Life at low temperatures: is disorder the driving force?

The thermodynamic characterization of various biological systems from psychrophiles points to a larger entropic contribution when compared to the corresponding mesophilic or (hyper) thermophilic counterparts, either at the level of the macromolecules (thermodynamic and kinetic stabilities) or of their function (ligand binding, catalytic activity). It is suggested here that in an environment characterized by a low heat content (enthalpy) and at temperatures that strongly slowdown molecular motions, the cold-adapted biological systems rely on a larger disorder to maintain macromolecular dynamics and function. Such pre-eminent involvement of entropy is observed in the experimental results and, from a macroscopic point of view, is also reflected for instance by the steric hindrances introduced by cis-unsaturated and branched lipids to maintain membrane fluidity, by the loose conformation of psychrophilic proteins or by the local destabilization of tRNA by dihydrouridine in psychrophilic bacteria.