Crystal structure of fully oxidized human thioredoxin

In addition to the active cysteines located at positions 32 and 35 in humans, mammalian cytosolic thioredoxin (TRX) possesses additional conserved cysteine residues at positions 62, 69, and 73. These non-canonical cysteine residues, that are distinct from prokaryotic TRX and also not found in mammalian mitochondrial TRX, have been implicated in biological functions regulating signal transduction pathways via their post-translational modifications. Here, we describe for the first time the structure of a fully oxidized TRX. The structure shows a non-active Cys62-Cys69 disulfide bond in addition to the active Cys32-Cys35 disulfide. The non-active disulfide switches the α3-helix of TRX, composed of residues Cys62 to Glu70, to a bulging loop and dramatically changes the environment of the TRX residues involved in the interaction with its reductase and other cellular substrates. This structural modification may have implications for a number of potential functions of TRX including the regulation of redox-dependent signaling pathways.