Thiokinase

The citric acid cycle One is generated directly in the cycle at the succinyl thiokinase step (Figure... 

Succinyl-CoA is converted to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). This is the only example in the citric acid cycle of substrate-level phosphorylation. Tissues in which glu-coneogenesis occurs (the hver and kidney) contain two isoenzymes of succinate thiokinase, one specific for GDP and the other for ADP. The GTP formed is used for the decarboxylation of oxaloacetate to phos-phoenolpymvate in gluconeogenesis and provides a regulatory hnk between citric acid cycle activity and the withdrawal of oxaloacetate for gluconeogenesis. Nongluconeogenic tissues have only the isoenzyme that uses ADP. 

As a result of oxidations catalyzed by the dehydrogenases of the citric acid cycle, three molecules of NADH and one of FADHj are produced for each molecule of acetyl-CoA catabohzed in one mrn of the cycle. These reducing equivalents are transferred to the respiratory chain (Figure 16-2), where reoxidation of each NADH results in formation of 3 ATP and reoxidation of FADHj in formation of 2 ATP. In addition, 1 ATP (or GTP) is formed by substrate-level phosphorylation catalyzed by succinate thiokinase. 

Citric acid cycle Succinate thiokinase Phosphorylation at substrate level 2... 

Synthesis of PHAMCL from fatty acids such as octanoic acid or from the corresponding alkanes such as octane was first detected in P. oleovorans [119]. The alkanes are oxidized to the fatty acids the latter are activated by thiokinases and then degraded via the fatty acid /1-oxidation pathway. Obviously intermediates of this pathway accumulate under conditions favorable for the synthesis of PHA and are subsequently converted into substrates for the PHA synthase. Many reactions for the conversion of an intermediate of the -oxidation cycle into R-(-)-3-hydroxyacyl-CoA were considered. These were ... 

Succinyl CoA synthetase (succinate thiokinase) catalyzes a substrate-level phosphorylation of GDP to GTP. 

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). 

In the last step, pyruvate kinase transfers this residue to ADP. The remaining enol pyruvate is immediately rearranged into pyruvate, which is much more stable. Along with step [7] and the thiokinase reaction in the tricarboxylic acid cycle (see p. 136), the pyruvate kinase reaction is one of the three reactions in animal metabolism that are able to produce ATP independently of the respiratory chain. 

This enzyme [EC 6.2.1.1], also referred to as acetate-CoA ligase or acetate thiokinase, catalyzes the reaction of acetate, coenzyme A, and ATP to form acetyl-CoA, AMP, and pyrophosphate. The enzyme will also utilize propanoate and propenoate as substrates. 

Succinyl-CoA synthetase (SCS), also known as succinate thiokinase (STK) or succinate CoA ligase ( 6.2.1.4-5), is so far the only known hydrogenosomal enzyme directly involved in energy conservation. The protein catalyzes the reversible, substrate-level phosphorylation of ADP or GDP to the respective triphosphate at the expense of the high-energy thioester bond of succinyl-CoA. Succinate and CoA are released in the reaction. The I vaginalis enzyme... 

Jenkins TM, Gorrell , Muller M, Weitzman PD (1991) Hydrogenosomal succinate thiokinase in Tritrichomonas foetus and Trichomonas vaginalis. Biochem Biophys Res Commun 179 892-896... 

Fig. 8 SDS-PAGE (a) and Western blot (b) analysis of the purified hydrogenosomal fractions isolated from the metronidazole-susceptible T. vaginalis strain TV 10-02 (P) and its metronidazole-resistant derivatives MR-3, MR-5, MR-30, MR-50, and MR-100 displaying the aerobic (3), early anaerobic (5), advanced anaerobic (30, 50), and fully developed anaerobic resistance (100) to metronidazole. Numbers in the designation of MR strains indicate the concentrations of metronidazole in ixg/ml at which the organisms multiply in culture. About 10 pg protein was loaded per line. PFOR pyruvate ferredoxin oxidoreduc-tase, a-STK a subunit of succinate thiokinase (hydrogenosomal enzyme not involved in metronidazole resistance used as control), Fdx ferredoxin. From Rasoloson et al. (2002) by courtesy of the Society of General Microbiology...

