Comparison of Backbone and Tryptophan Side-Chain Dynamics of Reduced and Oxidized Escherichia coli Thioredoxin Using ¹⁵N NMR Relaxation Measurements

The backbone and tryptophan side-chain dynamics of both the reduced and oxidized forms of uniformly ¹⁵N-labeled Escherichia coli thioredoxin have been characterized using inverse-detected two-dimensional ¹H-¹⁵N NMR spectroscopy. Longitudinal (T₁) and transverse (T₂)¹⁵N relaxation time constants and steady-state {¹H}-¹⁵N NOEs were measured for more than 90% of the protonated backbone nitrogen atoms and for the protonated indole nitrogen atoms of the two tryptophan residues. These data were analyzed by using a model free dynamics formalism to determine the generalized order parameter (S²), the effective correlation time for internal motions (τ_e), and ¹⁵N exchange broadening contributions (R_ex) for each residue, as well as the overall molecular rotational correlation time (τ_m). The reduced and oxidized forms exhibit almost identical dynamic behavior on the picosecond to nanosecond time scale. The W31 side chain is significantly more mobile than the W28 side chain, consistent with the positions of W31 on the protein surface and W28 buried in the hydrophobic core. Backbone regions which are significantly more mobile than the average include the N-terminus, which is constrained in the crystal structure of oxidized thioredoxin by specific contacts with a Cu²⁺ ion, the C-terminus, residues 20–22, which constitute a linker region between the first α-helix and the second β-strand, and residues 73–75 and 93–94, which are located adjacent to the active site. In contrast, on the microsecond to millisecond time scale, reduced thioredoxin exhibits considerable dynamic mobility in the residue 73–75 region, while oxidized thioredoxin exhibits no significant mobility in this region. The possible functional implications of the dynamics results are discussed.