The Globins of Cyanobacteria and Algae - Advances in Microbial Physiology - page 233

the aquomet state ( Pesce et al., 2000 ), serves as an appropriate model for this
species. It features Tyr B10 in the distal pocket, hydrogen bonded to the
axial water. The Tyr-Lys arrangement anticipated in CtrHb at high pH
( Das et al., 1999 ) requires the elimination of the water molecule and the col-
lapse of Tyr B10 O Z onto the haem iron. Furthermore, the ligation of Lys
E10 to the iron in the Leu B10 variant requires a substantial rearrangement of
the structure. Is such conformational change feasible? We will return to this
question in the discussion of Synechococcus and Synechocystis GlbN.
Cyanomet CtrHb revealed an unexpected property of the T globins.
The structure contains large internal apolar tunnels that lead from the surface
of the protein to the distal side of the iron ( Pesce et al., 2000 ). These
branched tunnels are also observed in the related protein from Mycobacterium
tuberculosis and are thought to control ligand access with the aid of gating
residues ( Milani et al., 2001; Milani, Pesce, et al., 2004 ). The long tunnel,
shown in Fig. 6.10 , has a volume of 400 ˚ 3 ( Milani et al., 2005 ) and is well
suited for the purpose of channelling diatomic molecules to the active site.
To probe the nature of the cavities, the X-ray structure was solved under
xenon pressure. Four atoms are located in the long tunnel, at locations that
appear conserved among several TrHb1s ( Milani, Pesce, et al., 2004 ).
Figure 6.10 The cavities of cyanomet CtrHb (PDB ID 1DLY, Pesce et al., 2000 ) calculated
with the program CAVER ( Petrek et al., 2006 ). Tunnels provide access to the distal side of
the haem. The structure is oriented as in Fig. 6.2 . This figure was prepared with PyMOL
( DeLano, 2002 ).