Among the many molecules of high-energy phosphate compounds formed as a result of the functioning of the citric acid cycle, one molecule is synthesized at the substrate level. In which of the following reactionsdoes this occur?: (A) Citrate - a-ketoglutarate, (B) a-ketoglutarate - succinate, (C) Succinate - fumarate, (D) Fumarate - malate
Answer is B. A molecule of guanosine triphosphate is synthesized from guanosine diphosphate and phosphate at the cost ofhydrolyzing 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 alpha-ketoglutarate to succinate is catalyzed by the a -ketoglutarate dehydrogenase complex and succinyl-CoA synthetase, respectively.