The organization of divalent cations in the active site of cadmium Escherichia coli fructose-1,6-bisphosphate aldolase

Previously determined crystal structures of the zinc enzyme Escherichia coli class II fructose-1,6-bisphosphate aldolase display good agreement for the protein structure but a differing metal-ion organization in the active site. The structure of the enzyme with Cd²⁺ in place of Zn²⁺ has now been determined to 2.0 Å resolution to facilitate cation identification. The protein structure was essentially identical to other structures and five Cd²⁺ positions were identified. Two of the cations are at the active site; one corresponds to the catalytic ion and the other provides a structural contribution. These Cd²⁺ sites are equivalent to two Zn²⁺ ions observed when the enzyme is complexed with a transition-state mimic and confirm our assignment of the roles played by these ions.