Protein Turnover and 4'-Phosphopantetheine Exchange in Rat Liver Fatty Acid Synthetase

The rate of turnover of rat liver fatty acid synthetase has been measured in animals tested under steady state conditions. The value obtained for the half-life varied from 71 to 108 hours. The exchange rate of the covalently bound prosthetic group, 4'-phosphopantotheine, with unbound pantothenate compounds was found to be more than an order of magnitude greater than the half-life of the enzyme complex. A model is proposed in which the prosthetic group, or some small portion of the enzyme complex containing the prosthetic group, is periodically removed from and replaced on the larger fatty acid synthetase moiety, such action taking place many times before an individual complex is catabolized. Moreover, this exchange is not part of the catalytic function of an enzyme complex, but may be a means of control of over all fatty acid synthetase activity.