Illustration of the enzyme aconitase (blue and green) in complex with messenger ribonucleic acid (mRNA, red and yellow). Aconitase is involved in the citric acid cycle but here it is performing a secondary function as an iron regulatory protein (IRP). It does this by binding to ferritin or transferrin receptor mRNA. Binding ferritin mRNA prevents its translation into the protein product (ferritin). Ferritin acts like a sponge and helps to protect cells from the toxic effects of excess iron. Binding transferrin receptor mRNA stabilises the mRNA increasing the amount of its protein product (transferrin receptor). The transferrin receptor imports iron into the cell. Therefore aconitase's actions increase the amount of iron in the cells