Nucleotide kinases, reaction catalyzed

UTP is formed from UMP by two reactions catalyzed by nucleotide kinases that use ATP as the phosphate donor. 

This appears to occur via a heme-sensitive eIF2a kinase whose catalytic activity is inhibited by heme.322 327 331 The [NAD1] /[NADH] ratio may also be a factor in controlling the nucleotide-exchange GEF-catalyzed reaction.332... 

Although purine nucleosides are intermediates in the catabolism of nucleotides and nucleic acids in higher animals and humans, these nucleosides do not accumulate and are normally present in blood and tissues only in trace amounts. Nevertheless, cells of many vertebrate tissues contain kinases capable of converting purine nucleosides to nucleotides. Typical of these is adenosine kinase, which catalyzes the reaction... 

Nucleoside triphosphates are the most common nucleotide used in metabolism. They are formed in the following manner. Recall that ATP is synthesized from ADP and P, during certain reactions in glycolysis and aerobic metabolism. ADP is synthesized from AMP in a reaction catalyzed by adenylate kinase ... 

Fluorouracil (5-FU) requires enzymatic conversion to the nucleotide (ribosylation and phosphorylation) in order to exert its cytotoxic activity. Several routes are available for the formation of floxuridine monophosphate (FUMP). 5-FU may be converted to fluorouridine by uridine phos-phorylase and then to FUMP by uridine kinase, or it may react directly with 5-phosphoribosyl-l-pyrophosphate (PRPP), in a reaction catalyzed by orotate phosphoribosyl transferase, to form FUMP. Many metabolic pathways are available to FUMP. As the triphosphate FUTP, it may be incorporated into RNA. An alternative reaction sequence... 

Fig. 41.10. Salvage of bases. The purine bases hypoxanthine and gnanine react with PRPP to form the nucleotides inosine and gnanosine monophosphate, respectively. The enzyme that catalyzes the reaction is hypoxanthine-gnanine phosphoribosyltransferase (HGPRT). Adenine forms AMP in a reaction catalyzed by adenine phosphoribosyltransferase (APRT). Nucleotides are converted to nucleosides by 5 -nucleotidase. Free bases are generated from nncleosides by purine nucleoside phosphorylase. Deamination of the base adenine occurs with AMP and adenosine deaminase. Of the purines, only adenosine can be directly phosphorylated back to a nucleotide, by adenosine kinase.

The addition of a third phosphate to any of the nucleoside pyrophosphates, such as UDP, is catalyzed by an enzyme found in yeast and several animal tissues. Since the reaction catalyzed by this enzyme appears to be nonspecific with respect to the base of the nucleotides, it has been named nucleoside diphosphate kinase (nudiki). The phosphate donor in this reaction may be any of the nucleoside triphosphates. As in the case of the nucleoside monophosphate kinases, there are no significant differences in the free energies of hydrolysis of the various nucleoside triphosphates, so all of the reactions are freely reversible with equilibrium constants near 1. Since the phosphorylation of nucleoside diphosphates is reversible, it is necessary that each of the corresponding triphosphates serve as phosphate donor. The phosphorylation of the monophosphates is similarly reversible, but in this case one of the sites on the enz3one appears to react only with adenine, and involves the conversion of ATP to ADP. The complementary reaction involves XMP XDP. Thus the nonadenine nucleotides are never equivalent to ATP, and the reaction is limited to the phosphorylation of a specific nucleoside monophosphate by ATP. 

Here, associated with the adenilate-kinase-like reaction (2ADP ATP -h AMP). This adenilatekinase-like reaction catalyzed by the chloroplast coupling factor CFi was demonstrated in the laboratory of Moudrianakis [181]. 

Draw the mechanism for the reaction catalyzed by nucleotide diphosphate kinase. 

These reactions are catalyzed by kinases, some of viiich have already been discussed in this chapter. The reaction may be virtually irreversible as in phosphate ester formation. Here the phosphate acceptor may be a hydroi l group of a carbohydrate (glucose, ycerol, fructose, nucleotides, etc.), (Moline, or pantetheine. The terminal phosphate may also be transferred to an acceptor without loss of high chemical potential, such as to nucleoside mono- or diphosphate or to a nitro n atom. These reactions are freely reversible. Nucleotide diphosphate may also donate its terminal phosphate as in the nucleotide monophosphate kinase reaction. 

