On the formation of the mixed pyrrole catalysed by porphobilinogen synthase from <i>Rhodobacter spheroides</i>

The enzyme porphobilinogen synthase (PBGS) catalyses the formation of porphobilinogen (PBG) from two molecules of 5-aminolevulinic acid (ALA). It has been claimed that the PBGS from <i>Rhodobacter spheroides</i> is able to form a mixed pyrrole, from one molecule of 5-aminolevulinic acid and one molecule of levulinic acid. The chemical synthesis of this mixed pyrrole allowed to show, that the compound formed from 5-aminolevulinic acid and levulinic acid with PBGS from <i>R. spheroides</i> has not the proposed structure. The putative enzyme catalysed formation of the mixed pyrrole had been used as an argument for the postulated mechanism of PBGS. In view of our results this line of arguments has to bee re-evaluated.