Isopenicillin N synthase; part of the gene cluster that mediates the biosynthesis of penicillin, the world's most important antibiotic (PubMed:3903520). 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:3903520, PubMed:3839755, PubMed:8076799, PubMed:8557701, PubMed:9418249, PubMed:9703965, PubMed:9826554, PubMed:9530516, PubMed:9841222). 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 pcbAB. The tripeptide ACV is then cyclized to isopenicillin N (IPN) by the isopenicillin N synthase pcbC that forms the beta-lactam nucleus. Finally, the alpha-aminoadipyl side chain is exchanged for phenylacetic acid by the isopenicillin N acyltransferase penDE to yield penicillin in the peroxisomal matrix (By similarity). 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. Belongs to the iron/ascorbate-dependent oxidoreductase family.