Structure of the high-valent FeIIIFeIV state in ribonucleotide reductase (RNR) of Chlamydia trachomatis—Combined EPR, 57Fe-, 1H-ENDOR and X-ray studies

A recently discovered subgroup of class I ribonucleotide reductase (RNR) found in the infectious bacterium Chlamydia trachomatis (C. trachomatis) was shown to exhibit a high-valent FeIIIFeIV center instead of the tyrosyl radical observed normally in all class I RNRs. The X-ray structure showed that C. trachomatis WT RNR has a phenylalanine at the position of the active tyrosine in Escherichia coli RNR. In this paper the X-ray structure of variant F127Y is presented, where the tyrosine is restored. Using 1H- and 57Fe-ENDOR spectroscopy it is shown, that in WT and variants F127Y and Y129F of C. trachomatis RNR, the FeIIIFeIV center is virtually identical with the short-lived intermediate X observed during the iron oxygen reconstitution reaction in class I RNR from E. coli. The experimental data are consistent with a recent theoretical model for X, proposing two bridging oxo ligands and one terminal water ligand. A surprising extension of the lifetime of the FeIIIFeIV state in C. trachomatis from a few seconds to several hours at room temperature was observed under catalytic conditions in the presence of substrate. These findings suggest a possible new role for the FeIIIFeIV state also in other class I RNR, during the catalytic radical transfer reaction, by which the substrate turnover is started.