Gamma-glutamyl cyclotransferase; part of the gene cluster that mediates the biosynthesis of an unusual class of epipolythiodioxopiperazines (ETPs) lacking the reactive thiol group important for toxicity (PubMed:27390873). Firstly, L-tyrosine is prenylated by tcpD, before undergoing condensation with L-glycine in a reaction catalyzed by the NRPS tcpP leading to the diketopiperazine (DKP) backbone (PubMed:27390873). Afterwards the alpha-carbon of tyrosine is oxidized by the cytochrome P450 tcpC to form a hydroxyl group (PubMed:27390873). However, in contrast other ETP biosynthesis pathways studied so far, tcpC is not able to bishydroxylate the DKP at both alpha-carbon positions, but hydroxylates the alpha-carbon of the tyrosine part and the nitrogen of the glycine part (PubMed:27390873). The next steps involve an alpha,beta-elimination reaction catalyzed by tcpI, a methylation by the methyltransferase tcpN the action of the four enzyme cascade tcpG/K/J/I (PubMed:27390873). Due to a dysfunctional cytochrome P450 monooxygenase tcpC, the pathway leads to the biosynthesis of probable non-toxic metabolites lacking the reactive thiol group (PubMed:27390873). an alpha-(gamma-L-glutamyl)-L-amino acid = 5-oxo-L-proline + an L-alpha-amino acid Secondary metabolite biosynthesis. Expression is positively regulated by the thioclapurine cluster-specific transcription factor tcpZ (PubMed:27390873). Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family.