Deoxyribonucleotide synthesis regulation

Purine Biosynthesis Is Regulated at Two Levels Pyrimidine Biosynthesis Is Regulated at the Level of Carbamoyl Aspartate Formation Deoxyribonucleotide Synthesis Is Regulated by Both Activators and Inhibitors... 

Deoxyribonucleotide Synthesis Is Regulated by Both Activators and Inhibitors... 

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. 

A summary of some of these processes is as follows synthesis of phospholipids and cholesterol de novo synthesis of ribonucleotides synthesis of RNA de novo synthesis of deoxyribonucleotides regulation of synthesis of deoxyribonucleotides salvage pathways duplication of DNA transcription and translation (polypeptide synthesis). After this series of topics, those of fuels and ATP generation, mitosis and, finally, regulation of the cycle, are described and discussed. 

Ribonucleotide reductase is responsible for maintaining a balanced supply of the deoxyribonucleotides required for DNA synthesis. To achieve this, the regulation of the enzyme is complex. In addition to the single active site, there are two sites on the enzyme involved in regulating its activity (Figure 22.13). 

Substrate specificity site The binding of nucleoside triphosphates to an additional allosteric site (known as the substrate specificity site) on the enzyme regulates substrate specificity, causing an increase in the conversion of different species of ribonucleotides to deoxyribonucleotides as they are required for DNA synthesis. 

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... 

Enzyme Synthesis Also Contributes to Regulation of Deoxyribonucleotides during the Cell Cycle Intracellular Concentrations of Nucleotides Vary According to the Physiological State of the Cell... 

Enzyme Synthesis Also Contributes to Regulation of Deoxyribonucleotides during the Cell Cycle... 

The Synthesis of Deoxyribonucleotides Is Controlled by the Regulation of Ribonucleotide Reductase... 

Regulation of ribonucleotide reductase is complex. The binding of dATP (deoxyadenosine triphosphate) to a regulatory site on the enzyme decreases catalytic activity. The binding of deoxyribonucleoside triphosphates to several other enzyme sites alters substrate specificity so that there are differential increases in the concentrations of each of the deoxyribonucleotides. This latter process balances the production of the 2 -deoxyribonucleotides required for cellular processes, especially that of DNA synthesis. 

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]. 

The concepts of an enzyme complex for DNA precursor and DNA synthesis and of different deoxyribonucleotide pools should soon promote new insights into notoriously difficult to apprehend processes of cell biology. Ribonucleotide reduction which is pivotal in deoxyribonucleotide metabolism has been described above as an enzyme system of extreme complexity but now basically understandable function and origin. Analysis of its integration into supramolecular structure and regulation will, not so soon, open a new chapter. 

The amount of nucleosides in cells is negligible also deoxyribonucleotides occur at insignificant levels in tissues. Ribonucleotides are important cellular components and occur as 5 -monophosphates, di-, tri-, and cyclic 3, 5 -phos-phates. Nucleoside phosphates function as energy carriers and serve as precursors for the synthesis of nucleic acids. Cyclic nucleotides (i.e., cyclic 3, 5 -AMP and cyclic-3, 5 -GMP) also play a role in metabolic regulation. 

Thymidine must first be converted to its triphosphate (TTP) before incorporation as the deoxyribonucleotide (dTTP) into the DNA molecule. Thus two enzymes must be present for thymidine to be utilized (1) thymidine kinase (ATP thymidine 5 phosphotransferase), for phosphorylation of thymidine and (2) DNA polymerase, for incorporation of deoxyribonucleotides on the DNA template. Limiting levels of either of these enzymes in seeds could obviously restrict DNA synthesis and, as a corollary, DNA synthesis could be regulated by either of these enzymes. 

---