Ribonucleotide reductase diferrous forms

The site in the active Fe ribonucleotide reductase contains two Fe(III) ions 3.3 A apart, bridged by one carboxylate from a glutamate residue and a water-derived oxo bridge (57). The function of this iron center appears to be the formation and stabilization of a free radical on a tyrosine about 5 A away. This radical is formed by reaction of the reduced, diferrous center with 02, probably through peroxide and ferryl intermediates. This unusually stable tyrosyl radical is thought to partic-... 

Atta, M., Andersson, K. K., Ingemarson, R., Thelander, L., and Gr%oslund, A., 1994, EPR studies of mixed-valent [Fe Fe ] clusters formed in the R2 subunit of ribonucleotide reductase from mouse or herpes simplex virus Mild chemical reduction of the diferric centers. J. Am. Chem. Soc. 116 6429n6430. 

Structure of the Iron Center Formation of the Iron Center and Tyrosyl Radical Spectroscopy of the Diferric Iron Center Spectroscopy of the Tyrosyl Radical Redox Properties of the Iron Center Mixed-Valent Form of the Iron Center Diferrous Form of the Iron Center Inhibitors to Iron-Containing Ribonucleotide Reductase Methane Monooxygenase A. Spectroscopy of the MMOH Cluster X-Ray Structure of MMOH... 

Fig. 1. Diferric iron clusters form hemer3fthrin, ribonucleotide reductase R2 subunit, and methane monooxygenase hydroxylase. The figure was made with the RasMol 2.0 program, and the protein coordinates as PDB files were obtained from Brookhaven Protein Data Bank. Only the amino acids (histidines, green carboxylates, black oxygen, red nitrogen, yellow acetate, blue iron, violet) coordinated to the iron cluster are shown, coordinated waters are not indicated. The first subunit containing the cluster is shown. Diferric Hr is from sipunculid worm Themiste dyscritra). The RNR-R2 is from E. coli. The MMOH is from Methvlococcus caosulatus (Bath).

Class I Ribonucleotide Reductases - The most widely distributed form of ribonucleotide reductase. It acts upon ribonucleoside diphosphates. The enzyme generates a free radical on a tyrosine residue, with the aid of a diferric oxygen bridge. 

The two-electron reduction of the diferric forms of hemerythrin (51), ribonucleotide reductase (27, 50), and methane monooxygenase (31) yields dioxygen-sensitive diferrous forms of the proteins. All three can be generated by dithionite treatment of the corresponding diferric forms, although the RRB2 reduction requires methyl viologen as mediator. The Fe(II) oxidation state is more difficult to probe spectroscopically, and only recently have methods been developed that allow this state to be characterized further. 

Rates of Fe binding/ oxidation by recombinant H and L ferritins differ over a 1000-fold. The L type of ferritin protein forms polynuclear complexes, as soon as the iron is oxidized, that are indistinguishable from the mineral (B. H. Huynh and E. C. Theil, unpublished results), whereas the H type ferritin proteins form a series of ferric intermediates that include a diferric-peroxo as the first product. The di-ferric peroxo species is similar to complexes that form in methane monoxygenase and ribonucleotide reductase (see Chapter 16). Thus, the Fe - - O2 inorganic chemistry... 

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]. 

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