Cytidine triphosphate synthesis

It was snbseqnently discovered that the first enzyme in the pathway for isoleucine synthesis, which is threonine deaminase, was inhibited by isoleucine in an extract of E. coli. No other amino acid caused inhibition of the enzyme. Threonine deaminase is, in fact, the rate-limiting enzyme in the pathway for isoleucine synthesis, so that this was interpreted as a feedback control mechanism (Fignre 3.13(a)). Similarly it was shown that the hrst enzyme in the pathway for cytidine triphosphate synthesis, which is aspartate transcarbamoylase, was inhibited by cytidine triphosphate (Fignre 3.13(b)). Since the chemical structures of isoleucine and threonine, or cytidine triphosphate and aspartate, are completely different, the qnestion arose, how does isolencine or cytidine triphosphate inhibit its respective enzyme The answer was provided in 1963, by Monod, Changenx Jacob. 

Figure 20.9 The positions in the pathway for de novo pyrimidine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. Glucose forms ribose 5-phosphate, via the pentose phosphate pathway (see chapter 6), which enters the pathway, after phosphorylation, as 5-phospho-ribosyl 1-pyrophosphate. Glutamine provides the nitrogen atom to synthesise carbamoylphos-phate (with formation of glutamate), and also to form cytidine triphosphate (CTP) from uridine triphosphate (UTP), catalysed by the enzyme CTP synthetase. It is the amide nitrogen of glutamine that is the nitrogen atom that is provided in these reactions.

Mutation of one of the two enzyme activities of UMP synthase leads to orotic aciduria, characterized by accumulation of its first substrate orotic acid and insufficient levels of the product UMP, which reduces availability of uridine triphosphate (UTP) and cytidine triphosphate (CTP) for use in nucleic acid synthesis. 

Figure 10.2. CTP luhibits ATCase. Cytidine triphosphate, an end product of the pyrimidine synthesis pathway, inhibits aspartate transcarbamoylase despite having little structural similarity to reactants or products.

The pathway for de novo synthesis of dolichol phosphate is the same as for cholesterol until production of famesyl pyrophosphate (Chapter 19). Two salvage pathways provide for the reutilization of dolichol and dolichol pyrophosphate once transfer of oligosaccharide from the dolichol to glycoprotein has occurred. A significant portion of the dolichol in liver is present as the free alcohol or as a fatty acid ester, and dolichol absorbed from the diet may enter this pool. Phosphorylation of dolichol is catalyzed by a dolichol kinase via cytidine triphosphate (CTP). Removal of one phosphate... 

The de novo pathways for phospholipid synthesis use cytidine triphosphate (CTP) for activation of intermediate species (analogous to the role of UTP in glycogen biosynthesis Chapter 15). The principal pathway of... 

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. 

Which is the rate-controlling step of pyrimidine synthesis that exhibits allosteric inhibition by cytidine triphosphate (CTP) ... 

This is the first step of the pathway that leads to the formation of cytosine, a building block for DNA synthesis. The form of cytosine that is used to synthesize DNA (and RNA) is the molecule cytidine triphosphate (CTP). When intracellular CTP concentrations are high, CTP molecules bind more often to the allosteric sites on aspartate transcarbamoylase molecules, causing a change in the shape of the enzyme that slows reaction 1 down markedly. Thus, CTP is an allosteric inhibitor of this enzyme. 

In the natural synthesis of phosphatidyl choline, the first step is believed to be the phosphorylation of choline, which itself has strongly basic properties of a tetraalkylammonium hydroxide. This is followed by reaction with cytidine triphosphate to give cytidine diphosphate choline, which in tnrn reacts with 1,2 diglyceride which is present (11.101). 

Roughan, P.G. (1985b) Cytidine triphosphate-dependent, acyl-CoA-indepen-dent synthesis of phosphatidylglycerol by chloroplasts isolated from spinach and pea. Biochim. Biophys. Acta 835, 527-532... 

The further conversion of UMP to cytidine nucleotides has not been demonstrated at the mononucleotide level. Apparently, uridine triphosphate was aminated by ammonia and ATP in E. coU to form cytidine triphosphate (CTP) no other nitrogen donors were able to perform the amination step (382, 383) (Fig. 22). Since the presence of glutamine was necessary for optimal synthesis of cytidine nucleotides from UMP in a homogenate of a hepatoma (384), it is possible that a different amination reaction exists in mammalian tissues. 

On the basis of the early observation that the increases in microsomal enzyme activity produced by phenobarbital and 3-methylcholanthrene were blocked by actinomycin-D, it was suggested that enzyme induction resulted from the synthesis of new enzyme protein which was, in turn, dependent upon the DNA-directed synthesis of a messenger-like RNA. Treatment of rats with 3-methylcholanthrene causes an increase of about 40% in the level of RNA in rat liver nuclei and the nuclear RNA from 3-methylcholanthrene-treated rats is more active in directing protein synthesis than RNA from control animals. Moreover, the in vitro incorporation of radioactive precursors such as orotic acid or cytidine triphosphate into nuclear RNA is 50 to 100% greater in preparations from 3-methylcholanthrene-treated animals than controls. It is of interest that treatment of rats with phenobarbital has been recently reported to result in a marked suppression of endogenous hepatic ribonuclease activity. 

While mammahan cells reutilize few free pyrimidines, salvage reactions convert the ribonucleosides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. ATP-dependent phosphoryltransferases (kinases) catalyze the phosphorylation of the nucleoside diphosphates 2 "-de-oxycytidine, 2 -deoxyguanosine, and 2 -deoxyadenosine to their corresponding nucleoside triphosphates. In addition, orotate phosphoribosyltransferase (reaction 5, Figure 34-7), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP). 

RNA polymerase, and the triphosphates of the purine ribonucleosides, uridine, and cytidine, and otherwise the same conditions, will prime the synthesis of RNA. The amounts of synthetic DNA or RNA are many fold greater than the amount of primer DNA the DNA product is nearly the same in most measurable ways as the primer DNA. The efficiency of the DNA in initiating these syntheses is known as the primer activity of the DNA, and can be affected by alterations of the bases which compose the nucleic acid, and by other factors. 

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