Sequences of putative soluble proteins from complete genomes of eight thermophiles and 12 mesophiles were analyzed to gain insight into determinants of protein thermostability. The predator algorithm was used to assign secondary structures to each protein sequence. Based on simple statistical tests, a set of stabilizing factors was identified. These include reduced protein size, increases in number of residues involved in hydrogen bonding, β-strand content and helix stabilization through ion pairs. There are also significant increases in the relative amounts of charged and hydrophobic β-branched amino acids and decreases in uncharged polar amino acids in proteins from thermophiles relative to mesophilic organisms. Factors such as the relative proportion of residues in loops, proline and glycine content and helix capping do not appear to be important.
