Halophilic adaptations of proteins

Halophilic organisms thrive in a wide variety of environments, ranging from mildly saline to saturated brine. Extremely saline conditions are very harsh to proteins and can result in problems such as loss of flexibility, which leads to loss of catalytic activity, or aggregation and precipitation due to salting-out effects. Halophilic proteins have therefore acquired specific adaptations to prevent such problems, and that is clearly reflected in their amino acid composition. As a consequence, halophilic proteins do not function properly in low salt conditions, and many even unfold and denature without sufficient amounts of salt. One of the most obvious adaptations found in halophilic proteins is an excess of acidic residues, while other adaptations include a low lysine content and an increase in small hydrophobic amino acids, the latter of which is at the expense of large hydrophobic residues. Different functions have been suggested for the role of the acidic residues. They are, most likely, important for binding of essential water molecules, but may play also a critical role in binding of salt or prevention of aggregation. The other changes in amino acid composition in halophilic proteins are probably important for the reduction of hydrophobicity, in order to prevent aggregation and maintain flexibility in high salt concentrations. Structural features that may be important for halophilic adaptation are also found, such as an increased number of salt-bridges. That is, however, not used by all halophilic proteins, indicating that different strategies have evolved to cope with very saline conditions.