The macrocyclization of tetratryptomycin A (3) was efficiently facilitated by the CttpC enzyme, enabling the rapid synthesis of tetratryptomycin A and B (1-2) in a concise manner. Despite an investigation of over 100 chemical and bio-oxidants, as previously mentioned, the macrocyclization of tetratryptomycin A (3) did not occur under conditions other than those involving CttpC. This noteworthy fact prompted an exploration of the reaction mechanism for the macrocyclization of tetratryptomycin A (3). To this end, a docking analysis was conducted between CttpC, whose amino acid sequence had already been elucidated, and tetratryptomycin A (3) (Figure ). In a comprehensive docking analysis, it is intriguing to note that significant changes in the conformation of tetratryptomycin A (3) were observed in the presence of CttpC (see the Supporting Information). Specifically, the predicted result with the best docking score revealed that the interatomic distance between C22-C18', which is involved in the macrocyclization of tetratryptomycin A (3), is computationally predicted to be 8.89 Å in the presence of CttpC, whereas in the absence of CttpC, the interatomic distance between C22-C18' in the most stable conformation of 3 is predicted to be 18.4 Å. Therefore, the computational results suggest that the presence of CttpC reduces the interatomic distance between C22-C18' of 3 by approximately half. This is also observed for the interatomic distance between N-C18' of 3. Specifically, in the absence of CttpC, the interatomic distance was 17.2 Å, while in the presence of CttpC, the distance was shortened to 9.96 Å, as shown by the calculations. Furthermore, the computational results also revealed that hemoglobin within the pocket of CttpC and 3 were in close proximity (Fe-C18': 9.17 Å).