An Obligatory Intermediate Controls the Folding of the α-Subunit of Tryptophan Synthase, a TIM Barrel Protein

The proposed kinetic folding mechanism of the α-subunit of tryptophan synthase (αTS), a TIM barrel protein, displays multiple unfolded and intermediate forms which fold through four parallel pathways to reach the native state. To obtain insight into the secondary structure that stabilizes a set of late, highly populated kinetic intermediates, the refolding of urea-denatured αTS from Escherichia coli was monitored by pulse-quench hydrogen exchange mass spectrometry. Following dilution from 8 M urea, the protein was pulse-labeled with deuterium, quenched with acid and mass analyzed by electrospray ionization mass spectrometry (ESI-MS). Hydrogen bonds that form prior to the pulse of deuterium offer protection against exchange and, therefore, retain protons at the relevant amide bonds. Consistent with the proposed refolding model, an intermediate builds up rapidly and decays slowly over the first 100 seconds of folding. ESI-MS analysis of the peptic fragments derived from αTS mass-labeled and quenched after two seconds of refolding indicates that the pattern of protection of the backbone amide hydrogens in this transient intermediate is very similar to that observed previously for the equilibrium intermediate of αTS highly populated at 3 M urea. The protection observed in a contiguous set of β-strands and α-helices in the N terminus implies a significant role for this sub-domain in directing the folding of this TIM barrel protein.