Activation: Fatty Acid ⇄ Fatty Acyl‐CoA
The first step in utilizing the fatty acid molecule for energy production is the conversion of the fatty acid to a CoA molecule in a two‐step process:
Note that the hydrolysis of two high‐energy phosphate bonds in ATP provides the energy source for the reaction. The inorganic pyrophosphate, PP i, is subsequently broken down to two phosphate ions by inorganic pyrophosphatase. The action of this enzyme means that very little PP i remains in the cell, making the synthesis of the fatty acyl‐CoA favored. This is an example of metabolic coupling, the process whereby a thermodynamically unfavored reaction is allowed because it shares an intermediate (in this case PP i) with a favored one.