Evidence of an associative intermediate on the myoglobin refolding pathway

Time-resolved small-angle x-ray scattering using the stopped-flow method has been applied successfully to investigate the refolding of myoglobin. This is the only method to date that yields direct information on protein physical dimensions during the folding process. It has the potential to detect and probe important processes, such as protein compaction and association, on a millisecond time scale. Initial experiments were performed with horse myoglobin denatured in high concentrations of urea. The denatured protein was diluted rapidly into a buffer containing no urea or low concentrations of urea. The time-course of the forward-scattered intensity shows a decrease in amplitude which is clearly not engendered by the compaction of the protein, but does correspond well to a dimer dissociation process. Initial and final radii of gyration correspond well to a dimer and a monomer, respectively. Kratky plots of the initial and final states also support the transient dimerization model. The apparent dissociation rate constant was obtainable directly from the data. An association rate constant and an equilibrium constant could be estimated. The dimerizing intermediate is speculated to be a globular non-native state with an exposed hydrophobic surface.