Formation of Succinyl Phosphate by Reaction of Phosphorylated Succinic Thiokinase with Succinate

Previous work in this laboratory showed that succinyl phosphate can be isolated from reaction mixtures containing relatively large amounts of succinic thiokinase (isolated from Escherichia coli), adenosine triphosphate (ATP), succinate, and magnesium ions. In addition, it was demonstrated that chemically synthesized succinyl phosphate was utilized by the enzyme to form both ATP and succinyl-CoA. It was shown that the enzyme can also be phosphorylated by chemically synthesized succinyl phosphate. However, it was not clear whether the formation of enzyme bound succinyl phosphate involved direct phosphorylation of succinate by ATP or whether the phosphorylated enzyme reacted with succinate. The present experiments demonstrate that the phosphorylated enzyme reacts with succinate to yield succinyl phosphate, which, in the absence of coenzyme A (CoA), dissociates from the enzyme. Incubation of the phosphorylated enzyme with both succinate and CoA leads to formation of inorganic phosphate accompanied by little, if any, formation of succinyl phosphate. The data support a reaction sequence in which the enzyme is phosphorylated by ATP and the phosphorylated enzyme reacts with succinate to yield enzyme-bound succinyl phosphate, which in turn reacts with CoA to give succinyl-CoA. As there is also evidence favoring an alternative mechanism (the “high-energy” nonphosphorylated pathway), it may be suggested that both reaction sequences are catalyzed by the enzyme.