 This reaction is the first common step in the biosynthesis of all biological tetrapyrroles.Zinc is essential for enzymatic activity.The structural basis for allosteric regulation of Porphobilinogen synthase (PBGS) is modulation of a quaternary structure equilibrium between octamer and hexamer (via dimers), which is represented schematically as 6mer* ↔ 2mer* ↔ 2mer ↔ 8mer.