The small protein contains a unique iron-sulfur center

The extreme sensitivity of the radical to oxygen explained why we lost it during purification and why the purified enzyme was inactive. However, with stricter adherence to anaerobic conditions, it was possible to prepare an enzyme that maintained a sizeable part of its glycyl radical. This was useful to identify the compounds of the system involved in the formation of the radical (activation) and those involved in the reduction of CTP. In fact, it appeared that CTP reduction by the radical-containing enzyme only required ATP, an allosteric effector, K+, Mg + and formate. As briefly described below, formate is the hydrogen donor during conversion of the ribose to deoxyribose [38]. 

4 The small protein contains a unique iron-sulfur center 

20 K resulted in broadening, with disappearance of the signal above 30 K. Both temperature and microwave power dependences were consistent with a [4Fe-4S] center. The EPR signal accounted for 50-100% of the protein-bound iron, with better yields when deazaflavin was used as the reducing agent. The EPR properties of the reduced cluster were basically retained within the ctlpl holoenzyme. 

The presence of a reduced [4Fe-4S] center was further confirmed by the observation, at large concentrations of the protein, of EPR signals at 5.3, 4.7, 2.95 and 1.34 (S. Ollagnier, E. Mulliez, J. Gaillard, T. A. Mattioli, M. Lutz, C. Meier, V. Schunemann, A. X. Trautwein and M. Fontecave, impublished results). Temperature-dependence studies showed that the g values at 4.7 and 2.95 are related and correspond to the ground Kramer s doublet of a S = spin state with D 0 and E/D = 0.17 and the less intense components at 5.3 and 1.34 have not yet been assigned. A S = spin state has only been observed with [4Fe-4S]+ clusters, so far. 

The former model had to be slightly modified when we discovered that the re- 

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