Evolutionary Conservation of the Copper-Dependent Thiol Oxidase Activity of Selenium-Binding Proteins

Human selenium-binding protein 1 (SELENBP1) is a methanethiol oxidase (MTO), converting methanethiol (MT) to hydrogen sulfide (H₂S), hydrogen peroxide (H₂O₂) and formaldehyde (HCHO). SELENBP1 has orthologs in all domains of life. RdMTO, an orthologous enzyme recently identified in the marine bacterium Roseobacter denitrificans, was postulated to require the cysteine residue closest to its C-terminus, Cys448, for MT binding and to oxidize MT to sulfane sulfur (S⁰) rather than to H₂S. SELENBP1 and RdMTO exhibit ~53% sequence identity, with Cys448 (numbered Cys466 in SELENBP1) and amino acids required for copper binding being conserved. Therefore, we here compared the MTO activity of recombinant SELENBP1 and RdMTO. We found that SELENBP1, like RdMTO, converts MT as well as structurally related alkyl thiols to form H₂S, H₂O₂ and, in the case of MT, HCHO in a strictly copper-dependent manner. MTO activity of both proteins was lowered but not abrogated upon mutation of their respective C-terminal cysteine residue. Thus, the catalytic mechanism of selenium-binding proteins that act as copper-dependent MTOs appears to be evolutionarily conserved from bacteria to humans. Presumably, this enzyme is an early evolutionary invention of prokaryotes, in order to cope with toxic thiols in their environment.