The folding characteristics of tryptophanase from Escherichia coli

The unfolding and refolding characteristics of Escherichia coli tryptophanase (tryptophan indole-lyase) [EC 4.1.99.1] in guanidine hydrochloride were studied. Tryptophanase unfolded by first dissociating its coenzyme, pyridoxal 5'-phosphate, from the active site. This dissociation caused a significant destabilization of structure, and global unfolding of the protein followed. During this global unfolding step, an intermediate was formed which had a strong tendency to aggregate irreversibly, as detected by light scattering experiments. Tryptophanase was unable to refold quantitatively after unfolding in 4 M guanidine hydrochloride. The low refolding yield was due to non-specific aggregation which occurs during refolding. Various conditions which limited this aggregation were probed, and it was found that by initiating the refolding reaction at low temperature, the aggregation of tryptophanase folding intermediates during the reaction could be avoided to a certain extent, and the refolding yield improved.