Nucleoside monophosphate kinases reaction

These reactions, nucleoside monophosphate kinase (reaction 1) and nucleoside diphosphate kinase (reaction 2), involve the formation and... 

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. 

Seagrave, J. Reyes, P. Pyrimidine nucleoside monophosphate kinase from rat bone marrow cells a kinetic analysis of the reaction mechanism. Arch. Biochem. Biophys., 254, 518-525 (1987)... 

ATP also brings about the formation of other nucleoside diphosphates by the action of a class of enzymes called nucleoside monophosphate kinases. These enzymes, which are generally specific for a particular base but nonspecific for the sugar (ribose or de-oxyribose), catalyze the reaction... 

The regeneration system for CMP-NeuAc can be employed both for a2,3-sialyltransferase-catalyzed reactions and for reactions mediated by a2,6-sialyltransferase. The system starts with NeuAc, the glycosyl acceptor, PEP, and catalytic amounts of ATP and CMP. CMP is converted to CDP by nucleoside monophosphate kinase (EC 2.7.4.4 NMK) in the presence of ATP, which is regenerated from the by-product ADP, catalyzed by PK in the presence ol PEP, then to CTP with PEP by PK. The CTP thus formed reacts with NeuAc, catalyzed b>... 

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... 

Other nucleoside triphosphate are synthesized in ATP-requiring reactions catalyzed by a series of nucleoside monophosphate kinases ... 

Energy-requiring reactions often generate the nucleoside diphosphate ADP. Adenylate kinase, an important enzyme in cellular energy balance, is a nucleoside monophosphate kinase that transfers a phosphate from one ADP to another ADP to form ATP and AMP ... 

Examine the reaction of nucleoside monophosphate kinase shown on page 252 of the primary text. Estimate what the equilibrium constant would be for the reaction under standard biochemical conditions. 

The acceptability of deoxyguanylate and dATP as substrates in the guanylate kinase reaction is noteworthy there does not appear to be a particular group of nucleoside monophosphate kinases specific for deoxyribonucleotides (see Chapter 14), apart from that for thymidylate and deoxyuridylate. 

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. 

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 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)... 

Consider as an example nucleoside monophosphate (NMP) kinase, an enzyme that we will examine in detail in the next chapter (Section 9.4). It catalyzes the following reaction ... 

During muscle contraction AMP deaminase activity increases. Nucleoside triphosphates are negative modulators, whereas nucleoside di- and monophosphates are positive modulators of the enzyme. The increased AMP deaminase activity prevents accumulation of AMP so that the adenylate kinase reaction favors the formation of ATP ADP - -ADP ATP -k AMP. 

Nucleoside kinases are a class of enzymes that catalyze the phosphorylation of nucleosides to make nucleoside monophosphates (Figure 22.2) as part of nucleotide biosynthetic salvage pathways. ATP provides the energy and phosphate for the reaction. Example enzymes include thymidine kinase, deoxycytidine kinase, and deoxyguanosine kinase. 

Pyrimidine bases are normally salvaged by a two-step route. First, a relatively nonspecific pyrimidine nucleoside phosphorylase converts the pyrimidine bases to their respective nucleosides (Fig. 41.17). Notice that the preferred direction for this reaction is the reverse phosphorylase reaction, in which phosphate is being released and is not being used as a nucleophile to release the pyrimidine base from the nucleoside. The more specific nucleoside kinases then react with the nucleosides, forming nucleotides (Table 41.2). As with purines, further phosphorylation is carried out by increasingly more specific kinases. The nucleoside phosphorylase-nucleoside kinase route for synthesis of pyrimidine nucleoside monophosphates is relatively inefficient for salvage of pyrimidine bases because of the very low concentration of the bases in plasma and tissues. 

The nucleoside monophosphates are converted to the triphosphates (the direct precursors of RNA) by two kinase reactions These kinases have a low specificity, and they catalyse the phosphorylation of nucleotides of adenine, guanine and the pyrimidines (Fig. 3). An alternative route for the synthesis of purine nucleotides is the Salvage pathway (see). 

This question was resolved by demonstrations of the intermediate formation of dTDP by Grav and Smellie (38) and by Ives 39). Ives studied the phosphorylation of thymidylate in extracts of the Novikoff rat hepatoma, in which nucleoside diphosphate kinase activity was about three orders of magnitude higher than that of the other enzymes of thymine metabolism present (thymidine and thymidylate kinases, thymidylate synthetase, and thymidylate phosphatase). Ives showed that dTDP was an intermediate in dTTP formation by a method which was independent of the monophosphate and diphosphate kinase reactions. When ATP[7 P] was employed as the phosphorylating agent in the tumor extracts, the dTTP product was labeled in both y- and /3-phosphates when ATP[/3- P] was employed, dTTP was essentially unlabeled ... 

---