The reaction catalyzed by choline kinase (E.C. 2.7.1.32 ATPtchollnephosphotransferase) is the first committed reaction of the nucleotide pathway for phosphatidylcholine (PtdCho) biosynthesis. Various theoretical considerations have led to the suggestion that this reaction is far from equilibrium and, along with the cytldylyltransferase reaction, may be rate-limiting for PtdCho biosynthesis in some animal tissues (1). As part of our studies on the regulation of PtdCho biosynthesis in plants we have investigated the properties of this enzyme and the in vivo concentrations of metabolites Involved in the reaction in postgermlnatlon castor bean endosperm. 

Hayaishi and colleagues, who devised the purification for the Brevibacter-ium liquefaciens enzyme, used it to characterize the reversibility of the adenylate cyclase reaction (Kurashina et ai, 1974) and found that the equilibrium constant for the reaction written in the direction of cyclic AMP formation is 0.12 Mat pH 7.3 at this pH the rates of the forward and reverse reactions are comparable but about the rate of the forward reaction measured at its pH optimum, pH 9. Our plan for determining the stereochemical course of the reaction is shown in Fig. 14. Since we had synthesized the diastereomers of cyclic [, 0]dAMP, we would use the cyclase to catalyze their pyrophosphorolysis and form the diastereomers of [a- 0, 0]dATP. However, the thermodynamics of the cyclase reaction prevents an efficient conversion of cyclic dAMP to dATP, so this reaction was coupled to the glycerol kinase reaction the kinase reaction utilizes the thermodynamic instability of the )J,y-anhydride bond to displace the overall equilibrium to favor the synthesis of the diastereomers of [a- 0, 0]dADP. Both the cyclase and glycerol kinase can utilize deoxyadenosine nucleotides as substrates, but only the cyclase reaction can alter the configuration of the chiral phosphorus atoms. 

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). 

This enzyme [EC 2.7.1.11], also known as phosphohexo-kinase and phosphofructokinase 1, catalyzes the reaction of ATP with D-fructose 6-phosphate to produce ADP and D-fructose 1,6-bisphosphate. Both D-tagatose 6-phosphate and sedoheptulose 7-phosphate can act as the sugar substrate. UTP, CTP, GTP, and ITP all can act as the nucleotide substrate. This enzyme is distinct from that of 6-phosphofructo-2-kinase. See also ATP GTP Depletion... 

ATP is the primary high-energy phosphate compound produced by catabolism, in the processes of glycolysis, oxidative phosphorylation, and, in photosynthetic cells, photophosphorylation. Several enzymes then cany phosphoryl groups from ATP to the other nucleotides. Nucleoside diphosphate kinase, found in all cells, catalyzes the reaction... 

Most kinases transfer chiral phospho groups with inversion and fail to catalyze partial exchange reactions that would indicate phosphoenzyme intermediates. However, nucleoside diphosphate kinase contains an active site histidine which is phosphorylated to form a phosphoenzyme.869 The enzyme catalyzes phosphorylation of nucleoside diphosphates other than ADP by a nucleotide triphosphate, usually ATP. 

Adenylate cyclase is considered as a second messenger that catalyzes the formation of cAMP (cyclic adenosine monophosphate) from ATP this results in alterations in intracellular cAMP levels that change the activity of certain enzymes—that is, enzymes that ultimately mediate many of the changes caused by the neurotransmitter. For example, there are protein kinases in the brain whose activity is dependent upon these cyclic nucleotides the presence or absence of cAMP alters the rate at which these kinases phosphorylate other proteins (using ATP as substrate). The phosphorylated products of these protein kinases are enzymes whose activity to effect certain reactions is thereby altered. One example of a reaction that is altered is the transport of cations (e.g., Na+, K+) by the enzyme adenosine triphosphatase (ATPase). 

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... 

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