Active sites of thioredoxin reductases: Why selenoproteins?

doi:10.1073/pnas.2134510100

Active sites of thioredoxin reductases: Why selenoproteins?

^*Biochemie-Zentrum Heidelberg, Heidelberg University, D-69120 Heidelberg, Germany; ^‡Medical Nobel Institute for Biochemistry, Department of Medical Biochemistry and Biophysics, Karolinska Institute, SE-17177 Stockholm, Sweden; and ^§Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, MI 48109-0606

Edited by Stephen J. Benkovic, Pennsylvania State University, University Park, PA, and approved August 29, 2003 (received for review July 18, 2003)

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 -Ser-Cys-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.

Footnotes

↵ † To whom correspondence should be addressed at: Biochemie-Zentrum Heidelberg, Heidelberg University, Im Neuenheimer Feld 504, D-69120 Heidelberg, Germany. E-mail: stephan{at}gromer-online.de.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: Trx, thioredoxin; TrxR, Trx reductase; DmTrxR, Drosophila melanogaster TrxR; Sec, selenocysteine; U, one-letter code for Sec.
↵ ¶ This assay is essentially a Trx assay, yet, due to the recycling of oxidized Trx by means of the nonenzymatic reduction of glutathione disulfide, a constant level of Trx is maintained.
↵ ∥ Note that mammalian TrxR and DmTrxR share only ≈55% identity. Thus, >200 residues (45%) are different. Attributing the difference in enzymatic activity, as done earlier, solely to the exchange of one amino acid (Sec ⇌ Cys) is therefore an obvious oversimplification. Our results clearly support this point.