|MadSci Network: Biochemistry|
The mechanism of the citrate synthase-catalyzed reaction between oxaloacetate and acetyl-CoA is a multi-step mechanism (~3 steps). The first step of the mechanism, after which an enol intermediate is formed, is considered to be the rate-limiting step of the mechanism because there is a large energetic barrier between the reactants and the intermediate.
You may have been told that the deltaG for this reaction is a large negative number (about -32 kJ/mol), but just remember that deltaG is the difference between the free energy of the reactants and products of a reaction, and says nothing about the pathway to get there. This reaction has a BIG energetic payoff, but getting there involves traversing at least one large energy barrier.
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