Reeves RE, Warren LG, Susskind B, Lo H-S (1977) An energy-conserving pyruvate-to-acetate pathway in Entamoeba histolytica. Pyruvate synthase and a new acetate thiokinase. J Biol Chem 252 726-731... 

Fig. 1 Enzymes localized to B. hominis mitochondrial-like organelle. The enzymes are 1 malic enzyme, 2 pyruvate NADP oxidoreductase, 3 acetate succinate CoA transferase, 4 succinate thiokinase, 5 a-ketoglutarate dehydrogenase, 6 isocitrate dehydrogenase, and 7 aconitase...

Oberlies, G. Fuchs, G. Thauer, R.K. Acetate thiokinase and the assimilation of acetate in methanobacterium thermoautotrophicum. Arch. Microbiol., 128, 248-252 (1980)... 

Fluoroacetate, prepared commercially for rodent control, is also produced by a South African plant. After entering a cell, fluoroacetate is converted to fluoroacetyl-CoA in a reaction catalyzed by the enzyme acetate thiokinase ... 

Succinate thiokinase (also called succinyl CoA synthetase) cleaves the high-energy thioester bond of succinyl CoA (see Figure 9.6). This reaction is coupled to phosphorylation of GDP to GTP. GTP and ATP are energetically interconvertible by the nucleoside diphos phate kinase reaction ... 

The generation of GTP by succinate thiokinase is another example of substrate-level phosphorylation (see p. 100). [Note Succinyl CoA is also produced from propionyl CoA derived from the metabolism of fatty acids with an odd number of carbon atoms (see p. 191), and from metabolism of several amino acids (see p. 264). 

Conversion of a free fatty acid to its activated form A fatty add must be converted to its activated form (attached to coenzyme A) before it can participate in TAG synthesis. This reaction, illustrated in Figure 15.6 (see p. 175), is catalyzed by a family of fatty acyl Co A synthetases (thiokinases). 

After a LCFA enters a cell, it is converted to the CoA derivative by long-chain fatty acyl CoA synthetase (thiokinase) in the cytosol (see p. 174). Because 0-oxidation occurs in the mitochondrial matrix, the fatty acid must be transported across the mitochon drial inner membrane. Therefore, a specialized carrier transports the long-chain acyl group from the cytosol into the mitochondrial matrix. This carrier is carnitine, and the transport process is called the carnitine shuttle (Figure 16.16). 

Succinyl CoA is cleaved by succinate thiokinase (also called succinyl CoA synthetase), producing succinate and ATP (or GTP). This is an example of substrate-level phosphory lation. Succinate is oxidized to fumarate by succinate dehydrogenase, producing FADH2. The enzyme is inhibited by oxaloacetate. Fumarate is hydrated to malate by fumarase (fumarate hydratase), and malate is oxidized to oxaloacetate by malate dehy drogenase, producing NADH. 

Fig. 5.22. Oxidation of acetyl-CoA via the tricarboxylic acid (TCA) cycle. Individual enzymes of the pathway are marked. 1, citrate synthase 2 and 3, cis-aconitate hydratase 4 and 3, isocitrate dehydrogenase 6, a-oxo glutarate dehydrogenas 7, succinate thiokinase 8, succinate...

Succinate Thiokinase Couples the Conversion of Succinyl-CoA to Succinate with the Synthesis of GTP... 

The reaction is complex and involves an intermediate in which a phosphate is attached to a histidine residue of the succinate thiokinase enzyme. Probably CoA is first displaced by inorganic phosphate, forming succinyl phosphate. A nitrogen atom of a specific histidine residue then attacks phosphorus, displacing succinate and forming an A-phos-phoryl derivative. In the final step GDP attacks the phosphorus atom of that derivative forming GTP. The role of GTP in this reaction is played by ATP in some organisms. 

---