Nonribosomal peptide synthetase; part of the gene cluster that mediates the biosynthesis of penicillin, the world's most important antibiotic (PubMed:2120195, PubMed:2110531). AatA catalyzes the exchange of the alpha-aminoadipyl side chain of isopenicillin N for phenylacetic acid to yield penicillin (PubMed:2120195, PubMed:2110531). This step occurs in the peroxisomal matrix and the penM and paaT transporters are involved in the isopenicillin N and phenylacetic acid import into the peroxisome, respectively (By similarity). 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 penDE to yield penicillin in the peroxisomal matrix (By similarity). H2O + isopenicillin N + phenylacetyl-CoA = CoA + H(+) + L-2-aminoadipate + penicillin G Antibiotic biosynthesis; penicillin G biosynthesis; penicillin G from L-alpha-aminoadipate and L-cysteine and L-valine: step 3/3. The active form of the enzyme results from processing of the 40-kDa monomeric precursor to a heterodimer containing subunits of 11 and 29 kDa. The unprocessed preprotein is translocated inside peroxisomes and regulates its self-processing. The pre-AAT protein is synthesized as 40 kDa precursor which is then self-processed into an 11 kDa (protein A) and a 29 kDa (protein B). The B protein carries AAT activity. Belongs to the peptidase C45 family.