The enzyme first catalyzes nucleophilic attack on the α-phosphate of ATP to form pyrophosphate and an acyl chain linked to AMP. The next step is formation of an activated thioester bond between the fatty acyl chain and Coenzyme A.
The balanced equation for the above is:
RCOO + CoASH + ATP → RCO-SCoA + AMP + PP <br />
This two-step reaction is freely reversible and its equilibrium lies near 1. To drive the reaction forward, the reaction is coupled to a strongly exergonic hydrolysis reaction: the enzyme inorganic pyrophosphatase cleaves the pyrophosphate liberated from ATP to two phosphate ions, consuming one water molecule in the process. Thus the net reaction becomes:
RCOO + CoASH + ATP → RCO-SCoA+ AMP + 2P