The Anaerobic Ribonucleotide Reductase from Escherichia coli
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For deoxyribonucleotide synthesis during anaerobic growth, Escherichia coli cells depend on an oxygen-sensitive class III ribonucleotide reductase. The enzyme system consists of two proteins: protein a, on which ribonucleotides bind and are reduced, and protein b, of which the function is to introduce
a catalytically essential glycyl radical on protein a. Protein b can assemble one [4Fe-4S] center per polypeptide enjoying both the [4Fe-4S]21 and [4Fe-4S]11 redox state, as shown by iron and sulfide analysis, Mo¨ssbauer spectroscopy (d 5 0.43 mmzs21 , DEQ 5 1.0 mmzs21 , [4Fe-4S]21), and EPR spectroscopy (g 5 2.03 and 1.93, [4Fe-4S]11). This iron center is sensitive to oxygen and can decompose into stable [2Fe-2S]21 centers during exposure to air. This degraded form is nevertheless active, albeit to a lesser extent because of the conversion of the cluster into [4Fe-4S] forms during the strongly reductive conditions of the assay. Furthermore, protein b has the potential to activate several molecules of protein a, suggesting that protein b is an activating enzyme rather than a component of an a2b2 complex as previously claimed.