Thioester Bonds are Energetic Too

Phosphoanhydride bonds like those in ATP are not the only energy-carrying bonds used by biology. In fact, phosphate was likely scarce during the early history of Earth and some researchers hypothesize that thioester bonds - bonds between a sulfur and a carbonyl carbon (R–S–CO–R’) - were used as energy currency during the evolution of early life (Goldford et al. 2017).

Thioacetate

Thioacetate is a simple example of a compound with a thioester bond.

Acetyl-CoA

Acetyl-CoA is the protypical example of a biological molecule containing a thioester bond. Can you locate the thioester? It might help to look at a larger image.

So how much energy is there in a thioester bond? We can use eQuilibrator to examine the hydrolysis of the common biological thioester, acetyl-CoA

Acetyl-CoA + H2O ⇌ Acetate + CoA

We find that this hydrolysis reaction has a ΔrG’m around -40 kJ/mol - very similar to the energetic scale of ATP hydrolysis. In fact, the thioester bond is sometimes exchanged with a phosphoester bond (like the one in ATP) during metabolism of acetyl-CoA

Acetyl-CoA + Pi ⇌ Acetyl-phosphate + CoA

Notice that this reaction has a positive ΔrG’m ≈ 10 kJ/mol, meaning that it would flow in the reverse direction (forming acetyl-CoA) if all the reactants had 1 mM concentrations. However, concentrations are of course not exactly 1 mM in cells. Cellular concentrations of inorganic phosphate are, for example, typically closer to 10 mM [1].

Try using eQuilibrator to set the phosphate concentration to 10 mM, leaving everything else at 1 mM. Notice that the ΔrG’m value changed by about 6 kJ/mol. As an exercise: show with some simple math that a 10-fold change in one reactant concentration will always alter the ΔrG’ by about 6 kJ/mol. [2]

[1]For typical concentrations in E. coli see BioNumbers ID 105540
[2]Hint: think about the formula for ΔrG’ = ΔrG’° + RT ln Q.