Ribonucleotide reduction triphosphates

The discoveries made with the E. coli system provided the basis for studies of ribonucleotide reduction in other microbial species and in animal cells. The mechanism of ribonucleotide reduction in rat tissues resembles that of E. coli in many ways, whereas ribonucleotide reduction in Lactobacillus leichmannii differs distinctively, in that coenzyme Bk takes part in a reduction accomplished at the nucleoside triphosphate level. Each of the three reductases that have been extensively purified reduces ribonucleotide substrates representing all four of the ribonucleosides of RNA and each displays remarkable allosteric regulatory properties (see below). 

The pathways of deoxyribonucleotide synthesis from ribonucleotides are summarized below. The trivial names of the enzymes of ribonucleotide reduction are as follows ribonucleoside diphosphate reductase ribonu-cleoside triphosphate reductase thioredoxin reductase. Enzyme Commission numbers and systematic names have not yet been assigned. 

Three types of enzymatic reactions depend upon alkyl corrin coenzymes. The first is the reduction of ribonucleotide triphosphates by cobalamin-dependent ribonucleotide reductase, a process involving intermo-lecular hydrogen transfer (Eq. 16-21). The second type of reaction encompasses the series of isomerizations shown in Table 16-1. These can all be depicted as in Eq. 16-34. Some group X, which may be attached by a C-C, C-O, or C-N bond, is transferred to an adjacent carbon atom bearing a hydrogen. At the same time,... 

Ribonucleotide reductases are discussed in Chapter 16. Some are iron-tyrosinate enzymes while others depend upon vitamin B12, and reduction is at the nucleoside triphosphate level. Mammalian ribonucleotide reductase, which may be similar to that of E. coli, is regarded as an appropriate target for anticancer drugs. The enzyme is regulated by a complex set of feedback mechanisms, which apparently ensure that DNA precursors are synthesized only in amounts needed for DNA synthesis.273 Because an excess of one deoxyribonucleotide can inhibit reduction of all... 

Ribonucleotide reductase and the thioredoxin system. In some lactobacilli, vitamin B12 is involved in the reduction of ribonucleotide triphosphate to deoxyribonucleotide. 

Another route to dUMP is the reduction of UDP to dUDP, followed by phosphorylation of dUDP to dUTP (or direct reduction of UTP to dUTP in some microorganisms). The dUTP is then hydrolyzed to dUMP. This circuitous route to dUMP is dictated by two considerations. First, the ribonucleotide reductase in most cells acts only on ribonu-cleoside diphosphates, probably because this permits better regulation of its activity. Second, cells contain a highly active deoxyuridine triphosphate diphosphohydrolase (dUT-Pase). It prevents the incorporation of dUTP into DNA by keeping intracellular levels of dUTP low by means of the reaction... 

FIGURE 6. The role of free radicals in the reduction of ribonucleotides to 2 -deoxyribonu-cleotides catalyzed by AdoCbl-dependent ribonucleotide triphosphate reductase. 

In all organisms, the ribonucleotide reductases play essential roles in the biosynthesis of DNA by catalyzing the reduction of nucleoside di- or triphosphates to the... 

Vitamin B12 (2 a) participates in the aqueous-phase biosynthesis of purine and pyrimidine bases, the reduction of ribonucleotide triphosphates, the conversion of methylmalonyl-coenzyme A to succinyl-coenzyme A, the biosynthesis of methionine from homocysteine, and the formation of myelin sheath in the nervous systems. 

DNA synthesis depends on a balanced supply of the four deoxyribonucleotides [1]. In all living organisms, with no exception so far, this is achieved by reduction of the corresponding ribonucleotides (substrates can be either ribonucleoside diphosphates NDP or ribonucleoside triphosphates NTP) by NADPH (Scheme 10-1), through a complex free radical chemistry. The substrate specificity is modulated by a sophisticated allosteric mechanism which makes it possible for a single protein to regulate the reduction of all four conunon ribonucleotides. This aspect will not be discussed here. Three well-characterized classes of ribonucleotide reductases (RNRs) have been described so far, which all are radical metalloenzymes [2-5]. 

For DNA synthesis to occur, the ribose moiety must be reduced to deoxyribose (Fig. 41.18). This reduction occurs at the dinucleotide level and is catalyzed by ribonucleotide reductase, which requires the protein thioredoxin. The deoxyribonucleo-side diphosphates can be phosphorylated to the triphosphate level and used as precursors for DNA synthesis (see Figs. 41.2 and 41.14). 

Table 5. Kinetic data and effector specificity for reduction of natural and synthetic, or modified ribonucleotides catalyzed by the triphosphate reductase of Lactobacillus leichmannii and diphosphate reductase of E. coll, respectively. (Data from Ref. 135,136, 142)... 

Reduction of ATP to the 2 -deoxy-compound has been carried out on a preparative scale using recombinant Lactobacillus leichmannii ribonucleotide triphosphate reductase. ... 

In all organisms, ribonucleotide reductases (RNRs) play an essential role in the biosynthesis of DNA by catalyzing the reduction of all four nucleoside di-or triphosphates to the corresponding 2 -deoxynucleotides [182,232]. Despite their central role in primary metabolism, RNRs have evolved to use a diverse array of cofactors to initiate the radical reaction, which eventually leads to nucleotide reduction (see Fig. 20) [233,234]. 

Fig.20 Schematic illustration of the reduction of ribonucleoside triphosphates to 2 -des-oxyribonucleoside triphosphates, as catalyzed by ribonucleotide reductases [182]...

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