Cytidine triphosphate hydrolysis

Some biosynthetic reactions are driven by the hydrolysis of nucleoside triphosphates that are analogous to ATP—namely, guanosine triphosphate (GTl ), uridine triphosphate (UTP), and cytidine triphosphate (CTP). The diphosphate forms of these nucleotides are denoted by GDP, UDP, and CDP, and the monophosphate forms are denoted by GMP, UMP, and CMP. Enzymes catalyze the transfer of the terminal phosphoryl group from one nucleotide to another. The phosphorylation of nucleoside monophosphates is catalyzed by a family of nucleoside monophosphate kinases, as discussed in Section 9.4. The phosphorylation of nucleoside diphosphates is catalyzed by 7iucleoside diphosphate kinase, an enzyme with broad... 

Synthesis from an Activated Diacylglycerol. The de novo pathway starts with the reaction of phosphatidate with cytidine triphosphate (CTP) to form the activated diacylglycerol, cytidine diphosphodiacylglycerol (CDP-diacylglycerol Figure 26.2), This reaction, like those of many biosyntheses, is driven forward by the hydrolysis of pyrophosphate. 

UMP is phosphorylated by two kinases to uridine triphosphate (UTP) via two sequential reactions with ATP. First, the diphosphate form UDP is produced, and further phosphorylation leads to UTP. Both steps are fueled by ATP hydrolysis. CTP (cytidine triphosphate) is subsequently formed by the amination of UTP by the catalytic activity of CTP synthetase. Glutamine is the NH3 donor, and also in this case the reaction is fueled by ATP hydrolysis. Cytidine monophosphate (CMP) is derived from CTP with subsequent loss of two phosphates. 

Closely related to ATP, and present in biological systems, are a number of other nucleoside triphosphates and their corresponding diphosphates and monophosphates. These are uridine triphosphate (UTP), cytidine triphosphate (CTP), guanosine triphosphate (GTP), and thymidine triphosphate (TTP), all of which have about the same free energy of hydrolysis as ATP (11.29). In addition there are the deoxy nucleoside triphosphates (Chapter 10.4). 

The de novo pathway to 2 -deoxythymidine monophosphate (dTMP) synthesis first requires the use of dUMP (2 -deoxyuridine-5 -monophosphate) from the metabolism of either UDP or CDP (cytidine diphosphate). The hydrolysis of dUTP (2 -deoxyuridine-5 -triphosphate) to dUMP and subsequent methylation at C-5 by the action of thymidylate synthase, using A, A i°-methylenetetrahydrofolate (THF) as the methyl donor, generate dTMP (Figure 6.54). The latter is subsequently phosphorylated to deoxy-thymidine triphosphate (dTTP) used in DNA synthesis and repair. 

It had previously been shown by a number of workers, and in a number of systems (138, 809-818) that the AMP moiety of labeled ATP was incorporated into soluble RNA, and that this incorporation occurred predominantly in a terminal position (as shown by recovery of the label, after alkaline hydrolysis of the RNA, as adenosine rather than as 2 - and 3 -AMP). Hecht and co-workers (193,813,814) showed that the incorporation of ATP was very small when preincubated RNA was used, but could be restored completely by either prior or simultaneous addition of C3rtidine triphosphate (CTP). This suggested that the terminal AMP had to be attached to a cytidine monophosphate (CMP) end group. This was further corroborated by the fact that in the absence of ATP, but not in its presence, most of the incorporated CTP was, itself, terminal. For every mole of ATP incorporated, there seemed to be two moles of CTP taken up. The terminal sequence of sRNA, therefore, emerged as adenylyl-5 3 -cytidylyl-5 -3 -cytidylyl-5 -3 -RNA (RNApCpCpA) and that the presence of this end group was, indeed, necessary for attachment of amino acids was shown by further detailed experiments of Hecht et al. (188). 

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