Substrate level phosphorylation in TCA cycle is in step?

ANSWER: (D) Succinate thiokinaseREF: Murray pp 130-135, Scriver, pp 1521-1552, http://en.wikipedia.org/wiki/Substratelevel_ phosphorylationSubstrate-level phosphorylation is a type of metabolism that results in the formation and creation of adenosine triphosphate (ATP) or guanosine triphosphate (GTP) by the direct transfer and donation of a phosphoryl (PO3) group to adenosine diphosphate (ADP) or guanosine diphosphate (GDP) from a phosphorylated reactive intermediate. Note that the phosphate group does not have to directly come from the substrate. By convention, the phosphoryl group that is transferred is referred to as a phosphate group.Once the pyruvate product of glycolysis is moved to the mitochondrial matrix, the pyruvate is converted to acetate and binds Coenzyme A to form Acetyl CoA to enter the Krebs cycle. While the Krebs cycle is oxidative respiration, one more instance of substrate-level phosphorylation occurs as Guanosine triphosphate (GTP) is created from GDP by transfer of a phosphate group during the conversion of Succinyl CoA to Succinate. This phosphate is transferred to ADP in another substrate-level phosphorylation event. The reaction is catalyzed by the enzyme Succinyl- CoA synthetase.A molecule of guanosine triphosphate is synthesized from guanosine diphosphate and phosphate at the cost of hydrolyzing succinyl-CoA to succinate and CoA. This constitutes substrate-level phosphorylation, and, in contrast to oxidative phosphorylation, this is the only reaction in the citric acid cycle that directly yields a high-energy phosphate bond. The sequence of reactions from a ketoglutarate to succinate is catalyzed by the a ketoglutarate dehydrogenase complex and succinyl-CoA synthetase, respectively. Succinate thiokinase is the only enzyme that generates ATP directly by substrate-level phosphorylation.a ketoglutarate + NAD+ + acetyl-CoA - succinyl-CoA + CO2 + NADHsuccinyl-CoA + Pi + GDP - succinate + GTP + acetyl-CoA