Thermodynamic bases for fatty acid ethyl ester synthase: catalyzed esterification of free fatty acid with ethanol and accumulation of fatty acid ethyl esters

Myocardial homogenates rapidly synthesize fatty acyl ethyl esters from nonesterified fatty acid and ethanol in the absence of coenzyme A or ATP, and the enzyme catalyzing this reaction, fatty acid ethyl ester synthase, has been purified 5400-fold to homogeneity [Mogelson, S., & Lange, L. G. (1984) Biochemistry (preceding paper in this issue)]. To define the factors permitting this de novo synthesis of ester bonds and the consequent accumulation of fatty acyl ethyl esters in myocardium, we determined thermodynamic parameters relevant to the kinetics and equilibria of this reaction and specifically characterized (1) the rates of synthesis of ethyl oleate, in both the presence and absence of purified enzyme catalyst, and (2) the physical properties of the product, ethyl oleate, in an aqueous milieu. Compared to the reaction of ethanol and oleate in the absence of catalyst, fatty acid ethyl ester synthase enhanced the rate of ethyl oleate synthesis by reducing the free energy of activation (delta G) from 32.5 to 19.9 kcal/mol, effected in large part by a positive entropy shift, delta Senz - delta S uncat = 23.9 cal/(mol.deg). Rate constants in the presence and absence of enzyme at 37 degrees C were 6 X 10(-2) s-1 and 7.8 X 10(-11) M-1 s-1, respectively, indicating a catalytic power of at least 10(8)M for this enzyme. Kinetic data indicated an enzymatic Vmax of 1.25 nmol/(mg.s) (37 degrees C). The equilibrium constant was calculated for the reaction oleate + ethanol in equilibrium ethyl oleate and was 0.095 M-1 at 37 degrees C.