Question

In the condensation reaction catalyzed by ß-ketoacyl-ACP synthase, a 4C unit is synthesized by the combination of a 2C unit and a 3C unit, with the release of CO₂. What is the (thermodynamic) advantage of this process over one that simply combines two 2C units?

Answer 1

The condensation reaction catalysed by the beta ketoacyl ACP synthase produces a 4C compound with the combination of a 2C compound (acetyl CoA) and 3C compound (malonyl CoA). This condensation reaction is favourable over the 2C compounds getting condensed thermodynamically because the malonyl CoA decarboxylation reaction is the driving force for this reaction. The energy captured in this carbon bond of malonyl CoA technically comes from the ATP that is used to form the bond in the 3C compound. If we use two acetyl CoA to condensation reaction, then there is no such thermodynamically favourable reactions that can occur because no decarboxylation or any side reaction can take place. Hence the thermodynamic advantage of the process of 3C+2C compound to form a 4C unit is the decarboxylation reaction which is thermodynamically equivalent to the ATP bond. Hence this is thermodynamically favourable.

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