The Globins of Cyanobacteria and Algae - Advances in Microbial Physiology - page 259

Electron transfer may also be a feature of enzymatic reactions of
hexacoordinate globins that can, like Synechococcus 7002 GlbN, bind ligands.
Additional studies of electron transfer rates will be necessary to define fully
the role that the globins play in the cell.
6.2.2 Structures and dynamic properties
The large conformational change that occurs upon ligand binding in endog-
enous hexacoordinate TrHb1s constitutes a structural aspect worthy of con-
sideration. The two extreme structures, illustrated by Synechocystis 6803 and
Synechococcus 7002 GlbNs in the bis-histidine and cyanomet states, are stabi-
lized either by the coordination of the distal histidine or by the formation of
the distal hydrogen bond network. Yet, the replacement of the distal histi-
dine ( Couture et al., 2000; Hvitved et al., 2001 ) or substitution of zinc for
iron ( Lecomte, Vu, & Falzone, 2005 ) leads to species with ambiguous struc-
tural properties. The determinants of flexibility are of interest in connection
with an enzymatic function requiring the timing of substrate access to the
haem, release of the product, and interactions with a reductase. Added to
NMR investigations, molecular dynamic studies as performed with other
globins (see, e.g. Nadra, Marti, Pesce, Bolognesi, & Estrin, 2008; Savard
et al., 2011 ) will shed some light on protein motions as they relate to enzy-
matic activity, explore the role of solvent in conditioning the properties of
the haem, and provide atomic level explanation for the consequences of spe-
cific amino acid replacements. It is also possible that large conformational
changes modulate interactions with other proteins by altering the surface
properties of the molecule. A search for specific partners once additional
physiological information restricts the functional possibilities may be
enlightening, particularly for the proteins that may require a dedicated
reductase for enzymatic turnover.
It should be stressed that there are large gaps in the structural descriptions.
Not only are few structures available ( Table 6.5 ), but they are all of the
T family and the same group within that family (TrHb1s). The structure
of the proteins from the M lineage would provide a basis for further
in vitro experiments. Also interesting would be a description of the globins
that lack the proximal histidine. Does haem bind to these proteins? It is
known that it can bind to a distal histidine and there is no a priori reason that
chemistry cannot occur on the 'dark side' of the structure ( Ascenzi,
Leboffe, & Polticelli, 2013 ). Thus, although the proteins without proximal
histidine have typically not been included in any extensive study, if there is