Selenium utilization in thioredoxin and catalytic advantage provided by selenocysteine

Biochem Biophys Res Commun. 2015 Jun 12;461(4):648-52. doi: 10.1016/j.bbrc.2015.04.082. Epub 2015 Apr 23.

Abstract

Thioredoxin (Trx) is a major thiol-disulfide reductase that plays a role in many biological processes, including DNA replication and redox signaling. Although selenocysteine (Sec)-containing Trxs have been identified in certain bacteria, their enzymatic properties have not been characterized. In this study, we expressed a selenoprotein Trx from Treponema denticola, an oral spirochete, in Escherichia coli and characterized this selenoenzyme and its natural cysteine (Cys) homologue using E. coli Trx1 as a positive control. (75)Se metabolic labeling and mutation analyses showed that the SECIS (Sec insertion sequence) of T. denticola selenoprotein Trx is functional in the E. coli Sec insertion system with specific selenium incorporation into the Sec residue. The selenoprotein Trx exhibited approximately 10-fold higher catalytic activity than the Sec-to-Cys version and natural Cys homologue and E. coli Trx1, suggesting that Sec confers higher catalytic activity on this thiol-disulfide reductase. Kinetic analysis also showed that the selenoprotein Trx had a 30-fold higher Km than Cys-containing homologues, suggesting that this selenoenzyme is adapted to work efficiently with high concentrations of substrate. Collectively, the results of this study support the hypothesis that selenium utilization in oxidoreductase systems is primarily due to the catalytic advantage provided by the rare amino acid, Sec.

Keywords: Disulfide reductase; Selenoprotein; Thioredoxin; Treponema denticola.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Binding Sites
  • Catalysis
  • Enzyme Activation
  • Protein Binding
  • Selenium / chemistry*
  • Selenocysteine / chemistry*
  • Structure-Activity Relationship
  • Substrate Specificity
  • Thioredoxins / chemistry*
  • Treponema denticola / enzymology*

Substances

  • Selenocysteine
  • Thioredoxins
  • Selenium