English: Shaded space-filling drawing of the ALAD enzyme (5-aminolaevulinic acid dehydratase) involved in the synthesis of the heme cofactor essential to oxygen binding for the hemoglobin in blood. Zn ions are essential to ALAD activity, but it can be displaced by the even tighter-binding lead ion -- therefore, inactivation of ALAD is one of the important factors in lead poisoning. The enzyme has 8 chains (in different shades of blue), and the pair of lead ions in each chaim are shown in gray. Drawn by David Goodsell, for the April 2016 Molecule-of-The-Month.