FIG 2.

Catalytic cycle of CysK2 and related cysteine synthases. The α-amino group of the acceptor substrate OPS displaces the ε-amino group of the active-site lysine residue (Lys65 in CysK2), resulting in the formation of an external aldimine. Elimination of the OPS β-phosphate and deprotonation of the Cα atom lead to the formation of the reactive aminoacrylate intermediate, the result of the first half-reaction. In the second half-reaction, a nucleophilic attack by the sulfur donor and reprotonation of Cα occur, with subsequent release of the reaction product.