Illustration of lead atoms (purple spheres) complexed with a yeast 5-aminolaevulinic acid dehydratase (ALAD) enzyme. This enzyme is involved in the biosynthesis of haem, a component of haemoglobin, the oxygen-carrying protein in red blood cells. Lead in the body can bind to ALAD in place of the usual zinc atoms, blocking the activity of the enzyme. This is one of the mechanisms by which lead poisoning has it affects.