Ribonucleotide reductase redox states

Studies on three different iron—sulfur enzyme systems which all require S-adenosylmethionine (SAM) — lysine 2,3-aminomutase, pyruvate-formate lyase, and anaerobic ribonucleotide reductase — have led to the identification of SAM as a major source of free radicals in living cells (for a recent review, see Atta et ak, 2010). As in the dehydratases, these systems have a [4Fe—4S] centre chelated by only three cysteines with one accessible coordination site. The cluster is active only in the reduced state [4Fe—4S] and appears to combine the two roles described previously, serving both as a ligand for substrate binding and as a redox catalyst (Figure 13.19). Their mechanism again requires that the exposed iron atom of the cluster shifts towards octahedral geometry as it binds... 

Fig. 3. Scheme of various redox states of ribonucleotide reductase protein R2, as well as known reactions to move between the states. Tyr O represents the tyrosyl radical Tyr OH is the normal tyrosyl residue (Y122 in E. coli ribonucleotide reductase),... 

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