Succinyl CoA synthase

D. Succinyl CoA is hydrolyzed to succinate and CoA in a reaction catalyzed by succinyl CoA synthase. 

Fig. 1.2. Maps of pyruvate metabolism in trichomonad hydrogenosomes published in 1976 (a) and 2003 (b). Metabolism of malate omitted. 1 pyruvate ferredoxin oxidoreductase, 2 hydroge-nase, 3 electron transport protein (unidentified in 1976, known to be ferredoxin in 2003), 4 acetate succinate coenzyme A (CoA) transferase, 5 succinyl-CoA synthase, 6 acetyl-CoA synthase (its presence was not confirmed later), (a From Fig. 2 in Muller 1976 b from Fig. 7.3 in Muller 2003)...

Four of the Krebs cycle reactions are considered irreversible citrate synthase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, and succinyl-CoA synthase. In two of these, COz is evolved. In one reaction, succinyl-CoA synthase, a substrate-level phosphorylation takes place, in which a high-energy compound, GTP, is generated. Note that in three of the reactions NADH is... 

The succinyl-CoA synthase reaction used succinyl-CoA, a high-energy intermediate, to produce succinate. This reaction also directly produces in mammals. 

In this complex reaction catalyzed by succinyl-CoA synthase (succinate thiokinase), the energy-rich thioester linkage of succinyl-CoA is hydrolyzed with release of free energy that is conserved in the substrate phosphorylation of GDP with phosphate to form GTP ... 

Reaction 5. Succinyl CoA is converted to succinate in this step, which once more is chemically very involved. The enzynne succinyl CoA synthase catalyzes a coupled reaction in which the high-energy thioester bond of succinyl CoA is hydrolyzed and an inorganic phosphate group is added to GDP to make GTP ... 

Succinyl-CoA is a high-energy thiol ester compound with a AG for hydrolysis of about -33-5 kJ (-8 kcal) mol", i.e. of the same order as that required for the synthesis of ATP from ADP. In the course of the cycle, cleavage of the thioester bond is coupled with the phosphorylation of guanosine diphosphate (GDP) and the AG of the overall reaction, which is reversible and is catalysed by succinyl-CoA synthase, is close to zero. 

The utilization of acetoacetate is controlled by the activity of the citric acid cycle. The reaction of acetoacetate succinyl CoA transferase provides an alternative to the reaction of succinyl CoA synthase (see Figure 5.18), and there will only be an adequate supply of succinyl CoA to permit conversion of acetoacetate to acetoacetyl CoA as long as the rate of citric acid cycle activity is adequate. 

AAB have the alternative TCA cycle that is modified as part of these enzymes. Although A. pasteurianus does not have any succinyl CoA synthase that involves interconversion between succinyl-CoA and succinic acid, AarC complements that function. AarC converts succinyl-CoA to succinate and transfers the CoA to acetic acid in the ceU simultaneously these are converted to acetyl-CoA and then converted to citrate by AarA. Thus, AAB crmsume acetic acid (Fig. 10.5). 

---