The Reaction Mechanism of Chromanol‐Ring Formation Catalyzed by tocopherol cyclase from <i>Anabaena variabilis</i> K<scp>ÜTZING</scp> (<i>Cyanobacteria</i>)

Abstract

Abstract Incubation of the synthetic 18 O‐labelled phytyl‐hydroquinone ( O 4 ‐ 18 O)‐ 2 with the tocopherol cyclase from Anabaena variabilis K ÜTZING ( Cyanobacteria ) in D 2 O furnished the doubly labelled γ‐tocopherol, (2 R ,3 S ,4′ R ,8′ R )‐(1‐ 18 O,3‐ 2 H)‐ 1 . The chirality at C(3) was determined by two independent routes providing interlocking evidence that the enzyme‐catalyzed ring closure proceeds by si ‐protonation of the double bond of 2 and concomitant re ‐attack of the phenolic O‐atom.