The Globins of Cold-Adapted Pseudoalteromonas haloplanktis TAC125: From the Structure to the Physiological Functions - Advances in Microbial Physiology - page 337

cold-shock proteins (CSPs) ( Cavicchioli et al., 2000 ) and molecular
chaperones ( Motohashi et al., 1999 ), solutes ( Pegg, 2007 ) and structural
modifications for maintaining membrane fluidity ( Chintalapati, Kiran, &
Shivaji, 2004; Russell, 1998 ). Moreover, cold-adapted organisms must
develop an effective and intricate network of defence mechanisms against
oxidative stress: an increasing number of oxidoreductases, superoxide dis-
mutases, catalases and peroxidases can be seen in this perspective ( Ayub
et al., 2009; Bakermans et al., 2007; Duchaud et al., 2007; M´digue
et al., 2005; Meth´ et al., 2005; Piette et al., 2010; Rabus et al., 2004 ).
In addition to adaptations at the cellular level, a key adaptive strategy is
the maintenance of adequate reaction rates at thermal extremes; therefore,
adequate features of catalytic processes become crucial. Enzyme catalytic
rates at low temperatures depend on increased protein flexibility and con-
comitant increase in thermolability ( Georlette et al., 2004 ).
In this respect, a suite of factors contribute tomaintaining enzyme molecular
flexibility in all cold-adapted organisms. In bacterial enzymes, Ser, Asp, Thr and
Ala are over-represented in the coil regions of secondary structures. On the
other hand, in the helical regions, aliphatic, basic, aromatic and hydrophilic
residues are generally under-represented ( Cavicchioli et al., 2002; D'Amico
et al., 2002; Ray et al., 1998; Rodrigues & Tiedje, 2008; Russell, 2000 ). More-
over, a reduction of surface, inter-domain, inter-subunit ionic linkages and a
decreased number of hydrogen bonds and salt bridges are key mechanisms to
induce an increasing of conformational flexibility of psychrophilic enzymes
( D'Amico et al., 2006; Feller & Gerday, 1997 ).
Cold adaptation is also strongly linked to the capacity of the organism to
sense temperature changes, perhaps by virtue of mechanisms linked with the
lipid composition of the cell membrane and alterations in the DNA and
RNA topology. The latter may enhance (or halt) the replication, transcrip-
tion and translation processes ( Eriksson, Hurme, & Rhen, 2002 ). Although
increased unsaturation and decreased chain length of fatty acids are the major
modifications of cell membranes, other membrane-associated components
may well play important roles in adaptation to low temperatures
( Jagannadham et al., 1991; Ray et al., 1998 ). Studies of Antarctic psy-
chrotrophic bacteria in vitro have shown that carotenoids may have a func-
tion in buffering membrane fluidity ( Jagannadham et al., 1991 ).
4. BACTERIAL GLOBINS
The traditional view of the exclusive role of haemoglobin (Hb) as O 2
carrier in vertebrates is obsolete. The discovery of globin genes in prokaryotic