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Table 1 An overview of suggested features for internal thermostability, selected from structural studies of homologues, along with some development approaches to introduce thermostability, and development of thermostable proteins.

From: Potential and utilization of thermophiles and thermostable enzymes in biorefining

Proposed features for internal stabilisation in thermostable proteins Contributing factors References
Helix stabilisation Low frequency of Cβ-branched amino acids (e.g. Val, Ile, Thr). Specific amino acids at helical ends (e.g. Pro) [16, 17]
Stabilising interactions in folded protein Disulfide bridges; Hydrogen bonds; Hydrophobic interactions; Aromatic interactions; Ion-pair networks (charged residues); Docking of loose ends [18–24]
Stabilising interactions between domains/subunits Oligomer formation via e.g. ion pair networks [17, 19, 25]
Dense packing Increase core hydrophobicit;, Fill cavities. Not a generally applicable feature as shown by Karshikoff & Ladenstein [21] [19]
Stable surface-exposed amino acids Low level of surface amino acids prone to deamidation (e.g. Gln, Asn) or oxidative degradation (e.g. Cys, Met) [17, 24]
Approaches to introduce internal thermostability in mesophilic proteins Engineering methodology  
Reducing length of or stabilising surface loops and turns Structure-based site directed mutagenesis. Promising results reported for: Loop deletions; Proline-stabilisation of loops; Docking of loose ends. [17, 24]
Introduce stabilising interactions Structure-based site directed mutagenesis. Success reported for introduction of ion-pairs, disulphide bridges, while core packing and helix stabilisation usually do not result in high stability gain. [17, 24]
Activity screen of diversified library at desired temperature Directed evolution and other random methods utilized successfully in several cases [24, 26]
Approaches to develop thermostable proteins   
Diversifying specificity (Structure-based) directed evolution by e.g. oligonucleotide randomisation in active site region, successfully utilized [27]
Improving activity at selected pH values Directed evolution [28]
Broadening temperature range for activity by introducing flexibility in active site region (Structure-based) directed evolution Patent by Diversa. Can be made e.g. by oligonucleotide randomisation in active site region. Saturation mutagenesis at selected positions also used. [29]
Substitution of surface-exposed amino acids to achieve long term stability Site directed or saturation mutagenesis at selected positions to reduce Gln, Asn, Cys, Met, suggested [16, 17]