Role of the Enoyl Reductase Domain in the Regulation of Fatty Acid Synthase Activity

Fatty acid synthase from the uropygial gland of goose was inhibited by increasing the [NADP]:[NADPH] ratio. NADP inhibition of the overall activity of fatty acid synthase, the ketoreductase, and the enoyl reductase were competitive with respect to NADPH with Ki values of 6 p ~ , 40 p ~ , and 260 pM, respectively. The other component activities of fatty acid synthase were not affected by NADP with one exception; the condensing activity was severely inhibited. This inhibition was noncompetitive with respect to malonyl-CoA and hexanoyl-CoA. The inhibition of condensing activity by NADP was aboiished by selective modification of the NADPH binding site of the enoyl reductase domain with pyridoxal phosphate. That the binding of NADPH, which triggers dimerization of fatty acid synthase peptides, occurs at the enoyl reductase domain was shown by the inability of NADPH to dimerize the pyridoxal phosphate-modified enzyme although this enzyme could be dimerized by high ionic strength. Proteolysis of the native enzyme was inhibited by NADPH but the proteolysis of the modified enzyme was not. These results strongly suggest that enoyl reductase domain of fatty acid synthase plays a key role in the interdomain interactions which regulate the activity of the enzyme via the pADP]:mADPH] ratio.