Why does ribonuclease irreversibly inactivate at high temperatures?

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Abstract
The mechanism of irreversible thermoinactivation of bovine pancreatic ribonuclease A in the pH range relevant to enzymatic catalysis has been elucidated. At 90.degree. C and pH 4, the enzyme inactivation is caused by hydrolysis of peptide bonds at aspartic acid residues (the main process) and deamidation of asparagine and/or glutamine residues. AT 90.degree. C and neutral pH (pH 6 and 8), the enzyme inactivation is caused by a combination of disulfide interchange (the main process), .beta.-elimination of cystine residues, and deamidation of asparagine and/or glutamine residues. These four processes appear to demarcate the upper limit of thermostability of enzymes.