Although fatty acid f-oxidation is catalyzed by a series of intramitochon-drial enzymes, and the fatty acyl chain is carried by CoA, fatty acid synthesis is catalyzed by a cytosolic-multienzyme complex in which the growing fatty acyl chain is bound by thioester linkage to an enzyme-bound 4 -phospho-pantetheine residue. This component of the fatty acid synthetase complex is ACP.
Apo-ACP is activated by a transferase, holo-ACP synthetase, which transfers 4'-phosphopantetheine from CoA to the hydroxyl group of a serine residue in the apoprotein, releasing ADP. ACP is inactivated by a hydrolase that releases 4'-phosphopantetheine, which can be reutilized for CoA synthesis.
There is a rapid turnover of phosphopantetheine between ACP and CoA, in response to the metabolic state, and the need for fatty acid synthesis (and thus ACP in the fed state) or fatty acid ^-oxidation (and thus CoA in the fasting state). Apo-ACP has a half-life of 6 to 7 days, whereas the prosthetic group turns over with a half-life of a few hours (Tweto and Larrabee, 1972; Volpe and Vagelos, 1973).
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