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 Stimulated by magnesium, inhibited by zinc. Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 1/4. Homooctamer. Does not seem to have a metal requirement for activity. Belongs to the ALAD family.