HIGH-RESOLUTION SOLUTION STRUCTURES OF OXIDIZED AND REDUCED ESCHERICHIA-COLI THIOREDOXIN

Background: Thioredoxin participates in thiol-disulfide exchange react ions and both oxidized thioredoxin (disulfide form) and reduced thiore doxin (dithiol form) are found under physiological conditions. Previou s structural studies suggested that the mio forms were extremely simil ar, although significant functional and spectroscopic differences exis t. Wie therefore undertook high-resolution solution structural studies of the two forms of Escherichia coli thioredoxin in order to detect s ubtle conformational differences. Results: The solution structures of reduced and oxidized thioredoxin are extremely similar. Backbone struc ture is largely identical in the two forms, with slight differences in the region of the active site, which includes Cys32 and Cys35. The si de chain sulfur atom of Cys32 is tilted away from that of Cys35 in the reduced form of the protein to accommodate the increase in S-S distan ce that occurs upon reduction of the disulfide, but the (chi)1 angles of the two cysteines remain the same in the two ions. Conclusions: Onl y subtle conformational changes occur upon changing the oxidation stat e of the active site cysteines, including the positions of some side c hains and in hydrogen bonding patterns in the active site region. Func tional differences between the two forms are probably therefore relate d to differences in local conformational flexibility in and near the a ctive site loop.