Structural diversity in the recognition between reduced thioredoxin and its oxidized enzyme partners

Abstract Thioredoxins (Trx) are ubiquitous proteins that are conserved in all living organisms from archaea to humans. These small proteins display various cellular roles, including functioning as reductases in redox processes. All Trxs share a similar, characteristic three-dimensional fold with the Cys-Pro-Gly-Cys motif that contains both the catalytic and the resolving cysteine (Cys) on the surface of the protein. Reaction of reduced Trx with its oxidized protein partners leads to formation of a transient interdisulfide intermediate. However, the short lifetime of this species hinders the characterization of the stabilizing interactions that occur between the partners. In this short review, the three-dimensional structures of four artificial covalent Trx-protein partner complexes are analyzed. The data show that interprotein stabilization is mainly due to hydrophobic contacts and main-chain hydrogen bonds but that no common recognition motif between Trx and its protein partners can be identified. In two cases, formation of the Trx-partner complex is accompanied by a significant conformational change of the protein target, although in no case does the conformation of Trx change significantly. The absence of a common recognition motif supports the idea that it is difficult to predict with confidence putative oxidized protein substrates of Trx using only soft docking and molecular simulation methods. Instead, biochemical methods including proteomic approaches remain the primary tools to identify novel protein substrates of Trx. The generality and relevance of methods used to identify which of the two Cys of the disulfide-oxidized protein partner forms the transient interdisulfide intermediate with Trx are also discussed.