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not at 25°C is a function of the adaptive changes of those molecules. Hochachka and
Somero (1984) suggested three central biochemical aims for cold adaptation within a cell:
1. Preservation of the structural integrity of macromolecules and assemblies such as
membranes.
2. Provision of the necessary energy, metabolites and intracellular environment for
metabolism.
3. Regulation of the metabolism in response to the changing needs of the organism.
1.5.3 -
Cold adaptation in proteins
The following sections outline the major adaptations in cellular proteins which
permit biochemical function at cold temperatures. Anti-freeze proteins will be considered
in detail because of their central role in this investigation.
1.5.3.1 -
Structural adaptations in psychrophilic enzymes
Protein stability depends on the combined effect of many weak non-covalent
interactions between the polypeptide backbone and between the amino-acid side chains
(Russell, 1990). The interactions include hydrogen bonds, salt bridges, van der Waals
interactions and hydrophobic bonding. Proteins inactivate and eventually destabilize due
to loss of conformational entropy between these forces (Mathews, 1987; Gerday et al.,
1997). Metabolic enzymes associated with psychrophilic organisms are heat labile
(Burton & Morita, 1963). However, proteins are shown to have thermal optima far in
excess of the upper growth-viable temperature limit of the organisms themselves (Morita,
1975). The ability of these enzymes to function at temperatures close to or at 0°C appears
to be inherent in their structure and not due to the action of other
factors (Russell &
Hamamoto, 1998). For example, studies of specific Vibrio strains have shown a series of
amylase enzymes which show deactivation at temperatures in excess of 30°C, optimal
activity at 25°C, whilst still providing 10-15% activity at 0°C (Hamamoto & Horikoshi,
1991).
Chessa et al. (2000) suggest that `cold adapted enzymes' are usually characterised
by a higher specific activity at low temperature when compared to their mesophilic
counterparts, and also by a lower stability against temperature and denaturing agents.
Russell & Hamamoto (1998) suggest that `enzymes from psychrophilic microorganisms
should be adapted to remain flexible at low temperatures in order to carry out the
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