Haem-Based Sensors: A Still Growing Old Superfamily - Advances in Microbial Physiology - page 23

ligand stabilization is linked to the ligand-discrimination process ( Sawai
et al., 2010 ).
2.2.3.5 The haem-containing phosphodiesterase A1 from
G. xylinus (AxPDEA1)
The c-di-GMP degradation and, as a consequence, the cellulose production
in G. xylinus are controlled by the phosphodiesterase 1 ( Ax PDEA1) ( Chang
et al., 2001 ). Ax PDEA1 displays a haem-binding PAS domain that controls
the activity of the fused PDE domain depending on environmental O 2 con-
centration. The deoxy-haem is the active form that leads to the hydrolysis of
the c-di-GMP phosphodiester bonds with the formation of linear pGpG
( Chang et al., 2001 ). On the other hand, the formation of c-di-GMP is
catalysed by Ax DCG2 ( Qi, Rao, Luo, & Liang, 2009 ).
Like Ax PDEA1, Ax DCG2 is a di-domain protein that comprises a sens-
ing PAS domain and an enzymatic DGC domain. But in this case, the PAS
domain binds an FAD cofactor non-covalently and is sensitive to the intra-
cellular redox state and/or O 2 concentration, as the PDE function is acti-
vated when FAD is oxidized ( Qi et al., 2009 ).
2.2.3.6 The haemerythrin-coupled diguanylate cyclase in V. cholerae
(VcBhr-DGC)
Haemerythrins form a class of non-haemdi-ironO 2 -binding proteins that dis-
play typical sequence and fold and are found only in invertebrates ( Vanin et al.,
2006 ). The presence of a chimeric protein has been recently reported in
V. cholerae , which consists of an N-terminal bacterial haemerythrin domain
fused to a C-terminal DGC domain ( Vc Bhr-DGC) ( Schaller et al., 2012 ).
The haemerythrin domain of Vc Bhr-DGC binds two non-haem iron
atoms and cycles between a di-ferric and a di-ferrous form. The di-ferrous
form shows high autoxidation rate when exposed to air, thus being stable
only under anaerobic conditions. Moreover, when the haemerythrin is in
the di-ferrous state, the activity of the DGC domain is enhanced 10-fold,
compared to the inactive di-ferric form ( Schaller et al., 2012 ).
Taking into account that V. cholerae forms biofilms and that the biofilm
formation is induced under microaerobic conditions, it can be inferred that
Vc Bhr-DGC is involved in this pathway as an environmental-signal
transducing molecule.
2.2.3.7 The haem NO-binding sensors
The bacterial ancestor of the animal soluble guanylyl cyclases (sGC) is rep-
resented by the bacterial HNOB sensors ( Iyer, Anantharaman, & Aravind,