Isopenicillin N synthase; part of the gene cluster that mediates the biosynthesis of penicillin, the world's most important antibiotic (PubMed:1368505, PubMed:1369045, PubMed:1588566). IpnA catalyzes the cyclization of the tripeptide N-[(5S)-5-amino-5-carboxypentanoyl]-L-cysteinyl-D-valine (LLD-ACV or ACV) to form isopenicillin N (IPN) that contains the beta-lactam nucleus (PubMed:1368505, PubMed:1369045, PubMed:1588566). The penicillin biosynthesis occurs via 3 enzymatic steps, the first corresponding to the production of the tripeptide N-[(5S)-5-amino-5-carboxypentanoyl]-L-cysteinyl-D-valine (LLD-ACV or ACV) by the NRPS acvA. The tripeptide ACV is then cyclized to isopenicillin N (IPN) by the isopenicillin N synthase ipnA that forms the beta-lactam nucleus. Finally, the alpha-aminoadipyl side chain is exchanged for phenylacetic acid by the isopenicillin N acyltransferase aatA to yield penicillin in the peroxisomal matrix (PubMed:1368505) (Probable). N-[(5S)-5-amino-5-carboxypentanoyl]-L-cysteinyl-D-valine + O2 = 2 H2O + isopenicillin N Binds 1 Fe(2+) ion per subunit. Antibiotic biosynthesis; penicillin G biosynthesis; penicillin G from L-alpha-aminoadipate and L-cysteine and L-valine: step 2/3. Monomer. Expression is repressed by glucose (PubMed:3096965). The transcription factor rfx1 controls penicillin biosynthesis through the regulation of the acvA, ipnA and aatA transcription (PubMed:22960281). Expression is also controlled by the transcription factor pacC that specifically recognizes the 5'-GCCARG-3' sequence in the promoter (PubMed:8736532). Belongs to the iron/ascorbate-dependent oxidoreductase family.