title: Thermal destruction processes in proteins involving cystine residues.
abstract: To ascertain the upper limit of protein thermostability and to evaluate the introduction of additional -S-S- bridges for the enhancement of protein thermostability by site-directed mutagenesis, the stability of cystine residues at 100 degrees C has been investigated in a number of unrelated proteins. All proteins studied (more than a dozen) undergo heat-induced beta-elimination of cystine residues in the pH range from 4 to 8. The time courses of this process closely follow first-order kinetics indicating that the stability of a cystine residue is not significantly affected by its position in the polypeptide chain. Furthermore, the first-order rate constants of beta-elimination of disulfide bonds at 100 degrees C are remarkably similar for different proteins--0.8 +/- 0.3 h-1 and 0.06 +/- 0.02 h-1 at pH 8.0 and 6.0, respectively. Thus, this process is relatively independent of both the primary structure and the elements of higher structures remaining in proteins in 100 degrees C. beta-Elimination of disulfides produces free thiols that cause yet another deleterious reaction in proteins--heat-induced disulfide interchange. This reshuffling of -S-S-bonds, which is much faster than beta-elimination, has also been quantitatively characterized.
mesh_terms: D003553:Cystine / Q000378:metabolism*; D004220:Disulfides / Q000378:metabolism; D004355:Drug Stability; D005978:Glutathione / Q000378:metabolism; D019791:Guanidine; D006146:Guanidines; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000378:metabolism; D007700:Kinetics; D009113:Muramidase / Q000378:metabolism; D011489:Protein Denaturation; D011506:Proteins / Q000378:metabolism*; D013438:Sulfhydryl Compounds / Q000378:metabolism; D014362:Trypsinogen / Q000378:metabolism
chemical_list: D004220:Disulfides; D006146:Guanidines; D007328:Insulin; D011506:Proteins; D013438:Sulfhydryl Compounds; D003553:Cystine; D014362:Trypsinogen; D009113:Muramidase; D005978:Glutathione; D019791:Guanidine