Dissection of complex protein dynamics in human thioredoxin

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Abstract
We report our direct study of complex protein dynamics in human thioredoxin by dissecting into elementary processes and determining their relevant time scales. By combining site-directed mutagenesis with femtosecond spectroscopy, we have distinguished four partly time-overlapped dynamical processes at the active site of thioredoxin. Using intrinsic tryptophan as a molecular probe and from mutation studies, we ascertained the negligible contribution to solvation by protein sidechains and observed that the hydration dynamics at the active site occur in 0.47-0.67 and 10.8-13.2 ps. With reduced and oxidized states, we determined the electron-transfer quenching dynamics between excited tryptophan and a nearby disulfide bond in 10-17.5 ps for three mutants. A robust dynamical process in 95-114 ps, present in both redox states and all mutants regardless of neighboring charged, polar, and hydrophobic residues around the probe, is attributed to the charge transfer reaction with its adjacent peptide bond. Site-directed mutations also revealed the electronic quenching dynamics by an aspartate residue at a hydrogen bond distance in 275-615 ps. The local rotational dynamics determined by the measurement of anisotropy changes with time unraveled a relatively rigid local configuration but implies that the protein fluctuates on the time scale of longer than nanoseconds. These results elucidate the temporal evolution of hydrating water motions, electron-transfer reactions, and local protein fluctuations at the active site, and show continuously synergistic dynamics of biological function over wide time scales.