Structure and mechanism of mammalian thioredoxin reductase: The active site is a redox-active selenolthiol/selenenylsulfide formed from the conserved cysteine-selenocysteine sequence

摘要

Mammalian thioredoxin reductases (TrxR) are homodimers, homologous to glutathione reductase (GR), with an essential selenocysteine (SeCys) residue in an extension containing the conserved C-terminal sequence -Gly-Cys-SeCys-Gly. In the oxidized enzyme, we demonstrated two nonflavin redox centers by chemical modification and peptide sequencing: one was a disulfide within the sequence -Cys⁵⁹-Val-Asn-Val-Gly-Cys⁶⁴, identical to the active site of GR; the other was a selenenylsulfide formed from Cys⁴⁹⁷-SeCys⁴⁹⁸ and confirmed by mass spectrometry. In the NADPH reduced enzyme, these centers were present as a dithiol and a selenolthiol, respectively. Based on the structure of GR, we propose that in TrxR, the C-terminal Cys⁴⁹⁷-SeCys⁴⁹⁸ residues of one monomer are adjacent to the Cys⁵⁹ and Cys⁶⁴ residues of the second monomer. The reductive half-reaction of TrxR is similar to that of GR followed by exchange from the nascent Cys⁵⁹ and Cys⁶⁴ dithiol to the selenenylsulfide of the other subunit to generate the active-site selenolthiol. Characterization of recombinant mutant rat TrxR with SeCys⁴⁹⁸ replaced by Cys having a 100-fold lower kcat for Trx reduction revealed the C-terminal redox center was present as a dithiol when the Cys⁵⁹-Cys⁶⁴ was a disulfide, demonstrating that the selenium atom with its larger radius is critical for formation of the unique selenenylsulfide. Spectroscopic redox titrations with dithionite or NADPH were consistent with the structure model. Mechanisms of TrxR in reduction of Trx and hydroperoxides have been postulated and are compatible with known enzyme activities and the effects of inhibitors, like goldthioglucose and 1-chloro-2,4-dinitrobenzene.