The Dos Family of Globin-Related Sensors Using PAS Domains to Accommodate Haem Acting as the Active Site for Sensing External Signals - Advances in Microbial Physiology - page 300

5.2.2 Dynamic change in coordination structure of haem in EcDos
Site-directed mutagenesis and X-ray crystal structural analyses reveal that the
ferric and ferrous haems in Ec Dos are coordinated with His77 and H 2 O (or
OH ), and His77 and Met95, respectively, as the axial ligands. The haem in
Ec Dos shows a redox-dependent ligand exchange between H 2 O (or OH )
and Met95. Though the ferrous haem coordinated with His77 and Met95
does not have a vacant site for an external ligand, it can bind an external
diatomic gas molecule such as O 2 , CO, and NO. When these gas molecules
are bound to the haem, another ligand exchange takes place between the
endogenous ligand, Met95, and the external ligand.
Ec Dos shows dynamic changes in its coordination structure of the haem
in response to the change in the oxidation state of haem or to the external
ligand. A similar dynamic ligand exchange was observed previously in cyto-
chrome cd1 and CooA. Cytochrome cd1 from Paracoccus pantotrophus (for-
merly Thiosphaera pantotropha ) is nitrite reductase, which has a c-type and
d1-type haems that act as an electron carrier and the enzymatic active site,
respectively. While the ferric haem c is coordinated with His17 and His69,
His17 is replaced by Met106 to form a His/Met-ligated haem upon the
reduction of the haem c ( Baker et al., 1997; F¨ l¨p, Moir, Ferguson, &
Hajdu, 1995; Williams et al., 1997 ). The haem d1 also shows a redox-
dependent ligand exchange. While the ferric d1 haem is coordinated with
His200 and Tyr25, Tyr25 is dissociated from the haem iron to form a
5-coordinate haem upon the reduction of the d1 haem ( Baker et al.,
1997; F¨ l¨p et al., 1995; Williams et al., 1997 ). These ligand exchanges will
be responsible for the fine tuning of the redox potentials of the haem c and
haem d1 to regulate the enzymatic activity. For the haem d1, this ligand
exchange is also responsible for the regulation of substrate (nitrite) binding
affinity. Though nitrite cannot bind to the 6-coordinate haem with His and
Tyr as the axial ligands, it can bind to the 5-coordinate ferrous d1 haem.
5.2.3 Structures of the haem-containing PAS domain in EcDos
The crystal structures of the haem-containing PAS domain of Ec Dos
( Ec DosH) have been reported for the ferric, ferrous, and oxy forms
( Kurokawa et al., 2004; Park, Suquet, Satterlee, & Kang, 2004 ), which show
a typical PAS fold consisting of five distinct a helices and five-stranded b
sheet. The haem is accommodated between the F helix and two b strands
(G b and H b ). The global structure of Ec Dos is similar to that of FixL
( Fig. 7.8 ).