Methane monooxygenase (MMO) catalyses the biological transformation of methane to methanol at a binuclear iron site. Guided by the three-dimensional structure of the R2 protein of E. coli ribonucleotide reductase (RNR), we have aligned the sequences of two different MMOs with the sequences of the iron coordinating four helix bundle in R2. The model suggests that the central four helix bundle of R2 is present also in MMO. The iron coordination is similar in MMO and R2 with two histidine ligands and four carboxyl ligands in both cases. The residues lining the proposed oxygen binding site in MMO are significantly smaller in MMO than in R2 allowing binding of both molecular oxygen and methane at this site. This binding site is lined by residues Cys151, Thr213, Ile217 and Ile(Val)239.