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The mechanism of cysteine oxygenation by cysteine dioxygenase enzymes.

We present here the first density functional theoretic study into the mechanism of cysteine dioxygenation by a model of cysteine dioxygenase enzymes. A large active site model containing the ligands bound to iron plus amino acid residues that are involved in hydrogen bonding interactions with the substrate is used. The reaction takes place via multi-state reactivity patterns on competing singlet, triplet, and quintet spin states, whereby the latter is the ground state in most complexes. Several new intermediates have been predicted, which have not been anticipated before. The dioxygen-bound complex is in a singlet spin ground state, and a state crossing to the quintet spin state leads to an FeOOS ring structure that splits into a cysteinyloxide radical that reorients and abstracts an electron from the iron center. In the final step, the oxoiron donates the oxygen atom to the substrate to produce cysteine sulfinic acid in a highly exothermic process. The rate-determining step is the initial step in the reaction mechanism on the quintet spin state surface.