Dynamism of activated-selenium species: Structural biological analysis of mechanism of biosynthesis of selenium-containing proteins

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Selenocysteine lyase (SCL) is an enzyme which specifically catalyzes the decomposition of selenocysteine to form alanine and selenium. The enzyme is proposed to be involved in an initial step of the biosynthesis of selenoproteins containing selenocysteine residues because it has an activity to deliver selenide for selenophosphate synthetase. Escherichia coli contains its homologous counterpart enzyme, cysteine desulfurase (CSD). Unlike SCL, CSD acts on both cysteine and selenocysteine as substrates. A conserved cysteine residue important for the decomposition of cysteine has been demonstrated to be not essential for the decomposition of selenocyteine. Mouse SCL has eight cysteine residues. We constructed mutant enzymes C101S, C129S, C177S, C304S, C346, C364S, C375, and C390S by site-directed mutagenesis. The mutant enzymes were purifued and characterized. The C375S and C375A mutants lost the selenocysteine lyase activity. Therefore, Cys375 is essential for the selenocysteine lyase activity of the enzyme, indicating that the mechanism of selenocysteine lyase reaction by SCL is different from that catalyzed by CSD.