The relationships between the aggregation state and the enzymatic activities of chicken
liver fatty acid synthase have been explored by monitoring the changes in light scattering, fluorescence, and the overall, beta-ketoacyl synthase, beta-ketoacyl
reductase and enoyl
reductase activities during dissociation and reassociation of the
enzyme. The data obtained indicate that the
enzyme dissociates at low temperature in both 0.1 M
potassium phosphate (pH 7.0), 1 mM
EDTA, and 5 mM
Tris(hydroxymethyl)aminomethane, 35 mM
glycine (pH 8.3) and 1 mM
EDTA, but the extent of dissociation is less in the
phosphate buffer. The assay conditions influence the assessment of the degree of dissociation and association: high temperatures,
phosphate (high
salt),
NADPH and
acetoacetyl-coenzyme A promote association of the monomeric
enzyme, whereas dilution in the Tris-
glycine buffer (low
salt) and low temperature promote dissociation. Both the rate and extent of association and dissociation are altered by substrates. The monomeric
enzyme does not possess beta-ketoacyl synthase and beta-ketoacyl
reductase activities. Results obtained with the
1,3-dibromo-2-propanone cross-linked
enzyme, which lacks beta-ketoacyl synthase activity, indicate that the
NADPH-binding site of beta-ketoacyl
reductase is disrupted at low ionic strength. In contrast, changes in ionic strength have little effect on the enoyl
reductase activity. The dimer is stabilized by both electrostatic and hydrophobic interactions, with the former being of special importance for maintenance of the beta-ketoacyl
reductase active site. site.