The O-2 cleavage by the iron dimer complex in RNR is experimentally found to be at least 400 times faster than the one in MMO, in spite of very similar iron dimers. The origin of this difference is investigated using the hybrid density functional method B3LYP with models containing about 60 atoms. The calculations support a suggestion that this difference could be due to the presence of a second sphere tryptophan (Trp48) in RNR. (C) 2002 Elsevier Science B.V. All rights reserved.