Squalene Synthetase. Inhibition by Ammonium Analogues of Carbocationic Intermediates in the Conversion of Presqualene Diphosphate to Squalene

Article abstract of DOI:10.1021/ja00192a036

Squalene synthetase (EC 2.5.1.21) catalyzes the formation of squalene (3) from farnesyl diphosphate (1) via presqualene diphosphate (2) in two steps.The mechanism of the rearrangement of 2 to 3 was studied with stable ammonium analogues 6 and 7 of primary and tertiary cyclopropylcarbinyl cations 4 and 5, respectively, proposed as intermediates.In non-pyrophosphate-containing buffers, 6 and 7 were not inhibitors.However, the combination of 6 or 7 with PP_i produced potent synergistic inhibition of squalene synthesis from 1 and 2.Amino acid 8, an analogue in whicha phosphonophosphate moiety was tethered to the amino group in 6, was a potent inhibitor of squalene synthesis in pyrophosphate-free buffers.When synthesis of 2 and 3 from 1 was measured simultaneously in the presence of 8, both rates were depressed in an identical manner.It was concluded that squalene synthetase has a single active site which catalyzes 1 --> 2 and 2 --> 3.The mechanism of the second reaction is discussed.