Spectroscopic, immunochemical, and thermodynamic properties of carboxymethyl(Cys6, Cys127)-hen egg white lysozyme

A three-disulfide form of hen egg white lysozyme with Cys6 and Cys127 blocked by carboxymethyl groups was prepared, purified, and characterized for eventual use in protein folding experiments. Trypsin digestion followed by proline-specific endopeptidase digestion facilitated the unambiguous assignment of the disulfide bond pairings and the modified residues in this derivative. 3SS-lysozyme demonstrated nearly full enzymatic activity at its pH optimum, pH 5.5. The 3SS-lysozyme derivative and unmodified lysozyme were shown to be identical by CD spectroscopy at pH 3.6. Immunochemical binding assays demonstrated that the conformation of lysozyme was perturbed predominantly only locally by breaking and blocking the disulfide bond between Cys6 and Cys127. Both 3SS-lysozyme and unmodified lysozyme exhibited reversible thermally induced transitions at pH 2.0, but the Tm of 3SS-lysozyme, 18.9 degrees C, was found to be 34 degrees lower than that of native lysozyme under the same conditions. The conformational chemical potential of the denatured form of unmodified lysozyme was determined from the transition curves to be approximately 6.7 kcal/mol higher than that of the denatured form of 3SS-lysozyme, at pH 2.0 and 35 degrees C, if the conformational chemical potential for the folded forms of both 3SS-lysozyme and unmodified lysozyme is arbitrarily assumed to be 0.0 kcal/mol. A calculation of the increase in the theoretical loop entropy of denatured 3SS-lysozyme resulting from the cleavage of the Cys6-Cys127 disulfide bond, however, yielded a value of only 5.4 kcal/mol for the difference in conformational chemical potential. This suggests that, in addition to the entropic component, there is also an enthalpic contribution to the difference in the conformational chemical potential corresponding to approximately 1.3 kcal/mol. Thus, it is concluded that the reduction and blocking of the disulfide bond between Cys6 and Cys127 destabilizes 3SS-lysozyme relative to unmodified lysozyme predominantly by stabilizing the denatured conformation by increasing its chain entropy.