In solution, a protein is in an equilibrate between the folded and the unfolded state. Irreversible chemical reactions are usually coupled with the unfolded state of a protein. Therfore, I proposed two strategy : 1, depression of the shift from the folded state to the unfolded state in a protein, 2, depression of irreversible reactions in the unfolded state of a protein. In this study, we used hen lysozyme as a model protein and obtained the following results.
As for 1, the denaturation temperatures (Tm) of lysozyme were measured using differential scanning calorimetry at pH 3 in the presence of several additives such as glycerol, glucose, galactose, mannose, trehalose, sucrose and sarcosine. In the presence of these additives, the Tm of lysozyme increased. Especially, addition of 1.5 M trehalose increased the Tm of lysozyme by 12ﾟC.Therefore, I found that the addition of these additives depressed the shift from the folded state to the unfolded state in lysozyme.
As for 2, inactivation experiments of lysozyme against heating in the presence of sucrose, trehalose and sarcosine were carried out. Inactivations of lysozyme at 1000ﾟC were observed even in the presence of these additives but the extents were less than those in the absence of additives. The reason why inactivations of lysozyme against heating were depressed in the presence of these additives was found to depend that these additives induced the unfolded state of lysozyme to be compact leading to the depression of unfavorable intermolecular interactions between lysozyme molecules. Moreover, these additives were also found to depress the irreversible chemical reactions such as deamidations or racemizations of amino acid residues in lysozyme. Therefore, I showed that these additives also play a role on the depression of the irreversible chemical reactions in a protein.