It should be mentioned that however stable a protein can be made by stabilizing the folded state, the ultimate limit of protein stability must come from covalent degradation. At high temperatures (80 - 120 °C) Asn and Gln are susceptible to deamidation, Asp-Xaa peptide bonds are susceptible to hydrolysis, disulphides bonds rupture, and Xaa-Pro peptide bonds undergo cis-trans isomerisation (where Xaa is any amino acid).
Interestingly, the upper limit for protein chemical thermostability may be higher than one would calculate from studies involving model mesophilic enzymes. Apart from the trivial response of just avoiding these residues (disulphides are absent and Asn/Gln content is reduced in hyper-thermophiles), it is observed that the deamidation rate of Gln and Asn residues is reduced, presumably by steric constraint, in fully folded hyper-thermophilic structures (Vielle & Zeikus, 1996 and references therein).
Kinetic Stability Evolution and StabilityBeginning