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 1 zinc ion per monomer. Inhibited by divalent lead ions. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 1/4. Homooctamer. Present with 11600 molecules/cell in log phase SD medium. Belongs to the ALAD family.