Protein thermostability in extremophiles

Thermostability of a protein is a property which cannot be attributed to th e presence of a particular amino acid or to a post synthetic modification. Thermostability seems to be a property acquired by a protein through many s mall structural modifications obtained with the exchange of some amino acid s and the modulation of the canonical forces found in all proteins such as electrostatic (hydrogen bonds and ion-pairs) and hydrophobic interactions. Proteins produced by thermo and hyperthermophilic microorganisms, growing b etween 45 and 110 degrees C are in general more resistant to thermal and ch emical denaturation than their mesophilic counterparts. The observed struct ural resistance may reflect a restriction on the flexibility of these prote ins, which, while allowing them to be functionally competent at elevated te mperatures, renders them unusually rigid at mesophilic temperatures (10-45 degrees C). The increased rigidity at mesophilic temperatures may find a st ructural determinant in increased compactness. In thermophilic proteins a n umber of amino acids are often exchanged. These exchanges with some strateg ic placement of proline in beta-turns give rise to a stabilization of the p rotein. Mutagenesis experiments have confirmed this statement. From the com parative analysis of the X-ray structures available for several families of proteins, including at least one thermophilic structure in each case, it a ppears that thermal stabilization is accompanied by an increase in hydrogen bonds and salt bridges. Thermostability appears also related to a better p acking within buried regions. Despite these generalisations, no universal r ules can be found in these proteins to achieve thermostability. (C) Societe francaise de biochimie et biologie moleculaire / Elsevier, Paris.