1-Aminocyclopropane-1-carboxylic oxidase(ACCO) is an on-heme iron(II)-containing enzyme involved in the biosynthesis of the phytohormone ethylene, which regulates fruit ripeninga nd flowering in plants. The active conformation of ACCO, and in particulart hat of the C-terminal part, remains unclear and open and closed conformationsh ave been proposed. In this work, ac ombined experimental and computationals tudy to understand the conformation and dynamics of the C-terminal part is reported.S ite-directed spin-labeling coupledt oe lectron paramagnetic resonance (SDSL-EPR)s pectroscopy was used. Mutagenesis experiments were performed to generate activee nzymesb earing two paramagnetic labels (nitroxide radicals) anchored on cysteiner esidues, one in the main core and one in the C-terminal part. Inter-spin distance distributions were measured by pulsed EPR spectroscopy and compared with the results of molecular dynamics simulations. The results revealt he existence of af lexibility of the C-terminal part. Thisf lexibility generates several conformations of the C-terminal part of ACCO that correspond neither to the existing crystal structures nor to the modelleds tructures. This highly dynamic region of ACCO raises questionso ni ts exact functiond uring enzymatic activity.