The purified multienzyme complex, fatty acid synthetase of yeast, produces palmityl-CoA and stearyl-CoA in equal amounts under standard conditions, with adequate NADPH, and a 1:2 ratio of acetyl-CoA and malonyl-CoA. Based on the known enzymatic properties of fatty acid synthetase a model which rationalizes the chain termination at the level of C16 and C18-acid is proposed.
The model is based on two assumptions that are supported by experimental evidence: (a) the probability of any covalently enzyme-bound saturated acyl residue forming a product by transfer to CoA is determined by the relative velocities of the condensing and transferring reactions. (b) The growing alkan chain interacts with the enzyme protein only after a chain length of 13 carbon atoms has been attained; this interaction changes the relative velocities in favour of product formation by an energy increment of -0.9 kcal for each additional methylene group beyond the 13th.
3 To calculate the probability of product release at a particular chain length, an equation was derived from the model describing quantitatively the observed product distribution.
4 The formula suggests conditions under which either short acyl-CoA derivatives or exclusively stearyl-CoA can be produced. Synthesis under these conditions was examined experimentally and results indicated that the formula can be applied to a wide range of experimental conditions.
5 Because the condensation reaction in fatty acid synthesis is the rate limiting step under normal conditions, saturated acyl residues have the longest life-time as intermediates and consequently the highest probability of leaving the enzyme complex as an endproduct. The reduction reactions can be made rate limiting when the NADPH level drops. Under such circumstances β-ketoacyl-CoA's and α,β-unsaturated acyl-CoA's appear as endproducts. With NADH as electron donor the first reduction is slowed to such an extent that predominantly β-ketoacyl-CoA's derivatives are synthesized.
6 The possibility that the enzyme complex catalyses synthesis of short chain fatty acids and and β-ketoacids in vivo is discussed.