Conformational changes in chemically modified Escherichia coli thioredoxin monitored by H/D exchange and electrospray ionization mass spectrometry

Hydrogen/deuterium (H/D) exchange in combination with electrospray ionization mass spectrometry and near-ultraviolet (UV) circular dichroism (CD) was used to study the conformational properties and thermal unfolding of Escherichia coli thioredoxin and its Cys32-alkylated derivatives in 1% acetic acid (pH 2.7). Thermal unfolding of oxidized (Oxi) and reduced (Red) -thioredoxin (TRX) and Cys-32-ethylglutathionyl (GS-ethyl-TRX) and Cys-32-ethylcysteinyl (Cys-ethyl-TRX), which are derivatives of Red-TRX, follow apparent EX1 kinetics as charge-state envelopes, H/D mass spectral exchange profiles, and near-UV CD appear to support a two-state folding/unfolding model. Minor mass peaks in the H/D exchange profiles and nonsuperimposable MS- and CD-derived melting curves, however, suggest the participation of unfolding intermediates leading to the conclusion that the two-state model is an oversimplification of the process. The relative stabilities as measured by melting temperatures by both CD and mass spectral charge states are, Oxi-TRX, GS-ethyl-TRX, Cys-ethyl-TRX, and Red-TRX. The introduction of the Cys-32-ethylglutathionyl Group provides extra stabilization that results from additional hydrogen bonding interactions between the ethylglutathionyl group and the protein. Near-UV CD data show that the local environment near the active site is perturbed to almost an identical degree regardless of whether alkylation at Cys-32 is by the ethylglutathionyl group, or the smaller, nonhydrogen-bonding ethylcysteinyl group. Mass spectral data, however, indicate a tighter structure for GS-ethyl-TRX.