Protein Data Bank in Europe

Abstract

Cadmium (Cd₂₊) is toxic to living organisms because it causes the malfunction of essential proteins and induces oxidative stress. NADP₊-dependent cytosolic isocitrate dehydrogenase (IDH) provides reducing energy to counteract oxidative stress via oxidative decarboxylation of isocitrate. Intriguingly, the effects of Cd₂₊ on the activity of IDH are both positive and negative, and to understand the molecular basis, we determined the crystal structure of NADP₊-dependent cytosolic IDH in the presence of Cd₂₊. The structure includes two Cd₂₊ ions, one coordinated by active site residues and another near a cysteine residue. Cd₂₊ presumably inactivates IDH due to its high affinity for thiols, leading to a covalent enzyme modification. However, Cd₂₊ also activates IDH by providing a divalent cation required for catalytic activity. Inactivation of IDH by Cd₂₊ is less effective when the enzyme is activated with Cd₂₊ than Mg₂₊. Although reducing agents cannot restore activity following inactivation by Cd₂₊, they can maintain IDH activity by chelating Cd₂₊. Glutathione, a cellular sulphydryl reductant, has a moderate affinity for Cd₂₊, allowing IDH to be activated with residual Cd₂₊, unlike dithiothreitol, which has a much higher affinity. In the presence of Cd₂₊-consuming cellular antioxidants, cells must continually supply reductants to protect against oxidative stress. The ability of IDH to utilise Cd₂₊ to generate NADPH could allow cells to protect themselves against Cd₂₊.