Mammalian fatty acid synthetase is a structurally and functionally symmetrical dimer.
We have explored a comprehensive experimental approach to determine whether the two condensing-enzyme active centers of the mammalian fatty acid synthetase are simultaneously functional. Our strategy involved utilization of trypsinized fatty acid synthetase, which is a nicked homodimer composed of two pairs of 125 + 95-kDa polypeptides. These core polypeptides lack the chain-terminating thioesterase domains but retain all other functional domains of the native enzyme and can assemble long-chain acyl moieties at a rate equal to that of the native enzyme. The 4'-phosphopantetheine content of these enzyme preparations, estimated from the amount of beta-alanine present, from the amount of taurine formed by performic acid oxidation and from the amount of carboxymethylcysteamine formed by alkylation with iodo[2-14C]acetate, was typically 0.86 mol/mol 95-kDa polypeptide. The stoichiometry of long-chain acyl-enzyme synthesis, measured with radiolabeled precursors, indicated that 0.84 mol acyl-chains were assembled/mol 95-kDa polypeptide. When the small amount of apoenzyme present is taken into account, this stoichiometry translates to 1.94 acyl chains per holoenzyme dimer. The 125-kDa polypeptide of one subunit could be cross-linked to the 95-kDa polypeptide of the other subunit by 1,3-dibromo-2-propanone yielding a single molecular species of 220 kDa. Cross-linking was accompanied by a loss of condensing-enzyme activity. This result is consistent with a structurally symmetrical model for the animal fatty acid synthetase [J.K. Stoops and S.J. Wakil (1981) J. Biol. Chem. 256, 5128-5133] in which the juxtaposed 4'-phosphopantetheine and cysteine thiols of opposing subunits that form the two potential catalytic centers for condensing activity are readily susceptible to cross-linking. Both half-maximal cross-linking and 50% inhibition of activity were observed with 1 mol 1,3-dibromo-2-propanone bound/mol enzyme. After assembly of long-chain acyl moieties on the 4'-phosphopantetheine residues, no vacant condensing-enzyme active sites were demonstrable either by cross-linking with 1,3-dibromo-2-propanone or by formation of carboxymethylcysteamine on treatment with iodoacetate. These results are consistent with a structurally and functionally symmetrical model for the mammalian fatty acid synthetase in which the two condensation sites are simultaneously active.
D000096:Acetone / Q000031:analogs & derivatives; D000478:Alkylation; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D055598:Chemical Phenomena; D002621:Chemistry; D002851:Chromatography, High Pressure Liquid; D004591:Electrophoresis, Polyacrylamide Gel; D064429:Fatty Acid Synthases / Q000302:isolation & purification*; D005260:Female; D006868:Hydrolysis; D007774:Lactation; D008099:Liver / Q000201:enzymology; D008321:Mammary Glands, Animal / Q000201:enzymology; D008970:Molecular Weight; D010204:Pantetheine / Q000031:analogs & derivatives / Q000032:analysis; D011247:Pregnancy; D051381:Rats; D014357:Trypsin
D000096:Acetone; D010204:Pantetheine; C047048:1,3-dibromo-2-propanone; D064429:Fatty Acid Synthases; D014357:Trypsin; C003129:4'-phosphopantetheine