Guanosine triphosphate reactions

The primary function of the dtric acid cycle is oxidation of acetyl CoA to carbon dioxide. The energy released from this oxidation is saved as NADH, FADHj, and guanosine triphosphate (GTP). The overall result of the cyde is represented by the following reaction ... 

Note that this overall reaction requires three coenzymes that we encountered as metabolites of vitamins in chapter 15 NAD+, derived from lucotiiuc acid or nicotinamide FAD, derived from riboflavin and coenzyme A(CoASH), derived from pantothenic acid. In the overall process, acetyl-SCoA is oxidized to two molecules of carbon dioxide with the release of CoASH. Both NAD+ and FAD are reduced to, respectively, NADH and FADH2. Note that one molecule of guanosine triphosphate, GTP, functionally equivalent to ATP, is generated in the process. 

Biaglow JE, Kachur AV (1997) The generation of hydroxyl radicals in the reaction of molecular oxygen with polyphosphate complexes of ferrous ion. Radiat Res 148 181-187 Biaglow JE, Field KD, Manevich Y, Tuttle S, Kachur A, Uckun F (1996) Role of guanosine triphosphate in ferric ion-linked Fenton chemistry. Radiat Res 145 554-562 Bielski BHJ (1991) Studies of hypervalent iron. Free Radical Res Commun 12/13 469-477 Bielski BHJ, Allen AO, Schwarz HA (1981) Mechanism of disproportionation of ascorbate radicals. J Am Chem Soc 103 3516-3518... 

The aminoacyl transfer reaction, one of the latter stages in protein synthesis, involves incorporation of amino acids from soluble ribonucleic acid-amino acid into ribosomal protein. This reaction requires guanosine triphosphate and a soluble portion of the cell. Evidence has been obtained with rat liver preparations that aminoacyl transfer is catalyzed by two protein factors, aminoacyl transferases (or polymerases) I and n, which have been resolved and partially purified from the soluble fraction. Transferase n activity has also been obtained from deoxycholate-soluble extracts of microsomes. With purified transferases I and n, incorporation is observed with relatively low levels of GTP its sulfhy-dryl requirement is met by a variety of compounds. The characteristics of this purified amino acid incorporating system, in terms of dependency on the concentration of its components, are described. 

Jhe synthesis of proteins, as characterized by the in vitro incorporation of amino acids into the protein component of cytoplasmic ribonu-cleoprotein, is known to require the nonparticulate portion of the cytoplasm, ATP (adenosine triphosphate) and GTP (guanosine triphosphate) (15, 23). The initial reactions involve the carboxyl activation of amino acids in the presence of amino acid-activating enzymes (aminoacyl sRNA synthetases) and ATP, to form enzyme-bound aminoacyl adenylates and the enzymatic transfer of the aminoacyl moiety from aminoacyl adenylates to soluble ribonucleic acid (sRNA) which results in the formation of specific RNA-amino acid complexes—see, for example, reviews by Hoagland (12) and Berg (1). The subsequent steps in pro-... 

Nucleotides play central roles in metabolism. They serve as sources of chemical energy (ATP and guanosine triphosphate (GTP)), participate in cellular signalling (cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP)) and are incorporated into important cofactors of enzymatic reactions. Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA (Scheme 3). 

The precursors for riboflavin biosynthesis in plants and microorganisms are guanosine triphosphate and ribulose 5-phosphate. As shown in Figure 7.3, the first step is hydrolytic opening of the imidazole ring of GTP, with release of carbon-8 as formate, and concomitant release of pyrophosphate. This is the same as the first reaction in the synthesis ofpterins (Section 10.2.4), but utilizes a different isoenzyme of GTP cyclohydrolase (Bacher et al., 2000, 2001). 

STEPS s-6 Hydrolysis and dehydrogenation of succinyl CoA. Succinyl CoA is hydrolyzed to succinate in step 5. The reaction is catalyzed by succinyl CoA synthetase and is coupled with phosphorylation of guanosine diphosphate (GDP) to give guanosine triphosphate (GTP). The overall transformation is similar to that of step 8 in glycolysis (Figure 29.4), in which a thiol ester is converted into an acyl phosphate and a phosphate group is then transferred to ADP. 

In this reaction, inosine triphosphate (ITP) can substitute for guanosine triphosphate (GTP), and the CO2 lost is the one fixed in the carboxylase reaction. The net result of these reactions is... 

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... 

This subsequent splitting of succinyl-CoA releasing CoA and succinic acid is used to drive a substrate level phosphorylation reaction, but this time it results in the formation, not of ATP but of another nucleoside triphosphate, guanosine triphosphate (GTP) from GDP. GTP can of course be utilized by the cell in exactly the same way as ATP, and can be converted to ATP directly at the expense of ADP, as shown below. 

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