Catalyzes an early step in the biosynthesis of tetrapyrroles. Binds two molecules of 5-aminolevulinate per subunit, each at a distinct site, and catalyzes their condensation to form porphobilinogen. 2 5-aminolevulinate = H(+) + 2 H2O + porphobilinogen Binds 8 zinc ions per octamer. Requires four zinc ions per octamer for full catalytic activity. Can bind up to 2 zinc ions per subunit. Can alternate between a fully active homooctamer and a low-activity homohexamer. A bound magnesium ion may promote the assembly of the fully active homooctamer. The magnesium-binding site is absent in the low-activity homohexamer. Inhibited by compounds that favor the hexameric state. Inhibited by divalent lead ions. The lead ions partially displace the zinc cofactor. Optimum pH is 6.8-7.3. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 1/4. Homooctamer; active form. Homohexamer; low activity form. Genetic variation in ALAD influences susceptibility to lead poisoning in individuals exposed to high amount of environmental lead. There are two common alleles: allele ALAD*1 and allele ALAD*2 resulting in 3 isozymes: ALAD 1-1, ALAD 1-2, and ALAD 2-2. Individuals with ALAD 1-2 or ALAD 2-2 isozymes have levels of blood lead higher than those in individuals with ALAD 1-1 isozyme. The sequence shown corresponds to allele ALAD*1. The disease is caused by variants affecting the gene represented in this entry. Belongs to the ALAD family.