The Globins of Cyanobacteria and Algae - Advances in Microbial Physiology - page 252

NO 2 þ 2H þ þ 1e ! NO þ H 2 O ð 6 : 2 Þ
This activity has been observed in other globins ( Gladwin, Grubina, &
Doyle, 2009 ), although it often proceeds at too slow a rate to be considered
of physiological relevance. The reaction uses nitrite and one (two) proton(s)
to generate NO and a hydroxyl ion (water molecule). The ferrous protein
provides the necessary electron and ends up in the ferric state. Thus, for
turnover to occur, the protein must be restored to the ferrous state, and
the issue of reduction is present for this particular activity as well as the NOD
activity. In vitro , the rates of nitrite consumption by Synechocystis 6803 GlbN
(as measured by the disappearance of ferrous protein) are faster than those for
cytoglobin ( Li, Hemann, Abdelghany, El-Mahdy, & Zweier, 2012 ) and
neuroglobin ( Petersen, Dewilde, & Fago, 2008 ). This reaction, however,
does not seem to be useful for generating free NO ( Sturms, DiSpirito,
et al., 2011; Tiso, Tejero, Kenney, Frizzell, & Gladwin, 2012 ).
Another reaction in the realm of nitrogen oxides is hydroxylamine
reduction.

3H þ þ 2e ! NH 4 þ þ NH 2 OH þ H 2 O ð 6 : 3 Þ

Hydroxylamine is an intermediate species in the reduction of nitrite to
the ammonium ion. When presented with hydroxylamine under anaerobic
conditions, Synechocystis 6803 GlbN releases an ammonium ion ( Sturms,
DiSpirito, Fulton, & Hargrove, 2011 ). Here as well, the in vitro reaction
is carried out faster by GlbN than other globins. The two-electron process
requires re-reduction of the protein. Although participation in anaerobic
respiration has been proposed, it is unclear how an energy-yielding process
could take place in the cytoplasm, and the nature of the electron donor is
not known.
Many proteins are also capable of peroxidase activity. Because of the
involvement of Synechococcus 7002 GlbN in protecting the cell from
ROS/RNS, its activity towards hydrogen peroxide was tested. Modest
activity was observed, which was not consistent with a physiological role
( Scott et al., 2010 ).

6. NEEDS AND OPPORTUNITIES

Since the pioneering 1992 publication first identified a globin in the
cyanobacterium N. commune ( Potts et al., 1992 ), the dominant questions in
the field have been: 'How prevalent are these proteins?', 'How do they differ