Diferric cluster

E. coli R2 protein have been reported (68). Parallel-mode EPR experiments showed a signal originating from a ferromagnetically coupled high-spin diferrous cluster, i.e., with S = 4. Azide was shown to bind to the iron cluster, and high concentrations of azide caused a shift as well as an increase in intensity of the EPR signal (68). 

The enzyme ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deox5nibonucleotides, which is the first rate-limiting step in DNA biosynthesis. On the basis of their cofactor compositions, RNRs may be grouped into four different classes [7]. Class I RNR from E. coli is comprised of two homodi-meric subunits, R1 and R2. The R1 subunit (2 x 86kDa) contains the substrate binding site and redox-active cysteine residues, which are involved in the reduction of the ribonucleotides. The R2 subunit (2 x 43 kDa) contains in its active form (R2act) a stable tyrosyl radical (Y122 ), which is necessary for catalytic activity. This tyrosyl radical is located in close proximity to a //-oxo diferric cluster and is embedded about 10 A away from the protein surface [38, 39]. 

DIFERRIC CLUSTER-TYROSYL RADICAL IN RIBONUCLEOTIDE REDUCTASE... 

Elgren TE, Hendrich MP, Que Jr L. 1993. Azide binding to the diferrous clusters of the R2 protein of ribonucleotide reductase from Escherichia coli. J Am Chem Soc 115 9291-9292. 

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