Kirk C. Aune

Kinetics of unfolding and refolding of proteins: III. Results for lysozyme

Abstract The kinetics of the reversible denaturation of lysozyme by guanidine hydrochloride have been studied over a wide range of pH and denaturant concentration. The results contrast sharply with those observed for cytochrome c . The interconversion of native (N) and denatured (D) states is strictly first-order in both directions under most conditions, showing that no intermediate forms of any kind accumulate to a significant extent during the reaction. At one pH (pH 2.6) experiments were extended to extreme denaturant concentrations, and under these conditions kinetic intermediates were observed. Analysis by the procedures of the first paper of this series Ikai & Tanford 1973 showed that the principal intermediate observed at low denaturant concentration must be different from that observed at high denaturant concentration, and that both must be on the direct pathway from N to D. This suggests that the over-all reaction mechanism is of the form and this mechanism is able to account for all of the observed results. The intermediate X 1 which accumulates transiently in the renaturation reaction at low concentrations of guanidine hydrochloride is spectrally very similar to the native state, i.e. it is probably a highly ordered state, but most of the interactions between surface acidic and basic groups, which are responsible for the anomalous titration behavior of native lysozyme, do not yet exist in this state. It is probable that the difference between the kinetics of refolding of lysozyme and cytochrome c may be ascribed to the existence of disulfide cross-links in lysozyme, which were intact in these experiments. These cross-links greatly limit the possible pathways for folding and thus make it much less likely that incorrectly folded states on dead-end pathways are readily accessible.