A kinetic study of the competition between renaturation and aggregation during the refolding of denatured-reduced egg white lysozyme

Abstract

The recovery of proteins following denaturation is optimal at low protein concentrations. The decrease in yield at high concentrations has been explained by the kinetic competition of folding and "wrong aggregation". In the present study, the renaturation-reoxidation of hen and turkey egg white lysozyme was used as a model system to analyze the committed step in aggregate formation. The yield of renatured protein for both enzymes decreased with increasing concentration in the folding process. In addition, the yield decreased with increasing concentrations of the enzyme in the denatured state (i.e., prior to its dilution in the renaturation buffer). The kinetics of renaturation of turkey lysozyme were shown to be very similar to those of hen lysozyme, with a half-time of about 4.5 min at 20 degrees C. The rate of formation of molecular species that lead to formation of aggregates (and therefore fail to renature) was shown to be rapid. Most of the reaction occurred in less than 5 s after the transfer to renaturation buffer, and after 1 min, the reaction was essentially completed. Yet, by observing the effects of the delayed addition of denatured hen lysozyme to refolding turkey lysozyme, it was shown that folding intermediates become resistant to aggregation only much more slowly, with kinetics indistinguishable from those observed for the appearance of native molecules. The interactions leading to the formation of aggregates were nonspecific and do not involve disulfide bonds. These observations are discussed in terms of possible kinetic and structural aspects of the folding pathway.