Institute for Chemical Research, Kyoto University^* Department of Chemistry, Graduate School of Science, Osaka City University, Osaka, Japan^**
○Rie Omi^* Suguru Kurokawa^* Hisaaki Mihara^* Tatsuo Kurihara^* Nobuyoshi Esaki^* Ikuko Miyahara^** Ken Hirotsu^**

Selenocysteine lyase (SCL), which is a pyridoxal 5'-phosphate (PLP) dependent enzyme, catalyzes the beta-elimination of L-selenocysteine to yield L-alanine and selenium. The reaction mechanism involves formation of an enzyme-bound cysteine-perselenide intermediate (Cys-S-Se^-) and the conserved cysteine residue (Cys375) is essential for catalysis. SCL is specific for L-selenocysteine and has no activity for L-cysteine, therefore it is known as the key enzyme in the specific selenium-delivery pathway for selenoprotein synthesis. In order to elucidate the strict discrimination between selenium and sulfur by SCL, we have determined the three-dimensional structures of native SCL and L-cysteine complex at 1.8 Å and 1.9 Å, respectively.
Overall and active site structure of SCL is similar to those of cysteine desulfurases which catalyze the same type of reaction as SCL but act on both cysteine and selenocysteine. Why SCL does not show activity on cysteine? In the case of SCL/L-cysteine complex structure, not the amino group of L-cysteine but the thiol group is located close to the C4A of PLP in the active site. This indicates that L-cysteine is incorporated into the active site but can not make an external aldimine with PLP. In addition, spectrum analysis shows that Cys375 plays an important role in the recognition of the L-selenocysteine. The mechanism for discrimination between selenium and sulfur by Cys375 will be presented on the basis of the structural comparison between SCL and cysteine desulfulase.