Active sites of thioredoxin reductases: Why selenoproteins?

Abstract

Selenium, an essential trace element for mammals, is incorporated into a selected class of selenoproteins as selenocysteine. All known isoenzymes of mammalian thioredoxin (Trx) reductases (TrxRs) employ selenium in the C-terminal redox center -Gly-Cys-Sec-Gly-COOH for reduction of Trx and other substrates, whereas the corresponding sequence in Drosophila melanogaster TrxR is -SerCys-Cys-Ser-COOH. Surprisingly, the catalytic competence of these orthologous enzymes is similar, whereas direct Sec-to-Cys substitution of mammalian TrxR, or other selenoenzymes, yields almost inactive enzyme. TrxRs are therefore ideal for studying the biology of selenocysteine by comparative enzymology. Here we show that the serine residues flanking the C-terminal Cys residues of Drosophila TrxRs are responsible for activating the cysteines to match the catalytic efficiency of a selenocysteine-cysteine pair as in mammalian TrxR, obviating the need for selenium. This finding suggests that the occurrence of selenoenzymes, which implies that the organism is selenium-dependent, is not necessarily associated with improved enzyme efficiency. Our data suggest that the selective advantage of selenoenzymes is a broader range of substrates and a broader range of microenvironmental conditions in which enzyme activity is possible.

Two Thioredoxin Reductases, trxr-1 and trxr-2, Have Differential Physiological Roles in Caenorhabditis elegans

“…These two mutants showed obvious different phenotypes in longevity, development, subcellular organelle activity, and gene expression, suggesting that those two TrxR genes have discrete functions in the nematode model system. Mitochondria are major ROS producers in eukaryotes and mitochondrial dysfunction that leads to overproduction of ROS has been postulated as one of the main causes for aging and related diseases (Gromer et al, 2003). Not trxr-1(jh143), but trxr-2(tm2047) and the double mutants of trxr-2(tm2047);trxr-1 (jh143) showed developmental defect and reduced life span under the environmental oxidative stress or at the high temperature, the conditions that generate excessive ROS.…”

Aminoacyl-tRNAs: setting the limits of the genetic code

“…Continuing progress in understanding the mechanism of selenocysteine insertion has now allowed for the first time a fairly complete description of the occurrences of selenoproteins in the living kingdom (Kryukov et al 2003;Castellano et al 2004), which may eventually explain why, even though selenoproteins are widespread, they are by no means ubiquitous. One factor in the rather idiosyncratic distribution of selenoproteins may be their broader substrate specificity and their efficiency in a wider range of microenvironmental conditions compared to equivalent proteins that instead contain cysteine in their active sites (Gromer et al 2003).…”

Effects of thioredoxin reductase-1 deletion on embryogenesis and transcriptome

“…Metazoan Txnrd enzymes generally contain the unusual amino acid selenocysteine (Sec) [16], which, together with the cysteine in the conserved C-terminal sequence Gly-Cys-SecGly, participates in redox activity [17][18][19]. Exceptions, for example in Drosophila, where two Cys residues, rather than a Cys and a Sec, are found at the C-terminal redox center [18], demonstrate that the Sec residue is not essential.…”

“…Exceptions, for example in Drosophila, where two Cys residues, rather than a Cys and a Sec, are found at the C-terminal redox center [18], demonstrate that the Sec residue is not essential. Indeed, recombinant human Txnrd1 in which the Sec is replaced with Cys, retains some reductase activity [20].…”