Cytidine nucleotide pathway

The synthesis of phosphatidylcholine (Ptd-choline) in animal tissues is carried out chiefly by the cytidine nucleotide pathway, although base-exchange reaction and stepwise methylation of preexisting phosphatidylethanolamine (Ptd-ethanolamine) also contribute to its formation "7 The N-methylation pathway, first demonstrated in liver by Bremer and Greenberg and successively described in this tissue by several authors, has not been however unequivocally demonstrated in brain, and conflicting data have been produced in this c onnection, ... 

Fig. 2. The pathway to phosphatidylcholine involving cytidine nucleotides (Kennedy, 1957).

Biosynthesis of uridine and cytidine nucleotides. This is shown in Fig.l. The first pyrimidine nucleotide to appear de novo in this pathway is uridine 5 -mono-phosphate (UMP, uridylic acid). This is phosphoryla-ted to uridine 5 -triphosphate (UTP). By donation of an amino group from ammonia or glutamine (in animal tissues), UTP is converted to cytidine 5 -triphos-phate (CTP). 

Perhaps the best-studied regulatory enzyme of this kind is aspartate transcarbamoy-lase (ATCase), which catalyzes the first steps in the biosynthetic pathway leading to uridine and cytidine nucleotides. Today, a great amount of information, structural, thermodynamical, and kinetic, is available on this system. Specifically it is known that cytidine triphosphate (CTP) inhibits ATCase. (Other molecules, such as ATP, activate the same enzyme. We shall focus, in this section, on inhibitory effectors only.)... 

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.

CMP-KDO synthetase (cytidine-5 -triphosphate cytidine-5 -mon-ophosphate-3-deoxy-D-manno-octulosonate cytidylyltransferase), the next enzyme in the pathway, catalyzes the formation of the nucleotide sugar, CMP-KDO from CTP and KDO. This enzyme was first studied by Ghalambor and Heath (IT). We have purified this enzyme to homogeneity (27). T i apparent K values for CTP nd KDO in the presence of 10 mM Mg were determined to be 2 x 10 M and 2.9 x 10 M, respectively. The enzy tic reaction was dependent upon the addition of CTP, KDO and Mg but did not require a reducing agent. The formation of CMP-KDO was not inhibited by the addition of CDP, CMP, KDO-8-phosphate or N-acetylneuraminic acid to the complete reaction mixture. In agreement with Ghalambor and Health (17), neither KDO-8-phosphate nor N-acetylneuraminic acid could substitute for KDO in the reaction mixture. Pyrophosphate, one of the end products, is a weak inhibitor of the reaction with an apparent Ijq value of 5.0 mM. The addition of CMP,CD or any of the other mono- or di-nucleotides did not inhibit the reaction. 

In accordance with the foregoing, i.e. that substitution at Nt does not affect the reduction pathway, the nucleoside and nucleotide react at the mercury electrode essentially like the bases36 37 but adsorb more strongly than cytosine at a potential more positive than —1.6 V 37,48). The EI/2 for the reduction wave in the cytosine series becomes more positive in the order base > nucleotide > nucleoside, and is linearly pH-dependent 37,53). The mechanism for electrochemical reduction of cytosine, cytidine, CMP and CpC have been considered in terms of their structure, association in solution and adsorption 37). It was concluded that the deamination step for CpC occurs very slowly or not at all 37). 

Aspartate transcarbamoylase catalyzes the first step in the biosynthesis of pyrimidines, bases that are components of nucleic acids. The reaction catalyzed by this enzyme is the condensation of aspartate and carbamoyl phosphate to form A-carbamoylaspartate and orthophosphate (Figure 10.1). ATCase catalyzes the committed step in the pathway that will ultimately yield pyrimidine nucleotides such as cytidine triphosphate (CTP). How is this enzyme regulated to generate precisely the amount of CTP needed by the cell ... 

The utilization of ammonia resulting from the combination of carbamyl phosphate with aspartic acid, the initial reaction for the synthesis of the pyrimidine nucleotides, continues only as long as there is a requirement for them (Fig. 3). Regulation of this biosynthetic pathway is probably by way of feedback inhibition of aspartate transcarbamylase. The rat liver enzyme is inhibited by uridine, cytidine or thymidine or such derivatives as CMP, UTP, or TMP, all intermediates or products of this pathway (B8). This is not the only enzyme of the pathway which may be subject to feedback regulation. Dihydroorotase from rat liver is also inhibited by some pyrimidines and purines (B9). 

Uridine-cytidine kinase is an important enzyme in pyrimidine nucleotide biosynthesis through the salvage pathway. In addition, this enzyme is also responsible for the 5 -phosphorylation of pyrimidine nucleoside analogs used... 

Scheme 22 The whole biosynthetic pathway of sugar nucleotides. ATP, adenosine triphosphate Gal-1 -P, galactose-1-phosphate UTP, uridine triphosphate UDP, uridine diphosphate NAD, nicotinamide adenine dinucleotide Fru, fructose AcCoA, acetyl coenzyme-A PEP, phosphoenolpyruvate CTP, cytidine triphosphate NADP, nicotinamide adenine dinucieotide phosphate GTP, guanosine triphosphate.

Deamination of purine and pyrimidine bases (Figures 6(b) and 6(c)) is an important reaction in nucleotide salvage pathways and RNA editing. In nucleosides, two of the most characterized reactions involve the conversion of adenosine and cytidine to form inosine and uridine (with the elimination of one molecule of... 

FIG. 6.13 Mammalian pyrimidine salvage and interconversion pathways. Enzymes listed in Figs 6.13-6.17 are as follows (1) deoxyCMP deaminase (2) thymidylate synthase (3) ribonucleotide reductase (4) deoxyuridine triphosphatase (5) CTP synthetase (6) nucleotide kinase (7) deoxyTMP kinase (8) nucleotide diphosphokinase (9) non-specific phosphatase or nucleotidase (10) cytidine kinase (11) pyrimidine phos-phorylase or hydrolase (12) uracil PRTase (13) cytidine deaminase (14) thymidine kinase (15) cytidine phosphotransferase (16) uridine phosphotransferase (17) thymidine phosphotransferase (18) deoxyribo-nucleotide phosphotransferase (19) cytosine PRTase. 

ATCase catalyzes the committed step in the pathway that will ultimately yield pyrimidine nucleotides such as cytidine triphosphate (GTP). How is this enzyme regulated to generate precisely the amount of CTP needed by the cell ... 

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