Nucleoside diphosphokinase and

The high-energy phosphate bonds of UTP, GTP, and CTP are energetically equivalent to ATP and are synthesized from ATP by nucleoside diphosphokinases and nucleoside monophosphokinases. For example, UTP is formed from UDP by a nucleoside diphosphokinase in the reaction ... 

My involvement with enzymes, from the beginning, focused on what Aey do and how they do it. Such concerns led Bill Joklik and me, as postdoctoral fellows in Herman Kal-ckar s laboratory, to the discovery of nucleoside diphosphokinase and the recognition of its essential role in using ATP for the formation of the then newly discovered other nucleoside triphosphates. 

UDP and UTP are selective agonists of certain of the P2Y receptors. It is not yet clear what factors control the release of uridine nucleotides into the extracellular space. UTP can be formed from UDP in the extracellular space by the action of the enzyme nucleoside diphosphokinase, which catalyzes the transfer of the -phosphate of nucleoside triphosphates to nucleoside diphosphates, e.g., ATP + UDP —> ADP + UTP. 

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)... 

Lam, S.C.T. Packham, M.A. Isolation and kinetic studies of nucleoside diphosphokinase from human platelets and effects of cAMP phosphodiesterase inhibitors. Biochem. Pharmacol., 35, 4449-4455 (1986)... 

Colomb, M.G. Cheruy, A. Vignais, P.V. Nucleoside diphosphokinase from beef heart cytosol. I. Physical and kinetic properties. Biochemistry, 11, 3370-3378 (1972)... 

Nakamura, H. Sugino, Y. Metabolism of deoxyribonucleotides. 3. Purification and some properties of nucleoside diphosphokinase of calf thymus. J. Biol. Chem., 241, 4917-4922 (1966)... 

The phosphorylation of GDP in this reaction is an example of substrate-level phosphorylation, and this is the only reaction in the citric acid cycle to produce a high-energy phosphate bond directly. The energy for this phosphorylation is derived from the hydrolysis of the thioester bond of succinyl-CoA. Subsequently, GTP phosphorylates ADP, catalyzed by nucleoside diphosphokinase, but this reaction... 

The GTP formed as described above yields ATP [via nucleoside diphosphokinase]. The reduced coenzymes (4 NADH and FADH2) feed electrons into the mitochondrial electron transport chain (ETC) to yield 14 ATP per pyruvate oxidized (12 ATP/4 NADH and 2 ATP/FADH2) through the process of oxidative phosphorylation as described in Section 13.5. This total yield of ATP corresponds to a total of 32 ATP per glucose oxidized plus a further 6 ATP from mitochondrial oxidation of NADH generated in glycolysis, that is, 38 ATP per glucose oxidized. 

What is the cost of converting glucose 6-phosphate into glycogen and back into glucose 6-phosphate The pertinent reactions have already been described, except for reaction 5, which is the regeneration of UTP. ATP phosphorylates UDP in a reaction catalyzed by nucleoside diphosphokinase. 

UDP is a nucleotide intermediate in pyrimidine biosynthesis. It is produced from UMP by UMP kinase, as shown in Figure 22.10 and is converted to UTP by nucleoside diphosphokinase (also in Figure 22.10). 

Pyruvate may enter the Krebs cycle by a pathway different from that involving the formation of acetyl-CoA. Indeed, the keto acid can be converted to a dicarboxylic acid by at least two different biochemical processes. One of them involves the carboxylation of pyruvic acid leading to the formation of oxaloacetic acid. The actual substrate in that reaction is not pyruvic acid, but phosphoenolpyruvic acid. The enzyme involved requires manganese, and IDP acts as the phosphorus acceptor. The reaction is coupled with the pyrophosphate transfer (catalyzed by nucleoside diphosphokinase) from ITP to ADP. 

These reactions are catalyzed by the enzymes nucleoside monophos-phokinase and nucleoside diphosphokinase, respectively. Note that these reactions are reversible, so that ATP may be synthesized at the expense of GTP or another nucleoside triphosphate. The precursor ADP (adenosine diphosphate) may also be synthesized from the reaction of AMP with ATP, catalyzed by the enzyme adenylate kinase ... 

Nucleoside diphosphokinase has been investigated in yeast, muscle, and intestinal mucosa 118, 119). The enzyme has been obtained free of nucleoside monophosphokinase and has been shown to catalyze the following reactions ... 

The adenine nucleotide carrier or translocase is specifically inhibited by atractyloside (competitive with respect to adenine nucleotide) and bongkrekic acid (noncompetitive). Atractyloside has been known for some time to be highly poisonous and its mechanism of action attests to the importance of the adenine nucleotide translocase. Other nucleotides, such as GTP, must first be converted to ATP by nucleoside diphosphokinase prior to transport out of the mitochondria. The outer mitochondrial compartment also contains nucleoside diphosphokinase for the conversion of ATP to GTP. 

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