The putative reaction mechanism of the macrocyclization by CttpC can be described as follows. The tetravalent iron complex, consisting of a heme moiety and iron, causes N-H abstraction of tetratryptomycin A (3), leading to the formation of radicals at the nitrogen indole. Subsequently, radical migration generates radicals at the C3 position of tryptophan. The nitrogen atom of diketopiperazine nucleophilically attacks the simultaneously formed imine, resulting in the formation of pyrroloindoline. The radical on the C3 of the tryptophan then couples with the radical on the nitrogen atom of the other tryptophan to form cyctetryptomycin B (2). On the other hand, it is assumed that the radical on the C3 of the tryptophan leads to the formation of cyctetryptomycin A (1) by radical addition to the C7 position of the other tryptophan.