Is Burst Hydrophobic Collapse Necessary for Protein Folding?

Folding of the lattice model of proteins is studied using Monte Carlo simulation. The amino acid sequence is designed to have a pronounced energy minimum for a given target (native) conformation. Our simulations reveal two possible scenarios. When the overall attraction between residues dominates, we find that folding to the native conformation is preceded by a rapid collapse into a burst intermediate which is a compact but structureless globule. Then, after a much longer time, an all-or-none transition from the globule to the native conformation occurs. In contrast, when the overall attraction is not strong, we do not observe a burst collapse stage. Instead, we find an all-or-none transition directly from the coil to the native conformation. Both scenarios yield comparable rates of folding. On the basis of these findings we discuss the role of intermediates in thermodynamics and kinetics of protein folding.