Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:P30044 (antioxidant enzyme)
 8,037 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Thioredoxin reductase (TrxR) is an essential enzyme required for the efficient maintenance of the cellular redox homeostasis, particularly in cancer cells that are sensitive to reactive oxygen species. In mammals, distinct isozymes function in the cytosol and mitochondria. Through an intricate mechanism, these enzymes transfer reducing equivalents from NADPH to bound FAD and subsequently to an active-site disulfide. In mammalian TrxRs, the dithiol then reduces a mobile C-terminal selenocysteine-containing tetrapeptide of the opposing subunit of the dimer. Once activated, the C-terminal redox center reduces a disulfide bond within thioredoxin. In this report, we present the structural data on a mitochondrial TrxR, TrxR2 (also known as TR3 and TxnRd2). Mouse TrxR2, in which the essential selenocysteine residue had been replaced with cysteine, was isolated as a FAD-containing holoenzyme and crystallized (2.6 A; R = 22.2%; R(free) = 27.6%). The addition of NADPH to the TrxR2 crystals resulted in a color change, indicating reduction of the active-site disulfide and formation of a species presumed to be the flavin-thiolate charge transfer complex. Examination of the NADP(H)-bound model (3.0 A; R = 24.1%; R(free) = 31.2%) indicates that an active-site tyrosine residue must rotate from its initial position to stack against the nicotinamide ring of NADPH, which is juxtaposed to the isoalloxazine ring of FAD to facilitate hydride transfer. Detailed analysis of the structural data in conjunction with a model of the unusual C-terminal selenenylsulfide suggests molecular details of the reaction mechanism and highlights evolutionary adaptations among reductases.
 ...
PMID:Crystal structures of oxidized and reduced mitochondrial thioredoxin reductase provide molecular details of the reaction mechanism. 1621 27

Thioredoxin reductase (TR) and thioredoxin (Trx) define a major cellular redox system that maintains cysteine residues in numerous proteins in the reduced state. Both cytosolic (TR1 and Trx1) and mitochondrial (TR3 and Trx2) enzymes are essential in mammals, but the function of the mitochondrial system is less understood. In this study, we characterized subcellular localization of three TR3 forms that are generated by alternative first exon splicing and that differ in their N-terminal sequences. Only one of these forms resides in mitochondria, whereas the two other isoforms are cytosolic. Consistent with this finding, TR3 did not have catalytic preferences for mitochondrial Trx2 versus cytosolic Trx1, both of which could serve as TR3 substrates. Similarly, TR1 was equally active with Trx1, Trx2, or a bacterial Trx. We generated recombinant selenoprotein forms of TR1 and TR3 and found that these enzymes were inhibited by zinc, but not by calcium or cobalt ions. We further developed a proteomic method for identification of targets of TRs in mammalian cells utilizing affinity columns containing recombinant TR3 forms differing in C-terminal sequences. Using this procedure, we found that Trx1 was the major target of TR3 in both rat and mouse liver cytosol. The truncated form of TR3 lacking selenocysteine was particularly efficient in binding Trx1, consistent with the previously observed role of truncated TR1 in apoptosis. Overall, these data establish that the function of TR3 is not limited to its role in Trx2 reduction.
 ...
PMID:Characterization of alternative cytosolic forms and cellular targets of mouse mitochondrial thioredoxin reductase. 1677 13