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 (By similarity). 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 (By similarity). Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 1/4. Homooctamer; active form. Homohexamer; low activity form (By similarity). Belongs to the ALAD family.