Guanosine-51-diphosphate kinase

The subsequent cleavage of the thio-ester succinylCoA into succinate and coenzyme A by succinic acid-CoA ligase (succinyl CoA synthetase, succinic thiokinase) is strongly exergonic and is used to synthesize a phosphoric acid anhydride bond ( substrate level phosphorylation , see p. 124). However, it is not ATP that is produced here as is otherwise usually the case, but instead guanosine triphosphate (CTP). However, GTP can be converted into ATP by a nucleoside diphosphate kinase (not shown). 

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... 

If one acetyl -CoA involved in TCA cycle gives = 10 ATPs ATPs due 8 acetyl-CoA = 8 X 10 = 80 ATPs due to 7 FADH2 = 1.5 x 7= 10.5 ATPS due 7 (NADH + H ) = 2.5 x 7.5 = 17.5. The total of number ATPs produced 108. During the initiation of the y - oxidation pathway a 2 ATPs converts into a 2 AMP and 2 Pi for the activation of fatty acid. So, net ATPs produced by palmitic acid are 106. These calculations assume that mitoehondrial oxidative phosphorlation produces 1.5 ATPS/FADH2 oxidized and 2.5 ATP/NADH2 oxidized. The Guanosine-5 -triphosphate (GTP) produced directly in the acid citric cycle yields ATP in the reaetion eatalyzed by nucleoside diphosphate kinase [99, 130]. 

Recently, a universal enzyme-coupled fluorescence assay for glycosyl transferases was developed. This method is extremely cost-effective and is based on the quantification of nucleotides produced in the glycosyl transfer reaction. The guanosine diphosphate (GDP), uridine diphosphate (UDP), and cytidine monophosphate (CMP) are phos-phorylated with nucleotide kinase in the presence of excess of ATP, generating ADP. Via coupled enzyme reactions involving ADP-hexokinase,glucose-6-phosphate dehydrogenase, and diaphorase, the ADP is utilized for the conversion of resazurin to resorufin, which is then quantified by fluorescence measurement. 

In most of these reactions adenine nucleotides are employed although reactions involving nucleotides of the uridine, cytidine, and guanosine series are well known. It should be noted that nucleosides themselves and nucleotide monophosphates are surprisingly inactive in biosynthetic processes. Thus, nucleoside kinases, monophosphate kinases, and diphosphate kinases, which convert nucleoddes and nucleoside monophosphates to the di- and triphosphate stage are incUqiensable for cellular bio yntheds. 

The antiviral mechanism of action of acyclovir has been reviewed (72). Acyclovir is converted to the monophosphate in herpes vims-infected cells (but only to a limited extent in uninfected cells) by viral-induced thymidine kinase. It is then further phosphorylated by host cell guanosine monophosphate (GMP) kinase to acyclovir diphosphate [66341 -17-1], which in turn is phosphorylated to the triphosphate by unidentified cellular en2ymes. Acyclovir triphosphate [66341 -18-2] inhibits HSV-1 viral DNA polymerase but not cellular DNA polymerase. As a result, acyclovir is 300 to 3000 times more toxic to herpes vimses in an HSV-infected cell than to the cell itself. Studies have shown that a once-daily dose of acyclovir is effective in prevention of recurrent HSV-2 genital herpes (1). HCMV, on the other hand, is relatively uninhibited by acyclovir. 

All NRTIs, as exemplified for AZT (Fig. 7), act in a similar fashion following their uptake by the cells, they are phosphorylated successively to their 5 -monophosphate, 5 -diphosphate, and 5 -triphosphate form (De Clercq 2002). Unlike the first phosphorylation step in the metabolic pathway of the acyclic guanosine analogues (see above), which is carried out by a virus-encoded enzyme (thymidine kinase), the first as well as the subsequent phosphorylations of the 2, 3 -dideoxynucleosides are carried out by cellular enzymes, that is, a 2 -deoxynucleoside (e.g., dThd) kinase, a 2 -deoxynucleotide (e.g., dTMP) kinase, and a (2 -deoxy)nucleoside 5 -diphosphate (NDP) kinase. 

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)... 

Fig. 11. Modes of action of fluorine on osteoblastic cells, (a) Tyrosine phosphatase hypothesis in osteoblastic cells, fluoride ion directly inhibits tyrosine phosphatase. Inhibition of this enzyme enhances the tyrosine phosphorylation of signalling molecules induced by receptor tyrosine kinase, which leads to activation of the extracellular signal-regulated kinase (ERK) through the Ras pathway and enhanced cell proliferation, (b) G-protein hypothesis in osteoblast-like cells, fluoride ions form a complex with aluminum, probably fluoroaluminate, which interacts with guanosine 5 -diphosphate (GDP) to form guanosine 5 -triphosphate (GTP)-like molecule. Activation of the G, protein stimulates the tyrosine phosphorylation of signalling molecules by a yet unknown tyrosine kinase (Tyr Kin) and activation of the ERK kinase through the Ras pathway leads to enhanced cell proliferation. (Reproduced by permission of Elsevier from Ref. [175] ...

CDK2, cell division kinase 2 cDNA, complementary DNA CDP, cytidine 5 -diphosphate CDPK, Ca2+-dependent protein kinase, calmodulin domain protein kinase CFTR, cystic fibrosis transmembrane conductance regulator cGMP, 3, 5 -c.yclic guanosine monophosphate cGMP PDE, cyclic GMP phosphodiesterase... 

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