Reaction mechanism and molecular basis for selenium/sulfur discrimination of selenocysteine lyase

J Biol Chem. 2010 Apr 16;285(16):12133-9. doi: 10.1074/jbc.M109.084475. Epub 2010 Feb 17.


Selenocysteine lyase (SCL) catalyzes the pyridoxal 5'-phosphate-dependent removal of selenium from l-selenocysteine to yield l-alanine. The enzyme is proposed to function in the recycling of the micronutrient selenium from degraded selenoproteins containing selenocysteine residue as an essential component. The enzyme exhibits strict substrate specificity toward l-selenocysteine and no activity to its cognate l-cysteine. However, it remains unclear how the enzyme distinguishes between selenocysteine and cysteine. Here, we present mechanistic studies of selenocysteine lyase from rat. ESI-MS analysis of wild-type and C375A mutant SCL revealed that the catalytic reaction proceeds via the formation of an enzyme-bound selenopersulfide intermediate on the catalytically essential Cys-375 residue. UV-visible spectrum analysis and the crystal structure of SCL complexed with l-cysteine demonstrated that the enzyme reversibly forms a nonproductive adduct with l-cysteine. Cys-375 on the flexible loop directed l-selenocysteine, but not l-cysteine, to the correct position and orientation in the active site to initiate the catalytic reaction. These findings provide, for the first time, the basis for understanding how trace amounts of a selenium-containing substrate is distinguished from excessive amounts of its cognate sulfur-containing compound in a biological system.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Substitution
  • Animals
  • Base Sequence
  • Catalytic Domain / genetics
  • Conserved Sequence
  • Crystallography, X-Ray
  • Cysteine / chemistry
  • DNA Primers / genetics
  • In Vitro Techniques
  • Lyases / chemistry*
  • Lyases / genetics
  • Lyases / metabolism*
  • Models, Molecular
  • Mutagenesis, Site-Directed
  • Protein Conformation
  • Protein Multimerization
  • Rats
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / genetics
  • Recombinant Proteins / metabolism
  • Selenium / metabolism*
  • Spectrometry, Mass, Electrospray Ionization
  • Substrate Specificity
  • Sulfur / metabolism*


  • DNA Primers
  • Recombinant Proteins
  • Sulfur
  • Lyases
  • selenocysteine lyase
  • Selenium
  • Cysteine