Biochemistry - Chemical Reactions in the Living Cell, Volume 2 - D. Metzler 1980

Types of reactions catalyzed by enzymes
Multiple displacement reactions and coupling of ATP cleavage reactions with endergonic processes
CoA transferases

Energy transfer often presents the following challenge: a Cell constantly has access to a thioester, such as succinyl-CoA, whose unstable bond stores the energy required to synthesize various other thioesters. The Cell could theoretically first synthesize ATP or GTP via a reverse thiokinase reaction, and then use the resulting ATP and GTP to synthesize a new bond, once again utilizing a thiokinase. However, special Enzymes known as CoA transferases accomplish this in a much more direct manner:

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The exact MECHANISM OF ACTION for these enzymes remains to be definitively established. How can CoA be transferred from one acyl group to another while preserving the high group-transfer potential of the acyl group?

The following experiments shed light on this question. Based on kinetic studies of the reaction catalyzed by succinyl-CoA: acetoacetate CoA-transferase, it was concluded that the reaction proceeds via a ping-pong mechanism (Chapter 6). Thus, the enzyme exists in two distinct forms, one of which has been shown to contain bound CoA [92]. The enzyme–CoA intermediate formed during the interaction between the enzyme and acetoacetate-CoA was reduced with ³H-labeled sodium borohydride, followed by complete acid Hydrolysis of the enzyme protein using HCl. This Procedure yielded tritiated α-amino-δ-hydroxyvaleric acid.

Since thioesters, much like standard esters, are cleaved to alcohols by sodium borohydride (via a two-step reaction), it was concluded that the enzyme–CoA intermediate is a thioester formed involving the side-chain carboxyl group of a glutamic acid residue [93]. Exchange studies using ¹⁸O-labeled succinate demonstrate that the ¹⁸O label from succinate is incorporated into both the enzyme–CoA intermediate and the carboxyl group of acetoacetate [94].

At least two mechanisms can be proposed to underlie this process. The first involves The formation of an anhydride intermediate and is mechanistically similar to the reactions discussed in previous sections [94a]. Students are encouraged to write out the sequential steps of this mechanism. Can this mechanism account for the data obtained from the ¹⁸O-exchange experiments? The second possible mechanism involves a four-center reaction:

Although the operation of mechanisms of this type has not been definitively proven for enzymatic reactions, they remain theoretically feasible.