Activation of class III ribonucleotide reductase from E-coli. The electrontransfer from the iron-sulfur center to S-adenosylmethionine

The anaerobic ribonucleotide :reductase (ARR) from E. coli is the prototype for enzymes that use the combination of S-adenosylmethionine (AdoMet) and an iron-sulfur center for generating catalytically essential free radicals. ARR is a homodimeric alpha2 protein which acquires a glycyl radical during anaerobic incubation with a [4Fe-4S]-containing activating enzyme (beta) a nd AdoMet under reducing conditions. Here we show that the ERR-active S = 1 /2 reduced [4Fe-4S](+) cluster is competent for AdoMet reductive cleavage, yielding 1 equiv of methionine and almost 1 equiv of glycyl radical. These data support the proposal that the glycyl radical results from a one-electr on oxidation of the reduced cluster by AdoMet. Reduced protein beta alone i s also able to reduce AdoMet but only in the presence of DTT. However, in t hat case, 2 equiv of methionine per reduced cluster was foamed. This unusua l stoichiometry and combined EPR and Mossbauer spectroscopic analysis are u sed to tentatively propose that AdoMet reductive cleavage proceeds by an al ternative mechanism involving catalytically active [3Fe-4S] intermediate cl usters.