The free coenzyme A requirement of animal fatty acid synthetase. Participation in the continuous exchange of acetyl and malonyl moieties between coenzyme a thioester and enzyme.

A hypothesis that the existence of common binding sites for acetyl and malonyl moieties on the animal fatty acid synthetase necessitates that free CoA be available continuously to facilitate unloading of inappropriately bound acetyl or malonyl moieties, allowing initial access of an acetyl moiety and subsequent access by malonyl moieties to the site of chain elongation, was formulated and tested. The unloading of acetyl or malonyl moieties from the enzyme was blocked by a CoA-scavenging system and the enzyme was unable to reload with the other substrate; the inhibition was relieved by the addition of CoA or pantetheine. The freely reversible nature of the loading/unloading reaction was established as follows. CoA or pantetheine, but not S-acetyl-N-acetylcysteamine, could act as donor or acceptor for acetyl moieties in the loading or unloading reactions; incubation of fatty acid synthetase, acetyl-CoA, and [G-3H]CoA resulted in the formation of acetyl-[G-3H]CoA in an amount consistent with the predicted equilibrium; and addition of a high concentration of CoA shifted the equilibrium toward unloading, leaving most of the substrate-binding sites vacant. These results support our hypothesis and provide a plausible explanation both for the requirement of free CoA by the fatty acid synthetase and for the observed inhibition of fatty acid synthesis by high concentrations of CoA.