Exploring hyperthermophilic proteins under pressure: theoretical aspects and experimental findings

Proteins from hyperthermophilic microorganisms are generally capable of withstanding temperatures close to, or even higher than the boiling point. As a rule, these proteins are strongly piezostable as well, although exceptions have been also reported. This observation has a theoretical relevance, as the understanding of the effects of pressure and temperature on protein stability is equally important to develop a comprehensive model for their thermodynamic stability. Nevertheless, the structural features justifying the correlation between heat resistance and pressure resistance are poorly understood. Actually, most reports do not exceed the phenomenological level. Only in the case of the small protein Sso7d from Sulfolobus solfataricus, characterisation of wild-type and some mutants showed that both properties are largely accounted for by a network of aromatic residues found in the hydrophobic core of the molecule. Current knowledge, however, does not allow to establish to what extent this finding may be generalised. In a biotechnological perspective, hyperthermophilic enzymes seem to be more suitable for bioprocesses at high pressure with respect to their mesophilic counterparts. Indeed, thanks to their higher resistance towards pressure and temperature, they may be exploited in a much broader range of working conditions for tuning activity and specificity. Furthermore, they are often activated by increasing pressure, although it cannot be established, to date, to what extent this is a common feature.