The first step is believed to involve an E1 elimination of ammonia from porphobilinogen, generating a carbocation intermediate (1). This intermediate is then attacked by the dipyrrole cofactor of porphobilinogen deaminase, which after losing a proton yields a trimer covalently bound to the enzyme (2).
The initial step in this process is thought to involve an E1 elimination of ammonia from porphobilinogen, producing a carbocation intermediate (1), which then reacts with the dipyrrole cofactor of porphobilinogen deaminase, forming a covalent bond with the enzyme (2).