Biochemistry of Selenium Compounds and Pyridoxal 5'-phosphate enzymes

Abstract

Pyridoxal 5'-phosphate-dependent enzymes, selenocysteine lyase (SCL) and cysteine desulfurase, occur in a variety of organisms. SCL catalyzes the decomposition of L-selenocysteine to form L-alanine and an enzyme-bound cysteine perselenide (Cys-Se) intermediate. Cysteine desulfurase catalyzes the same type of reaction as SCL but acts on L-cysteine as a substrate to form an cysteine persulfide (Cys-SH) on the active site residue. We carried out comparative studies between SCL and cysteine desulfurase to investigate functional and mechanistic characteristics of the family of the enzymes. Spectrum analyses showed that L-cysteine enters into the active site pocket of SCL but can not form an external aldimine complex with PLP. The formation of an external aldimine complex between SCL and L-selenocysteine requires the active-site nucleophile Cys375. Thus, Cys375 of SCL plays a central role in the discrimination between selenium and sulfur in a substrate and its analog.