Refolding of lysozyme by quasistatic and direct dilution reaction paths: A first-order-like state transition

A first-order-like state transition model is considered to be a global reaction mechanism to directly folded proteins from an unfolded state to its native form. In order to verify the general applicability of this mechanism, we used lysozyme as a model protein. It was fully unfolded by 4.5 M urea, 0.1 M dithiothreitol (DTT) in pH 3 and refolded to its native form by way of an overcritical reaction path (a quasistatic process) or directly crossing transition boundary path (a directly dilution process). In addition to the two states coexisting in the direct folding path, lyzosyme might be trapped in a glassy state. However, it can escape from the glassy state by concentration twice. This indicates the existence of a state transition line or boundary in the direct folding reaction. However, lysozyme can continuously fold from unfolded to native by an overcritical reaction path. During the overcritical path, four stable folding intermediates and native lysozyme were obtained. The secondary structures, particle size distributions, thermal stabilities, and oxidation state of disulfide bonds of folding intermediates were analyzed by circular dichroism spectra, dynamic light scattering, differential scanning calorimetry, and Raman spectra, respectively. According to the data, the intermediates of both the overcritical reaction and the direct crossing transition boundary paths can be described by a common concept pertaining to a model that undergoes collapse, sequential, and first-order-like state transition. This indicated that protein folding by way of different reaction paths might follow a similar folding mechanism-i.e., a mechanism of overcritical folding of intermediates. A protein folding reaction diagram is postulated and discussed. In spite of a global interaction mechanism the alpha-helix is formed prior to the beta-sheet, which may indicate that protein folding is initiated by local interactions.