Initially, we expected that the fragment Y153-R204 would be disordered since this region is missing in homologous proteins of microbial origin and our homologous modelling did not predict any secondary structure elements in the long loop . However, our structural data showed for the first time that the domain Y153-R204 of AcmB not only forms secondary structures but also closes around the product, further tightening its binding (Figure ). The domain is composed of four helices (H6-H9) together with an antiparallel β-sheet D (B7-B8).