Metabolic 2 -deoxyribonucleoside synthesi

Most of the carbon that flows through nucleotide synthetic pathways goes into ribonucleotide triphosphates (rNTPs - ATP, CTP, GTP, and UTP). A relatively small fraction is diverted to the synthesis of deoxyribonucleoside triphosphates (dNTPs). rNTPs are synthesized in excess of dNTPs because most cells contain 5-10 times as much RNA as DNA and because rNTPs have multiple metabolic roles, whereas dNTPs are used only to make DNA. 

The ribonucleoside di- and triphosphates (NDPs, NTPs) and the deoxyribonucleoside di- and triphosphates (dNDPs, dNTPs) have important metabolic functions in the cell. They operate as energy carriers in many reactions and are also precursors in the synthesis of nucleic acids. 

As we have already noted, the deoxyribonucleoside phosphates occur in animal tissues in small amounts and their concentrations are increased in cells engaged in DNA synthesis. The triphosphates of the four deoxyri-bonucleosides represented in DNA have been demonstrated in cell extracts, as have the mono- and diphosphates of thymidine and deoxycytidine. The mono-, di-, and triphosphates of deoxyuridine are known as intermediary compounds in the metabolism of the pyrimidine deoxyribonucleotides. 

The 2-deoxy-D-ribose 5-phosphate aldolase (RibA or DERA EC 4.1.2.4) is a class I enzyme that, in vivo, catalyzes the reversible addition of acetaldehyde to D-glyceraldehyde 3-phosphate (34 Figure 5.57) in the metabolic degradation of 127 from deoxyribonucleosides [269], ivith an equilibrium constant for synthesis of 2 x lO m [56]. It is, therefore, unique among the aldolases in that it uses an aldehyde rather than a ketone as the aldol donor. RibA has been isolated from eukaryotic and prokaryotic sources [270, 271],... 

---