III Ribonucleotide Reductases

RADICAL TRANSFER REACTIONS IN CLASS II AND III RIBONUCLEOTIDE REDUCTASES 

Are there long-range RTPs in class II and III RNRs No, in neither of these two elasses is the radieal chain initiator site separated very far from the aetive site and the eysteine residue that transiently forms a thiyl radical. 

The three-dimensional structure of the class III RNR from bacteriophage T4 was recently solved to high resolution for the mutant enzyme G580A, which has an alanine residue at the position of the stable glycyl radical in the native enzyme (Logan et al., 1999). This prevents oxygen-dependent irreversible truncation of the polypeptide chain (Young et al., 1996). The three-dimensional structure of the mutant enzyme showed that 

AdoMet and Gly580 of NrdD, and that also the RTP for activation of class III RNR is short. 

Andersson, M. E., H gbom, M., Rinaldo-Matthis, A., Andersson, K. K., Sj berg, B.-M., and Nordlund, P., 1999, The crystal structnre of an azide complex of the diferrous R2 subunit of ribonucleotide reductase displays a novel carboxylate shift with important mechanistic implications for diiron-catalyzed oxygen activation. J. Amer. Chem. Soc. 121 2346n2352. 

---

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

Cho KB, V Pelmenschikov, A Graslund, PEM Siegbahn (2004) Density functional calculations on class III ribonucleotide reductase Substrate reaction mechanism with two formates. J. Phys. Chem. B 108 (6) 2056-2065... 

D.T. Logan, J. Andersson, B.M. Sjoberg, and P. Nordlund. 1999. A glycyl radical site in the crystal structure of a class III ribonucleotide reductase Science 283 1499-1504. (PubMed)... 

Figure 7.32 Propagation steps of (a) Types I and II ribonucleotide reductases, (b) Type III ribonucleotide reductase. (c) Suicide enz5une inactivation by 2 -deoxy-2 -halo- or -pseudohaloribonucleotides.

Class III Ribonucleotide Reductases - Found only in facultative or obligate anaerobes. 

Class III ribonucleotide reductases - Found only in facultative or obligate anaerobes, the Class III enzyme acts on ribonucleoside triphosphate substrates. It uses adenosylmethionine and an iron-sulfur center to generate the catalytically essential radical on a glycine residue. 

Treatment via chelation has been observed for 2-acetylpyridine thiosemi-carbazone derivatives, which have been found to possess inhibitory activity for the RNA-polymerases of the influenza virus [133]. The iron(III) complexes were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase (no added iron) compared to uncomplexed thiosemi-carbazone [128]. Raina and Srivastava [134] prepared and characterized low spin iron(III) complexes of 2-acetylpyridine thiosemicarbazone, [Fe(8-H)2A] (A = NO3, OH, Cl, N3, NCS or NO2), which were proposed as being seven-coordinate. However, all but the azide complex are 1 1 electrolytes in DMF and their solid ESR spectra are rhombic with the g-values being about 2.20,2.15 and 2.00. Of the six complexes, the azide ion seems to interact ihost strongly with the iron(III) center. 

The iron(III) complexes of 21 and 22 were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase than un-complexed thiosemicarbazones [128]. The mechanism of antitumor action by these complexes still remains largely speculative, although some excellent preliminary studies have appeared. It has been postulated [148] that tridentate... 

The general influence of covalency can be qualitatively explained in a very basic MO scheme. For example, we may consider the p-oxo Fe(III) dimers that are encountered in inorganic complexes and nonheme iron proteins, such as ribonucleotide reductase. In spite of a half-filled crystal-field model), the ferric high-spin ions show quadrupole splittings as large as 2.45 mm s < 0, 5 = 0.53 mm s 4.2-77 K) [61, 62]. This is explained... 

To what extent are the binuclear units in the purple acid phosphatases analogous to those found in hemerythrin and ribonucleotide reductase There are similarities and differences. The oxidized form is purple and EPR silent with strong antiferromagnetic coupling between the two Fe(III) centers the reduced form is pink and EPR active (gav = 1.7-1.8) with weak antiferromagnetic coupling between the Fe(III)-Fe(II) centers (3,72,73). 

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-... 

---