SINGLE VERSUS PARALLEL PATHWAYS OF PROTEIN-FOLDING AND FRACTIONAL FORMATION OF STRUCTURE IN THE TRANSITION-STATE

Protein engineering and kinetic experiments indicate that some regions of proteins have partially formed structure in the transition state f or protein folding. A crucial question is whether there is a genuine s ingle transition state that has interactions that are weakened in thos e regions or there are parallel pathways involving many transition sta tes, some with the interactions fully formed and others with the struc tural elements fully unfolded. We describe a kinetic test to distingui sh between these possibilities. The kinetics rule out those mechanisms that involve a mixture of fully formed or fully unfolded structures f or regions of the barley chymotrypsin inhibitor 2 and barnase, and so those regions are genuinely only partially folded in the transition st ate. The implications for modeling of protein folding pathways are dis cussed.