Metal vs. chalcogen competition in the catalytic mechanism of cysteine dioxygenase

Why the cysteine dioxygenase (CDO) cannot catalyze the oxidation of selenocysteine (Sec) but that of cysteine (Cys) is still an open question. In order to solve this question, the CDO model complex, the active site of CDO, and their selenium-substituted complexes have been selected as the computational models in this work. The stepwise donation of electron density during the first two reaction steps has been explored. In the first step, the electron density-donor ability of Se to donate to Fe is stronger than that of S; in the second step, S has the better electron density-donor ability to donate to O(2) than Se. Under the influence, in the Cys-bound complexes, the change of the oxidation state for the Fe center is II -> III -> II, while the Fe center in the Sec-bound complexes remains in the II oxidation state throughout. Considering that the ferric-superoxo species is an active oxidant and exhibits high reactivity in such reaction, it is speculated that the valence change of the Fe center makes the Cys-bound complexes effectively catalyze the oxidation of Cys, while the Sec-bound complexes cannot catalyze the oxidation of Sec. The competition for donation of electron density determines the valence change and the reaction ability. (C) 2013 Elsevier Inc. All rights reserved.