The Globins of Cyanobacteria and Algae - Advances in Microbial Physiology - page 240

obtained in the original preparation of N. commune GlbN ( Thorsteinsson
et al., 1996 ) corresponds to the coordination of His E10 on the distal side.
However, this also indicates that the degree of endogenous coordination
cannot be anticipated from the primary structure. In the absence of exoge-
nous ligand, it is not obvious whether modelling based on the open confor-
mation of CtrHb or the closed conformation of Synechocystis 6803 GlbN (or
any intermediate state) should be selected. Sequence retrieval and alignment
using CtrHb as the query show that lysine is a common residue at position
E10 and that tyrosine is a conserved residue at B10. In this group of proteins
as well, some degree of endogenous hexacoordination may be expected.

5.1.5.3 Tunnels and cavities
With a similar degree of reservation, it is reasonable to assume that TrHb1s
display access tunnels when they are in the conformation containing an
exogenous ligand. The volume of the tunnels and cavities and the identity
of gating residues allowing or blocking access to the haem distal side may
vary. For example, Phe E15 is a gating residue in M. tuberculosis trHbN
(a TrHb1; Milani, Pesce, et al., 2004 ), but it is not conserved in the
cyanobacterial and algal set. Whether gating also occurs in other structures
is not known.

5.1.5.4 Distal hydrogen bond network
In the TrHb1s that contain hydrogen-bonding residues at positions B10, E7
and E11, it is likely that the hydrogen bond network observed in cyanomet
CtrHb (PDB ID 1DLY, Pesce et al., 2000 ), cyanomet Synechocystis 6803
GlbN (PDB ID 1S69, Trent et al., 2004 ) or cyanomet Synechococcus 7002
GlbN (PDB ID 4L2M) is formed. The conservation of these residues in
TrHb1-1s supports this expectation. The strength of the network and its
exact geometry, however, are difficult to predict. The geometry of the
TrHb1-2s bound state is unknown since these proteins have a hydrophobic
residue in place of Tyr or His B10.

5.1.5.5 Covalent haem attachment
The same words of caution hold for the formation of the histidine-haem
bond described for Synechococcus 7002 and Synechocystis 6803 GlbNs.
A histidine is found at the identical position in several cyanobacterial
TrHb1s ( Fig. 6.13 ), and the modification follows a simple mechanism
( Preimesberger, Wenke, Gilevicius, Pond, & Lecomte, 2013 ;

see