Ribonucleotide reductase , studied with

Three classes of analogs have been studied a) analogs in which the 5,6-dimethylbenzimidazole moiety of the nucleotide has been replaced, b) analogs with modifications on the periphery of the corrin ring and c) analogs in which the S -deoxyadenosyl moiety has been altered or replaced. Coenzymes in which the 5,6-dimethylbenzimidazole moiety has been replaced by benzimidazole, 5-methoxybenzimidazole, adenine and 2-methyladenine have been tested with ribonucleotide reductase. Adenosylcobamides with benzimidazole and adenine function as coenzymes in the L. leichmannii system (52, 80), while those with 5-methoxybenzimidazole and 2-methyladenine are active with the R. meliloti reductase (82). In contrast adenosylcobinamide, which lacks the entire nucleotide moiety does probably not function as a coenzyme because it did not yield the rapidly appearing paramagnetic intermediate (77). 

Sjbberg B-M (1997) Ribonucleotide Reductases - A Group of Enzymes with Different Metallosites and a Similar Reaction Mechanism. 88 139-174 Slebodnick C, Hamstra BJ, Pecoraro VL (1997) Modeling the Biological Chemistry of Vanadium Structural and Reactivity Studies Elucidating Biological Function. 89 51-108 Smit HHA, see Thiel RC (1993) 81 1-40... 

Metalloenzymes with non-heme di-iron centers in which the two irons are bridged by an oxide (or a hydroxide) and carboxylate ligands (glutamate or aspartate) constitute an important class of enzymes. Two of these enzymes, methane monooxygenase (MMO) and ribonucleotide reductase (RNR) have very similar di-iron active sites, located in the subunits MMOH and R2 respectively. Despite their structural similarity, these metal centers catalyze very different chemical reactions. We have studied the enzymatic mechanisms of these enzymes to understand what determines their catalytic activity [24, 25, 39-41]. 

A common way to benefit from the ability to combine different molecular orbital methods in ONIOM is to combine a DFT or ab-initio description of the reactive region with a semi-empirical treatment of the immediate protein environment, including up to 1000 atoms. Due to the requirement for reliable semi-empirical parameters, as discussed in Section 2.2.1, this approach has primarily been used for non-metal or Zn-enzymes. Examples include human stromelysin-1 [83], carboxypeptidase [84], ribonucleotide reductase (substrate reaction) [85], farnesyl transferase [86] and cytosine deaminase [87], Combining two ab-initio methods of different accuracy is not common in biocatalysis applications, and one example from is an ONIOM (MP2 HF) study of catechol O-methyltransferase [88],... 

Since HU needs to be taken lifelong in the treatment of non-neoplastic conditions such as SCA (since the inhibition of ribonucleotide reductase is reversible) a major concern is its long term secondary effects. Several studies have shown the potential leukomegenic effect of HU in myeloproliferative disorders [22], [23]. Such concern becomes quite legitimate in sickle cell patients with permanently expanded erythropoiesis in whom the use of HU is at the limit of marrow toxicity. Hence alternative therapies must be sought for. 

Studies on three different iron-sulfur enzyme systems, which all require S-adenosyl methionine—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. 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... 

Proteins with dinuclear iron centres comprise some well studied representatives like ribonucleotide reductase (RNR), purple acid phosphatase (PAP), methane monooxygenase hydroxylase (MMOH), ruberythrin and hemerythrin. The latter is an oxygen carrier in some sea worms it has been first characterized within this group and has thus laid the foundation to this class of iron coordination motif. Ruberythrin is found in anaerobic sulfate-reducing bacteria. Its name implies that, in addition to a hemerythrin-related diiron site another iron is coordinated in a mononuclear fashion relating to rubredoxin which is an iron-... 

The manner in which the reduction of ribonucleotides to deoxyribonucleotides is regulated has been studied with reductases from relatively few species. The enzymes from E. coli and from Novikoff s rat liver tumor have a complex pattern of inhibition and activation (fig. 23.25). ATP activates the reduction of both CDP and UDP. As dTTP is formed by metabolism of both dCDP and dUDP, it activates GDP reduction, and as dGTP accumulates, it activates ADP reduction. Finally, accumulation of dATP causes inhibition of the reduction of all substrates. This regulation is reinforced by dGTP inhibition of the reduction of GDP, UDP, and CDP and by dTTP inhibition of the reduction of the pyrimidine substrates. Because evidence suggests that ribonucleotide reductase may be the rate-limiting step in deoxyribonucleotide synthesis in at least some animal cells, these allosteric effects may be important in controlling deoxyribonucleotide synthesis. 

Inhibition of DNA synthesis is brought about by the action of dTTP as an allosteric inhibitor of ribonucleotide reductase (Reichard et al., 1961 Moore and Hurlbert, 1966 Brown and Reichard, 1969 Rummer et al., 1978). This enzyme is responsible for reducing all four ribonucleoside diphosphates (NDP) to the corresponding de-oxyribonucleoside diphosphates (dNDP). It is subject to a complex allosteric control which has been most studied with the bacterial enzyme. Most studies with the mammalian enzyme show it to be similar to the bacterial enzyme (Fig.11.7). 

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