Kinetic characterization of early intermediates in the folding of E. coli tryptophan-synthase beta 2 subunit

This report describes the use of fluorescence energy transfer between an intrinsic energy donor (tryptophan 177) and two chemically added acceptors to study intermediates in the folding of the beta 2 subunit of E. coli tryptophan-synthase. Two early folding steps are thus identified and characterized. One is very rapid (its rate constant at 12 degrees C is 0.02 sec-1) and corresponds to the folding of the N-terminal domain into a structure whose overall features approximate well those of the native domain. The second step is somewhat slower (its rate constant at 12 degrees C is 0.008 sec-1) and involves a conformational rearrangement of the N-terminal domain brought about by the interactions between the N- and C-terminal domains within a monomeric beta chain. This brings to five the number of intermediates which have been identified and ordered on the folding pathway of the dimeric beta 2 subunit.