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] |