Nucleoside monophosphate kinases phosphorylation

All of these triphosphates take part in phosphorylations in the cell. Similarly, specific nucleoside monophosphate kinases catalyze the formation of nucleoside diphosphates from the corresponding monophosphates. 

Nucleoside Monophosphate Kinases Catalyzing Phosphoryl Group Exchange between Nucleotides Without Promoting Hydrolysis... 

Figure 9.45. Phosphoryl Group Transfer by Nucleoside Monophosphate Kinases. These enzymes catalyze the interconversion of a nucleoside triphosphate (here, ATP) and a nucleoside monophosphate (NMP) into two nucleoside diphosphates by the transfer of a phosphoryl group (shown in red).

How is the other major pyrimidine ribonucleotide, cytidine, formed It is synthesized from the uracil base of UMP, but UMP is converted into UTP before the synthesis can take place. Recall that the diphosphates and triphosphates are the active forms of nucleotides in biosynthesis and energy conversions. Nucleoside monophosphates are converted into nucleoside triphosphates in stages. First, nucleoside monophosphates are converted into diphosphates by specific nucleoside monophosphate kinases that utilize ATP as the phosphoryl-group donor (Section 9.4). For example, UMP is phosphorylated to UDP by UMP kinase. 

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... 

NUCLEOSIDE MONOPHOSPHATE KINASES CATALYZING PHOSPHORYL GROUP EXCHANGE BETWEEN NUCLEOTIDES WITHOUT PROMOTING HYDROLYSIS... 

FIGURE 9.45 Phosphoryl group transfer by nucleoside monophosphate kinases. 

AMP and GMP can be phosphorylated to the di- and triphosphate levels. The production of nucleoside diphosphates requires specific nucleoside monophosphate kinases, whereas the production of nucleoside triphosphates requires nucleoside diphosphate kinases, which are active with a wide range of nucleoside diphosphates. The purine nucleoside triphosphates are also used for energy-requiring processes in the cell and also as precursors for RNA synthesis (see Fig. 41.2). 

Because the majority of the known nucleoside monophosphate kinase reactions require an adenosine phosphate as one of the substrates, they may be classified as those which (1) are specific for adenylate as a phosphoryl acceptor, and (2) as those which require ATP as a phosphoryl donor. (The adenylate kinase reaction obviously may be placed in either category.) There may exist an additional class of nucleoside monophosphate kinase reactions in which adenosine phosphates do not participate however, such enzyme activity has not been unequivocally demonstrated. 

The nucleoside monophosphate kinases do not appear to catalyze the phosphorylation of inosinate. Hershko et al. 19) did not detect inosinate phosphorylation in stroma-free hemolysates of human erythrocytes. 

Nucleoside monophosphate kinases have been isolated which are specific for adenylate as the phosphoryl acceptor, but will accept various nucleoside triphosphates as phosphoryl donors. Such an enzyme activity was partly purified from hog kidney by Gibson et at. 20). Heppel et al. 21) have separated a phosphokinase activity from calf liver which accepts only adenylate as the nucleoside monophosphate substrate, but uses ATP, GTP, CTP, UTP, or ITP as the phosphate donor. 

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. 

The regeneration system for CMP-NeuAc is more complicated than that for NDP-sugars (Scheme 7) [24]. An additional phosphorylation step must be incorporated, because CMP, a nucleoside monophosphate, is released after reaction with the sialyltransferase. For recycling purposes, nucleoside monophosphate kinase (NMK EC 2.7.4.4) or myokinase (MK EC 2.7.4.3) is added for the conversion of CMP to CDP. In this reaction, the phosphoryl donor is ATP. Subsequently, both CDP and ADP must be re-phosphorylated to CTP and ATP, respectively. Thus, for regeneration of CMP-NeuAc, an additional kinase and two equivalents of PEP are required. The condensation of NeuAc with CTP is catalyzed by CMP-NeuAc synthetase (EC 2.7.7.43). This system was used for the large-scale synthesis of 6 -sialyl-LacNAc(6 -SLN) from LacNAc catalyzed by a2,6-SiaT (EC 2.7.7.43) in 97% yield. 

Entecavir, telbivudine, clevudine, and the other nucleoside analogues (Fig. 4aa) need to be phosphorylated to their 5 -triphosphate form to be antivirally active (Fig. 8). This again implies three phosphorylation steps based successively on a nucleoside kinase, nucleoside 5 -monophosphate kinase, and nucleoside 5 -diphosphate kinase. These reactions have been characterized only in a few cases, that is, thymidylate kinase in the metabolism of clevudine (Hu et al. 2005). 

In the preceding sections the conversion of purines and purine nucleosides to purine nucleoside monophosphates has been discussed. The monophosphates of adenosine and guanosine must be converted to their di- and triphosphates for polymerization to RNA, for reduction to 2 -deoxyribonucleoside diphosphates, and for the many other reactions in which they take part. Adenosine triphosphate is produced by oxidative phosphorylation and by transfer of phosphate from 1,3-diphosphoglycerate and phosphopyruvate to adenosine diphosphate. A series of transphosphorylations distributes phosphate from adenosine triphosphate to all of the other nucleotides. Two classes of enzymes, termed nucleoside mono-phosphokinases and nucleoside diphosphokinases, catalyse the formation of the nucleoside di- and triphosphates by the transfer of the terminal phosphoryl group from adenosine triphosphate. Muscle adenylate kinase (myokinase)... 

The synthesis of purine nucleotides (1) starts from IMP. The base it contains, hypoxanthine, is converted in two steps each into adenine or guanine. The nucleoside monophosphates AMP and CMP that are formed are then phos-phorylated by nucleoside phosphate kinases to yield the diphosphates ADP and GDP, and these are finally phosphorylated into the triphosphates ATP and CTP. The nucleoside triphosphates serve as components for RNA, or function as coenzymes (see p. 106). Conversion of the ribonucleotides into deoxyribo-nucleotides occurs at the level of the diphosphates and is catalyzed by nucleoside diphosphate reductase (B). 

Adenylate kinase performs the essential function of recovering AMP formed by many enzymatic processes and converting it to ADP (Eq. 6-65) which can be reconverted to ATP by oxidative or substrate level phosphorylation. The enzyme is present in all organisms. In vertebrates different isoenzymes function in the cytosol, mitochondrial intermembrane space, and mitochondrial matrix.862 863 A group of other nucleotide and deoxynucleotide kinases convert nucleoside monophosphates into diphosphates.864 865 Some of them, e.g., uridylate kinase are similar in structure and properties to adenylate kinase.866 867 Another member of the adenylate kinase family is phosphoribulokinase, an important photosynthetic enzyme (see Fig. 17-14, step a).868... 

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