Stoichiometry of substrate binding to rat liver fatty acid synthetase

Two rat liver fatty acid synthetase preparations, containing 1.6 and 2.0 mol of 4'-phosphopantetheine/mol of synthetase, showed specific activity of 2006 and 2140 nmol of NADPH oxidized/min per mg of protein respectively. The two synthetase preparations could be loaded with either 3.3-4.4 mol of [1-14] acetate or 2.9-3.7 mol of [2-14C]malonate, by incubation with either [1-14C] acetyl-CoA or [2-14C]malonyl-CoA. The 4'-phosphopantetheine site could be more than 90% saturated and the serine site about 80% saturated with malonate derived from malonyl-CoA. However, with acetyl-CoA as substrate, binding at both the 4'-phosphopantetheine and cysteine thiol sites did not reach saturation. We interpret these results to indicate that, whereas the equilibrium constant for transfer of substrates between the serine loading site and the 4'-phosphopantetheine site is close to unity, that for transfer of acetyl moieties between the 4'-phosphopantetheine and cysteine sites favours formation of the 4'-phosphopantetheine thioester. Thus, despite the apparent sub-stoichiometric binding of acetate, the results are consistent with a functionally symmetrical model for the fatty acid synthetase which permits simultaneous substrate binding at two separate active centres.