Cytidine triphosphate synthetase

Cysteine proteases 482 Cytidine deaminase 359 Cytidine triphosphate synthetase 298 Cytochrome c, folding kinetics 551... 

Some of the reactions of PO3- parallel enzymatic reactions promoted by adenosine triphosphate (ATP). Pyruvate kinase catalyzes the equilibration of ATP and pyruvate with adenosine diphosphate (ADP) and phosphoenol pyruvate (11,12). In a formal sense, this reaction resembles the preparations of enol phosphate (eqs. 6 and 7). Cytidine triphosphate synthetase catalyzes the reaction of uridine triphosphate with ammonia to yield cytidine triphosphate (13). In a formal sense, this reaction resembles the replacement of the ester carbonyl group of ethyl acetate by the nitrogen of aniline (eq. 8). 

Hendriks, E. F., O Sullivan, W. J., and Stewart, T. S. (1998). Molecular cloning and characterization of the Plasmodium falciparum cytidine triphosphate synthetase gene. Biochim. Biophys. Acta 1399, 213-218. 

Cytidine triphosphate synthetase (CTPS) mediates the conversion of uridine triphosphate (UTP) into cytidine... 

Aspartate carbamyltransferase 2 dihydroorotase 3 orotate reductase 4 orotate phosphoribosyl-transferase 5 orotidine-5 -phosphate decarboxylase 6 cytidylate kinase, nucleotide diphosphate kinase 7 cytidine triphosphate synthetase 8 nucleoside monophosphate kinase, ribonucleoside diphosphate reductase, phosphatase 9 thymidylate synthase... 

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.

Cytidine triphosphate (CTP) is produced by amination of UTP by CTP synthetase (Figure 22.22). [Note The nitrogen is provided by glutamine—another example of a reaction in nucleotide biosynthesis in which this amino acid is required.]... 

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. 

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. 

Scheme L Synthesis of a2,64inked sialyl-N-acetyllactosamine using a one-pot multi-enzyme system with in situ regeneration of CMP-Neu5Ac. Abbreviations for enzymes CSS, CMP-sialic acid synthetase NMK, nucleoside monophosphate kinase PK, pyruvate kinase PPase, pyrophosphatase. Abbreviations for compounds PEP, phosphoenolpyruvate ADP, adenosine 5 -diphosphate ATP, adenosine 5 -triphosphate CMP, cytidine 5-monophosphate CDP, cytidine 5 -diphosphate CTP, cytidine 5-triphosphate LacNAc, N-acetyllactosamine NeuSAc, N-acetylneuraminic acid PPi, inorganic pyrophosphate.

Regulatory control of bacterial thymidylate synthetases by nucleotides has not been reported however, Lorenson et al. (7) investigated that of the chick embryo thymidylate synthetase and found that the 5 -mono-, di-, and triphosphates of adenosine, cytidine, guanosine, and uridine, and of their deoxyribosides, did not inhibit activity. Thus, the activity of the synthetase, from this source at least, does not seem to be subject to allosteric regulation by nucleotides. With crude extracts of Ehrlich ascites... 

CMP-Neu5Ac synthetase cytidine-5 -triphosphate Inorganic pyrophosphatase... 

---