Question 2PDB's (Protein Data Bank) molecule of the month for September is citrate synthase, one of the enzymes of the citric acid cycle. Read the PDB website to find out more about this interesting enzyme. [Hat Tip: Philip J at Biocurious]
Citrate synthase catalyzes the following reaction,
One of the most interesting things about this reaction is that the standard Gibbs free energy change of the reaction (ΔG°′) is −31.5 kJ mol-1. Here's a question that I ask of my second year students.
Like most reactions in vivo, the actual Gibbs free energy change for this reaction is zero. Normally you might expect that such a large negative standard Gibbs free energy change would indicate that the forward reaction is coupled to the synthesis of ATP. Indeed, the hydrolysis of the similar thioester bond in succinyl CoA (Step 5 of the citric acid cycle) is coupled to the synthesis of GTP (or ATP). However, in the case of the citrate synthase reaction, the available energy is used for a different purpose. What is this purpose?
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