Mode of Inhibition of β-Hydroxydecanoyl Thioester Dehydrase by 3-Decynoyl-N-acetylcysteamine

The alkynoic substrate analogue 3-decynoyl-N-acetylcysteamine (NAC) was previously shown to inhibit β-hydroxydecanoyl thioester dehydrase irreversibly. 2,3-Decadienoyl-NAC prepared by chemical isomerization of 3-decynoyl-NAC inhibits dehydrase activity even more effectively. The allenic and the acetylenic inhibitors both inactivate dehydrase by combining with a histidine residue at the active site. α-Dideutero-3-decynoyl-NAC reacts more slowly with dehydrase than its hydrogen analogue: kh:kd = 2.60. No kinetic isotope effect is observed for the inhibition of enzyme by α-d-2,3-decadienoyl-NAC. It is concluded that 3-decynoyl-NAC as such is not the true inhibitor, but that the dehydrase isomerizes it to the allene which in turn inactivates the enzyme by a chemical reaction. The finding that free 2,3-decadienoic acid inhibits dehydrase, whereas 3-decynoic acid does not, supports the postulated mode of action.