A general method for the prediction of the three dimensional structure and folding pathway of globular proteins: Application to designed helical proteins

Starting from amino acid sequence alone, a general approach for simulating folding into the molten globule or rigid, native state depending on sequence is described. In particular, the 3D folds of two simple designed proteins have been predicted using a Monte Carlo folding algorithm. The model employs a very flexible hybrid lattice representation of the protein conformation, and fast lattice dynamics. A full rotamer library for side group conformations, and potentials of mean force of short and long range interactions have been extracted from the statistics of a high resolution set of nonhomologous, 3D structures of globular proteins. The simulated folding process starts from an arbitrary random conformation and relatively rapidly assembles a well defined four helix bundle. The very cooperative folding of the model systems is facilitated by the proper definition of the model protein hydrogen bond network, and multibody interactions of the side groups. The two sequences studied exhibit very different behavior. The first one, in excellent agreement with experiment, folds to a thermodynamically very stable four helix bundle that has all the properties postulated for the molten globule state. The second protein, having a more heterogeneous sequence, at lower temperature undergoes a transition from the molten globule state to the unique native state exhibiting a fixed pattern of side group packing. This marks the first time that the ability to predict a molten globule or a unique native state from sequence alone has been achieved. The implications for the general solution of the protein folding problem are briefly discussed.