Structure and stability of a thioredoxin reductase from Sulfolobus solfataricus: a thermostable protein with two functions

Recent investigations have demonstrated that disulfide bridges may play a crucial role in the stabilization of proteins in hyperthermophilic organisms. A major role in the process of disulfide formation is played by ubiquitous proteins belonging to the thioredoxin superfamily, which includes thioredoxins (Trx), thioredoxin reductases (TrxR), and disulfide oxidases/isomerases (PDO/PDI). Here we report a characterization of the structure and stability of the TrxR (SsTrxRB3) isolated from the archaeon Sulfolobus solfataricus. This protein is particularly interesting since it is able to process different substrates (Trxs and PDO) and it is endowed with an additional NADH oxidase activity. The crystal structure of the wild-type enzyme, of its complex with NADP and of the C147A mutant provides interesting clues on the enzyme function. In contrast to what is observed for class II TrxRs, in the structure of the oxidized enzyme, the FAD binding site is occupied by a partially disordered NAD molecule. In the active site of the C147A mutant, which exhibits a marginal NADH oxidase activity, the FAD is canonically bound to the enzyme. Molecular modeling indicates that a FAD molecule can be accommodated in the site of the reduced SsTrxRB3. Depending on the oxidation state, SsTrxRB3 can bind a different cofactor in its active site. This peculiar feature has been related to its dual activity. Denaturation experiments followed by circular dichroism indicate that electrostatic interactions play an important role in the stabilization of this thermostable protein. The analysis of the enzyme 3D-structure has also provided insights into the bases of SsTrxRB3 stability.