Coenzyme A Transferase

The following evidence supports the conclusion that the reaction catalyzed by succinyl coenzyme A:3-keto acid coenzyme A transferase proceeds through two half-reactions in a ping-pong mechanism with an enzyme-CoA intermediate. (a) Double reciprocal plots of the initial rates against substrate concentration for the reaction in each direction are best described by a series of parallel lines. (b) Inhibition by each of the acid products is competitive with respect to the other acid substrate and noncompetitive with respect to the acyl-CoA substrate. (c) Four expected Haldane relationships for this mechanism are obeyed within experimental error. (d) At a given concentration of acetoacetyl-CoA the maximal rate of the acetoacetate-acetoacetyl-CoA exchange reaction is the same as the maximal rate of the over-all reaction with succinate. (e) At saturating concentrations of acetoacetate the amount of inhibition of the reaction by succinate is equal to the amount of labeled succinate incorporated into succinyl-CoA; i.e. inhibition occurs by reversal of the first half-reaction. (f) At high concentrations of succinate the amount of inhibition by acetoacetate is equal to the amount of exchange of acetoacetate into acetoacetyl-CoA. At low succinate concentrations, at which the first half-reaction can proceed faster than the second, exchange is greater than inhibition. The enzyme is inhibited upon incubation with dilute solutions of acetylating agents or with acetoacetyl-CoA and certain bivalent metals. It is inhibited by monovalent anions, with an order of effectiveness which follows the Hofmeister series. Inhibition by sodium chloride is predominantly competitive with respect to succinate and acetoacetate.