Na⁺/K⁺ exchange switches the catalytic apparatus of potassium-dependent plant L-asparaginase

Three crystal structures of potassium-dependent plant L-asparaginase were solved in complexes with K⁺, with Na⁺ and with both cations. A novel alkali metal-binding loop Val111–Ser118 (the activation loop) changes its conformation upon K⁺/Na⁺ exchange, leading to a reconfiguration of three key residues, His117, Arg224 and Glu250 (the catalytic switch), to allow or prevent substrate binding in the active site of the enzyme.