturally appears among the functions proposed for
cyanobacterial and algal globins because these organisms have to cope with
NO as a product of nitrate and nitrite metabolism ( Mallick, Rai, Mohn, &
Soeder, 1999; Sakihama, Nakamura, & Yamasaki, 2002 ) or other metabolic
processes. The identification of a reductase remains a difficult step in solid-
ifying the hypothesis of NOD function ( Smagghe et al., 2008 ). In one case,
that of the assimilatory nitrate reductase of the raphidophyte Heterosigma
akashiwo , a TrHb1 domain is present between a cytochrome b 5 domain
and a FAD-binding domain ( Stewart & Coyne, 2011 ). NO dioxygenation
is facilitated by the proximity of the latter domain. For globins that are
not chimeric, the problem remains. It is possible that ferredoxin reductases
play a role ( Gardner, 2012 ), along with the pool of ferredoxin ( Scott
et al., 2010 ).
In an effort to explore the functional possibilities of Synechocystis 6803
GlbN, Hargrove and co-workers examined the response to NO challenge
of an engineered E. coli strain deprived of its FHb and harbouring the glbN
gene. GlbN rescues the wild-type phenotype as an indication that the protein
is capable of protecting its heterologous host. A lag in the response also sug-
gests that a reductase is needed to be produced for detoxification to occur;
however, the nature of this reductase is not known. Although this experiment
demonstrates the participation of GlbN in protection, it unfortunately does
not inform on its role in the Synechocystis cell ( Smagghe et al., 2008 ).
5.4.4 Other reactions
In addition to the NO dioxygenase activity, Synechocystis 6803 GlbN is capa-
ble of nitrite reduction under anaerobic conditions ( Sturms, DiSpirito, &
Hargrove, 2011 ).
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