Polyprenyl diphosphate synthase from mulberry leaves: Stereochemistry of hydrogen elimination in the prenyltransferase reaction

A polyprenyltransferase catalysing the formation of Z-double bonds was partially purified from mulberry leaves, Morus bombysis. The enzyme catalysed a consecutive condensation of isopentenyl diphosphate with geranyl- geranyl diphosphate as an allylic primer to produce a series of ficaprenol-type Z,E-mixed polyprenyl diphosphates with carbon chain length ranging from C40 to C60. Not only all-E-geranylgeranyl diphosphate but also E,E-farnesyl- and geranyl diphosphates were accepted as substrates. Addition of Triton X-100 stimulated the enzymatic activity. The stereochemistry of hydrogen elimination from the 2-position of isopentenyl diphosphate during the Z-prenyl chain formation was examined directly by experiments using this synthase and (S)-[1-14C, 2-3H]isopentenyl diphosphate and it was demonstrated that the 2-pro-S hydrogen was lost. Feeding experiments of stereospecifically 3H-labelled mevalonic acid with intact mulberry leaves, however, showed that mevalonic acid was incorporated into polyprenols with elimination of the 4-pro-S hydrogen of the acid.